what is the name of the high-energy compound that cells use to fuel chemical reactions?

General Concepts

Heterotrophic Metabolism

Heterotrophic metabolism is the biologic oxidation of organic compounds, such as glucose, to yield ATP and simpler organic (or inorganic) compounds, which are needed by the bacterial cell for biosynthetic or assimilatory reactions.

Respiration

Respiration is a type of heterotrophic metabolism that uses oxygen and in which 38 moles of ATP are derived from the oxidation of 1 mole of glucose, yielding 380,000 cal. (An additional 308,000 cal is lost every bit heat.)

Fermentation

In fermentation, another type of heterotrophic metabolism, an organic chemical compound rather than oxygen is the terminal electron (or hydrogen) acceptor. Less energy is generated from this incomplete grade of glucose oxidation, but the process supports anaerobic growth.

Krebs Cycle

The Krebs cycle is the oxidative process in respiration past which pyruvate (via acetyl coenzyme A) is completely decarboxylated to CO_two. The pathway yields xv moles of ATP (150,000 calories).

Glyoxylate Cycle

The glyoxylate wheel, which occurs in some bacteria, is a modification of the Krebs wheel. Acetyl coenzyme A is generated straight from oxidation of fatty acids or other lipid compounds.

Electron Transport and Oxidative Phosphorylation

In the terminal stage of respiration, ATP is formed through a series of electron transfer reactions inside the cytoplasmic membrane that drive the oxidative phosphorylation of ADP to ATP. Leaner utilize diverse flavins, cytochrome, and non-heme iron components equally well as multiple cytochrome oxidases for this procedure.

Mitchell or Proton Extrusion Hypothesis

The Mitchell hypothesis explains the energy conservation in all cells on the basis of the selective extrusion of H⁺ ions across a proton-impermeable membrane, which generates a proton motive force. This energy allows for ATP synthesis both in respiration and photosynthesis.

Bacterial Photosynthesis

Bacterial photosynthesis is a light-dependent, anaerobic mode of metabolism. Carbon dioxide is reduced to glucose, which is used for both biosynthesis and energy product. Depending on the hydrogen source used to reduce CO_ii, both photolithotrophic and photoorganotrophic reactions exist in bacteria.

Autotrophy

Autotrophy is a unique class of metabolism found only in bacteria. Inorganic compounds are oxidized directly (without using sunlight) to yield energy (e.g., NH₃, NO₂ ^–, Southward_two, and Feⁱⁱ⁺). This metabolic mode also requires energy for CO₂ reduction, like photosynthesis, merely no lipid-mediated processes are involved. This metabolic mode has also been called chemotrophy, chemoautotrophy, or chemolithotrophy.

Anaerobic Respiration

Anaerobic respiration is another heterotrophic mode of metabolism in which a specific chemical compound other than O₂ serves as a terminal electron acceptor. Such acceptor compounds include NO_three ^–, Then₄ ^two–, fumarate, and even CO_ii for methane-producing bacteria.

The Nitrogen Wheel

The nitrogen wheel consists of a recycling process past which organic and inorganic nitrogen compounds are used metabolically and recycled among bacteria, plants, and animals. Important processes, including ammonification, mineralization, nitrification, denitrification, and nitrogen fixation, are carried out primarily by bacteria.

Introduction

Metabolism refers to all the biochemical reactions that occur in a cell or organism. The study of bacterial metabolism focuses on the chemical diversity of substrate oxidations and dissimilation reactions (reactions by which substrate molecules are broken down), which unremarkably office in bacteria to generate energy. Also inside the telescopic of bacterial metabolism is the study of the uptake and utilization of the inorganic or organic compounds required for growth and maintenance of a cellular steady land (assimilation reactions). These respective exergonic (energy-yielding) and endergonic (energy-requiring) reactions are catalyzed inside the living bacterial cell past integrated enzyme systems, the end event being cocky-replication of the prison cell. The capability of microbial cells to live, role, and replicate in an appropriate chemic milieu (such every bit a bacterial culture medium) and the chemical changes that effect during this transformation constitute the telescopic of bacterial metabolism.

The bacterial jail cell is a highly specialized energy transformer. Chemical energy generated by substrate oxidations is conserved past germination of high-energy compounds such every bit adenosine diphosphate (ADP) and adenosine triphosphate (ATP) or compounds containing the thioester bond

(acetyl ~ SCoA) or succinyl ~ SCoA. ADP and ATP represent adenosine monophosphate (AMP) plus i and two high-energy phosphates (AMP ~ P and AMP ~ P~ P, respectively); the energy is stored in these compounds as high-energy phosphate bonds. In the presence of proper enzyme systems, these compounds tin can be used as free energy sources to synthesize the new circuitous organic compounds needed past the cell. All living cells must maintain steady-land biochemical reactions for the formation and use of such high-energy compounds.

Kluyver and Donker (1924 to 1926) recognized that bacterial cells, regardless of species, were in many respects similar chemically to all other living cells. For example, these investigators recognized that hydrogen transfer is a common and fundamental feature of all metabolic processes. Bacteria, similar mammalian and institute cells, utilise ATP or the high-energy phosphate bond (~ P) equally the primary chemical free energy source. Bacteria besides require the B-complex vitamins equally functional coenzymes for many oxidation-reduction reactions needed for growth and energy transformation. An organism such as Thiobacillus thiooxidans, grown in a medium containing only sulfur and inorganic salts, synthesizes large amounts of thiamine, riboflavine, nicotinic acid, pantothenic acid, pyridoxine, and biotin. Therefore, Kluyver proposed the unity theory of biochemistry (Die Einheit in der Biochemie), which states that all basic enzymatic reactions which support and maintain life processes within cells of organisms, had more similarities than differences. This concept of biochemical unity stimulated many investigators to use bacteria as model systems for studying related eukaryotic, plant and beast biochemical reactions that are essentially "identical" at the molecular level.

From a nutritional, or metabolic, viewpoint, three major physiologic types of leaner be: the heterotrophs (or chemoorganotrophs), the autotrophs (or chemolithotrophs), and the photosynthetic bacteria (or phototrophs) (Table iv-one). These are discussed below.

Table four-1

Nutritional Variety Exhibited by Physiologically Different Bacteria.

Heterotrophic Metabolism

Heterotrophic bacteria, which include all pathogens, obtain free energy from oxidation of organic compounds. Carbohydrates (particularly glucose), lipids, and protein are the most commonly oxidized compounds. Biologic oxidation of these organic compounds by leaner results in synthesis of ATP as the chemical energy source. This process as well permits generation of simpler organic compounds (precursor molecules) needed by the bacteria cell for biosynthetic or assimilatory reactions.

The Krebs wheel intermediate compounds serve every bit precursor molecules (edifice blocks) for the energy-requiring biosynthesis of complex organic compounds in bacteria. Deposition reactions that simultaneously produce energy and generate precursor molecules for the biosynthesis of new cellular constituents are chosen amphibolic.

All heterotrophic leaner require preformed organic compounds. These carbon- and nitrogen-containing compounds are growth substrates, which are used aerobically or anaerobically to generate reducing equivalents (eastward.g., reduced nicotinamide adenine dinucleotide; NADH + H⁺); these reducing equivalents in turn are chemical energy sources for all biologic oxidative and fermentative systems. Heterotrophs are the almost commonly studied bacteria; they grow readily in media containing carbohydrates, proteins, or other complex nutrients such as blood. Also, growth media may be enriched past the addition of other naturally occurring compounds such as milk (to report lactic acid bacteria) or hydrocarbons (to study hydrocarbon-oxidizing organisms).

Respiration

Glucose is the well-nigh common substrate used for studying heterotrophic metabolism. Most aerobic organisms oxidize glucose completely by the following reaction equation:

This equation expresses the cellular oxidation process called respiration. Respiration occurs within the cells of plants and animals, unremarkably generating 38 ATP molecules (equally energy) from the oxidation of 1 molecule of glucose. This yields approximately 380,000 calories (cal) per style of glucose (ATP ~ x,000 cal/mole). Thermodynamically, the consummate oxidation of ane mole of glucose should yield approximately 688,000 cal; the energy that is not conserved biologically as chemical free energy (or ATP formation) is liberated every bit rut (308,000 cal). Thus, the cellular respiratory process is at best nigh 55% efficient.

Glucose oxidation is the most unremarkably studied dissimilatory reaction leading to energy production or ATP synthesis. The complete oxidation of glucose may involve iii fundamental biochemical pathways. The offset is the glycolytic or Embden- Meyerhof-Parnas pathway (Fig. iv-i), the 2nd is the Krebs cycle (besides called the citric acid cycle or tricarboxylic acid bike), and the third is the series of membrane-jump electron transport oxidations coupled to oxidative phosphorylation.

Respiration takes place when any organic chemical compound (commonly carbohydrate) is oxidized completely to CO₂ and H₂O. In aerobic respiration, molecular O_iiserves as the terminal acceptor of electrons. For anaerobic respiration, NO₃ ^–, SO₄ ^two–, CO₂, or fumarate can serve as final electron acceptors (rather than O_ii), depending on the bacterium studied. The end event of the respiratory procedure is the complete oxidation of the organic substrate molecule, and the stop products formed are primarily CO₂ and H_twoO. Ammonia is formed also if poly peptide (or amino acid) is the substrate oxidized. The biochemical pathways normally involved in oxidation of diverse naturally occurring organic compounds are summarized in Effigy 4-2.

Effigy 4-two

Heterotrophic metabolism, general pathway.

Metabolically, leaner are unlike blue-green alga (blueish-green algae) and eukaryotes in that glucose oxidation may occur by more than one pathway. In leaner, glycolysis represents one of several pathways by which leaner can catabolically attack glucose. The glycolytic pathway is most unremarkably associated with anaerobic or fermentative metabolism in bacteria and yeasts. In bacteria, other minor heterofermentative pathways, such as the phosphoketolase pathway, also be.

In improver, two other glucose-catabolizing pathways are found in bacteria: the oxidative pentose phosphate pathway (hexose monophosphate shunt), (Fig. iv-3) and the Entner-Doudoroff pathway, which is almost exclusively constitute in obligate aerobic bacteria (Fig. four-iv). The highly oxidative Azotobacter and well-nigh Pseudomonas species, for example, utilize the Entner-Doudoroff pathway for glucose catabolism, because these organisms lack the enzyme phosphofructokinase and hence cannot synthesize fructose ane,6-diphosphate, a key intermediate compound in the glycolytic pathway. (Phospho-fructokinase is also sensitive to molecular O₂ and does non function in obligate aerobes). Other bacteria, which lack aldolase (which splits fructose-ane,6-diphosphate into two triose phosphate compounds), likewise cannot have a functional glycolytic pathway. Although the Entner-Doudoroff pathway is unremarkably associated with obligate aerobic bacteria, it is present in the facultative anaerobe Zymomonas mobilis (formerly Pseudomonas lindneri). This organism dissimilates glucose to ethanol and represents a major alcoholic fermentation reaction in a bacterium.

Glucose dissimilation also occurs by the hexose monophosphate shunt (Fig. 4-3). This oxidative pathway was discovered in tissues that actively metabolize glucose in the presence of 2 glycolytic pathway inhibitors (iodoacetate and fluoride). Neither inhibitor had an event on glucose dissimilation, and NADPH + H⁺ generation occurred direct from the oxidation of glucose-half dozen-phosphate (to vi-phosphoglucono-δ-lactone) by glucose-6phosphate dehydrogenase. The pentose phosphate pathway subsequently permits the direct oxidative decarboxylation of glucose to pentoses. The capability of this oxidative metabolic system to featherbed glycolysis explains the term shunt.

The biochemical reactions of the Entner-Doudoroff pathway are a modification of the hexose monophosphate shunt, except that pentose sugars are not directly formed. The two pathways are identical upward to the formation of half-dozen-phosphogluconate (see Fig. four-4) and so diverge. In the Entner-Doudoroff pathway, no oxidative decarboxylation of 6-phosphogluconate occurs and no pentose compound is formed. For this pathway, a new six carbon chemical compound intermediate (2-keto-three-deoxy6-phosphogluconate) is generated by the action of 6-phosphogluconate dehydratase (an Fe²⁺– and glutathione-stimulated enzyme); this intermediate compound is and then directly cleaved into the triose (pyruvate) and a triose-phosphate compound (glyceraldehyde-three-phosphate) by the 2-keto-3-deoxy6-phosphogluconate aldolase. The glyceraldehyde-3-phosphate is further oxidized to another pyruvate molecule by the same enzyme systems that catalyze the last glycolytic pathway (see Fig. 4-4).

The glycolytic pathway may exist the major ane existing concomitantly with the minor oxidative pentose phosphate - hexose monophosphate shunt pathway; the Entner-Doudoroff pathway also may function as a major pathway with a small-scale hexose monophosphate shunt. A few bacteria possess simply one pathway. All cyanobacteria, Acetobacter suboxydans, and A. xylinum possess only the hexose monophosphate shunt pathway; Pseudomonas saccharophilia and Z. mobilis possess solely the Entner-Doudoroff pathway. Thus, the terminate products of glucose dissimilatory pathways are every bit follows:

The glucose dissimilation pathways used by specific microorganisms are shown in Table iv-ii.

Table 4-ii

Glucose Dissimilation Pathways Utilized by Bacteria, Cyanobacteria, and Yeasts.

All major pathways of glucose or hexose catabolism take several metabolic features in common. First, there are the preparatory steps by which key intermediate compounds such every bit the triose-PO4, glyceraldehyde-three-phosphate, and/or pyruvate are generated. The latter ii compounds are near universally required for further assimilatory or dissimilatory reactions inside the cell. Second, the major source of phosphate for all reactions involving phosphorylation of glucose or other hexoses is ATP, non inorganic phosphate (Pi). Actually, chemical energy contained in ATP must be initially spent in the first step of glucose metabolism (via kinase-type enzymes) to generate glucose-vi-phosphate, which initiates the reactions involving hexose catabolism. Third, NADH + H⁺or NADPH + H⁺is generated as reducing equivalents (potential energy) directly by i or more of the enzymatic reactions involved in each of these pathways.

Fermentation

Fermentation, another instance of heterotrophic metabolism, requires an organic chemical compound as a terminal electron (or hydrogen) acceptor. In fermentations, simple organic end products are formed from the anaerobic dissimilation of glucose (or some other chemical compound). Energy (ATP) is generated through the dehydrogenation reactions that occur equally glucose is cleaved down enzymatically. The elementary organic end products formed from this incomplete biologic oxidation process also serve every bit final electron and hydrogen acceptors. On reduction, these organic end products are secreted into the medium as waste metabolites (normally alcohol or acid). The organic substrate compounds are incompletely oxidized past bacteria, nevertheless yield sufficient energy for microbial growth. Glucose is the most common hexose used to study fermentation reactions.

In the belatedly 1850s, Pasteur demonstrated that fermentation is a vital process associated with the growth of specific microorganisms, and that each type of fermentation tin can be divers by the principal organic end product formed (lactic acrid, ethanol, acerb acid, or butyric acrid). His studies on butyric acid fermentation led directly to the discovery of anaerobic microorganisms. Pasteur concluded that oxygen inhibited the microorganisms responsible for butyric acrid fermentation because both bacterial mobility and butyric acrid formation ceased when air was bubbled into the fermentation mixture. Pasteur also introduced the terms aerobic and anaerobic. His views on fermentation are made clear from his microbiologic studies on the production of beer (from Etudes sur la Biere, 1876):

In the experiments which we take described, fermentation by yeast is seen to exist the directly event of the processes of nutrition, assimilation and life, when these are carried on without the agency of gratis oxygen. The estrus required in the accomplishment of that work must necessarily accept been borrowed from the decomposition of the fermentation matter…. Fermentation by yeast appears, therefore, to be essentially connected with the property possessed by this minute cellular plant of performing its respiratory functions, somehow or other, with the oxygen existing combined in sugar.

For well-nigh microbial fermentations, glucose dissimilation occurs through the glycolytic pathway (Fig. 4-one). The simple organic compound most commonly generated is pyruvate, or a compound derived enzymatically from pyruvate, such as acetaldehyde, α-acetolactate, acetyl ~ SCoA, or lactyl ~ SCoA (Fig. 4-v). Acetaldehyde can then be reduced past NADH + H⁺ to ethanol, which is excreted by the cell. The end product of lactic acrid fermentation, which occurs in streptococci (eastward.g., Streptococcus lactis) and many lactobacilli (e.g., Lactobacillus casei, L. pentosus), is a single organic acrid, lactic acid. Organisms that produce simply lactic acrid from glucose fermentation are homofermenters. Homofermentative lactic acid bacteria dissimilate glucose exclusively through the glycolytic pathway. Organisms that ferment glucose to multiple stop products, such as acetic acid, ethanol, formic acrid, and CO₂, are referred to as heterofermenters. Examples of heterofermentative bacteria include Lactobacillus, Leuconostoc, and Microbacterium species. Heterofermentative fermentations are more than mutual among bacteria, as in the mixed-acid fermentations carried out by bacteria of the family Enterobacteriaceae (e.g., Escherichia coli, Salmonella, Shigella, and Proteus species). Many of these glucose fermenters usually produce CO₂ and H₂ with different combinations of acrid end products (formate, acetate, lactate, and succinate). Other bacteria such as Enterobacter aerogenes, Aeromonas, Serratia, Erwinia, and Bacillus species also form CO₂ and H₂ as well as other neutral end products (ethanol, acetylmethylcarbinol [acetoin], and 2,3-butylene glycol). Many obligately anaerobic clostridia (e.g., Clostridium saccharobutyricum, C. thermosaccharolyticum) and Butyribacterium species ferment glucose with the product of butyrate, acetate, CO₂, and H_ii, whereas other Clostridum species (C. acetobutylicum and C. butyricum) also form these fermentation stop products plus others (butanol, acetone, isopropanol, formate, and ethanol). Similarly, the anaerobic propionic acid bacteria (Propionibacterium species) and the related Veillonella species ferment glucose to form CO₂, propionate, acetate, and succinate. In these bacteria, propionate is formed by the fractional reversal of the Krebs cycle reactions and involves a CO₂fixation by pyruvate (the Wood-Werkman reaction) that forms oxaloacetate (a four-carbon intermediate). Oxaloacetate is then reduced to malate, fumarate, and succinate, which is decarboxylated to propionate. Propionate is as well formed by some other iii-carbon pathway in C. propionicum, Bacteroides ruminicola, and Peptostreptococcus species, involving a lactyl ~ SCoA intermediate. The obligately aerobic acetic acrid bacteria (Acetobacter and the related Gluconobacter species) can also ferment glucose, producing acetate and gluconate. Figure 4-v summarizes the pathways by which the various major fermentation end products grade from the dissimilation of glucose through the mutual intermediate pyruvate.

Effigy iv-5

Fermentative pathways of leaner and the major end products formed with the organism blazon conveying out the fermentation.

For thermodynamic reasons, leaner that rely on fermentative process for growth cannot generate as much energy equally respiring cells. In respiration, 38 ATP molecules (or approximately 380,000 cal/mole) can exist generated as biologically useful free energy from the complete oxidation of ane molecule of glucose (assuming 1 NAD(P)H = 3 ATP and i ATP → ADP + Pi = 10,000 cal/mole). Table four-3 shows comparable bioenergetic parameters for the lactate and ethanolic fermentations by the glycolytic pathway. Although just 2 ATP molecules are generated by this glycolytic pathway, this is apparently enough energy to permit anaerobic growth of lactic acrid bacteria and the ethanolic fermenting yeast, Saccharomyces cerevisiae. The ATP-synthesizing reactions in the glycolytic pathway (Fig. 4-1) specifically involve the substrate phosphorylation reactions catalyzed past phosphoglycerokinase and pyruvic kinase. Although all the ATP molecules available for fermentative growth are believed to be generated by these substrate phosphorylation reactions, some energy equivalents are besides generated by proton extrusion reactions (acid liberation), which occur with intact membrane systems and involve the proton extrusion reactions of energy conservation (Fig. 4-9) as it applies to fermentative metabolism.

Table 4-3

Energy Obtained from Bacterial Fermentations by Substrate Phosphorylations.

Figure iv-9

Mitchell hypotheses, a chemiosmotic model of free energy transduction.

Krebs Cycle

The Krebs cycle (also called the tricarboxylic acrid cycle or citic acid cycle) functions oxidatively in respiration and is the metabolic procedure by which pyruvate or acetyl ~ SCoA is completely decarboxylated to CO2. In bacteria, this reaction occurs through acetyl ~ SCoA, which is the first production in the oxidative decarboxylation of pyruvate past pyruvate dehydrogenase. Bioenergetically, the following overall exergonic reaction occurs:

If 2 pyruvate molecules are obtained from the dissimilation of 1 glucose molecule, then thirty ATP molecules are generated in full. The decarboxylation of pyruvate, isocitrate, and α-ketoglutarate accounts for all CO₂ molecules generated during the respiratory procedure. Figure 4-6 shows the enzymatic reactions in the Krebs wheel. The chemical energy conserved past the Krebs cycle is contained in the reduced compounds generated (NADH + H⁺, NADPH + H⁺, and succinate). The potential free energy inherent in these reduced compounds is not bachelor as ATP until the final step of respiration (electron transport and oxidative phosphorylation) occurs.

Effigy 4-6

Krebs cycle (also tricarboxylic acid or citric acrid bike).

The Krebs cycle is therefore another preparatory phase in the respiratory process. If 1 molecule of pyruvate is oxidized completely to three molecules of CO₂, generating xv ATP molecules, the oxidation of 1 molecule of glucose will yield as many as 38 ATP molecules, provided glucose is dissimilated by glycolysis and the Krebs cycle (farther bold that the electron ship/oxidative phosphorylation reactions are bioenergetically identical to those of eukaryotic mitochondria).

Glyoxylate Cycle

In general, the Krebs wheel functions similarly in bacteria and eukaryotic systems, merely major differences are found among bacteria. One difference is that in obligate aerobes, L-malate may be oxidized directly by molecular O₂ via an electron transport concatenation. In other bacteria, only some Krebs cycle intermediate reactions occur considering α-ketoglutarate dehydrogenase is missing.

A modification of the Krebs cycle, normally called the glyoxylate wheel, or shunt (Fig. 4-vii), which exists in some bacteria. This shunt functions similarly to the Krebs wheel but lacks many of the Krebs cycle enzyme reactions. The glyoxylate cycle is primarily an oxidative pathway in which acetyl~SCoA is generated from the oxidation, of acetate, which usually is derived from the oxidation of fat acids. The oxidation of fatty acids to acetyl~SCoA is carried out by the β-oxidation pathway. Pyruvate oxidation is not direct involved in the glyoxylate shunt, even so this shunt yields sufficient succinate and malate, which are required for energy production (Fig. 4-seven). The glyoxylate cycle also generates other precursor compounds needed for biosynthesis (Fig. 4-7). The glyoxylate cycle was discovered every bit an unusual metabolic pathway during an attempt to learn how lipid (or acetate) oxidation in leaner and constitute seeds could lead to the direct biosynthesis of carbohydrates. The glyoxylate cycle converts oxaloacetate either to pyruvate and CO₂ (catalyzed past pyruvate carboxylase) or to phosphoenolpyruvate and CO_ii (catalyzed by the inosine triphosphate [ITP]-dependent phosphoenolpyruvate carboxylase kinase). Either triose compound can then be converted to glucose past reversal of the glycolytic pathway. The glyoxylate bicycle is found in many bacteria, including Azotobacter vinelandii and particularly in organisms that grow well in media in which acetate and other Krebs cycle dicarboxylic acrid intermediates are the sole carbon growth source. I primary function of the glyoxylate cycle is to furnish the tricarboxylic and dicarboxylic acid intermediates that are normally provided by the Krebs cycle. A pathway whose primary purpose is to replenish such intermediate compounds is called anaplerotic.

Electron Ship and Oxidative Phosphorylation

The terminal phase of respiration occurs through a series of oxidation-reduction electron transfer reactions that yield the energy to bulldoze oxidative phosphorylation; this in turn produces ATP. The enzymes involved in electron transport and oxidative phosphorylation reside on the bacterial inner (cytoplasmic) membrane. This membrane is invaginated to form structures called respiratory vesicles, lamellar vesicles, or mesosomes, which function equally the bacterial equivalent of the eukaryotic mitochondrial membrane.

Respiratory electron transport bondage vary greatly amid leaner, and in some organisms are absent. The respiratory electron transport chain of eukaryotic mitochondria oxidizes NADH + H⁺, NADPH + H⁺, and succinate (as well every bit the coacylated fat acids such as acetyl~SCoA). The bacterial electron transport chain also oxidizes these compounds, but it can as well directly oxidize, via non-pyridine nucleotide-dependent pathways, a larger variety of reduced substrates such as lactate, malate, formate, α-glycerophosphate, H_two, and glutamate. The respiratory electron carriers in bacterial electron transport systems are more varied than in eukaryotes, and the chain is commonly branched at the site(s) reacting with molecular O₂. Some electron carriers, such as nonheme atomic number 26 centers and ubiquinone (coenzyme Q), are common to both the bacterial and mammalian respiratory electron transport chains. In some leaner, the naphthoquinones or vitamin K may be constitute with ubiquinone. In notwithstanding other bacteria, vitamin K serves in the absence of ubiquinone. In mitochondrial respiration, only one cytochrome oxidase component is plant (cytochrome a + a_iii oxidase). In leaner at that place are multiple cytochrome oxidases, including cytochromes a, d, o, and occasionally a + a_iii (Fig. 4-8)

In bacteria cytochrome oxidases usually occur as combinations of a_ane : d: o and a + a_iii :o. Bacteria also possess mixed-part oxidases such every bit cytochromes P-450 and P-420 and cytochromes c' and c'c', which also react with carbon monoxide. These various types of oxygen-reactive cytochromes undoubtedly have evolutionary significance. Bacteria were nowadays earlier O₂ was formed; when O_two became available as a metabolite, bacteria evolved to employ it in different ways; this probably accounts for the diversity in bacterial oxygen-reactive hemoproteins.

Cytochrome oxidases in many pathogenic bacteria are studied by the bacterial oxidase reaction, which subdivides Gram-negative organisms into two major groups, oxidase positive and oxidase negative. This oxidase reaction is assayed for by using N,North,N', North'-tetramethyl-p-phenylenediamine oxidation (to Wurster's blueish) or by using indophenol blue synthesis (with dimethyl-p-phenylenediamine and α-naphthol). Oxidase-positive bacteria contain integrated (cytochrome c type:oxidase) complexes, the oxidase component most oft encountered is cytochrome o, and occasionally a + a₃ . The cytochrome oxidase responsible for the indophenol oxidase reaction complex was isolated from membranes of Azotobacter vinelandii, a bacterium with the highest respiratory rate of any known cell. The cytochrome oxidase was plant to be an integrated cytochrome c₄ :o complex, which was shown to be present in Bacillus species. These Bacillus strains are also highly oxidase positive, and well-nigh are found in morphologic group II.

Both bacterial and mammalian electron transfer systems can acquit out electron transfer (oxidation) reactions with NADH + H⁺, NADPH + H⁺, and succinate. Energy generated from such membrane oxidations is conserved inside the membrane and then transferred in a coupled manner to bulldoze the formation of ATP. The electron transfer sequence is achieved entirely by membrane-leap enzyme systems. As the electrons are transferred by a specific sequence of electron carriers, ATP is synthesized from ADP + inorganic phosphate (Pi) or orthophosphoric acrid (H_iiiPO₄) (Fig. 4-viii).

In respiration, the electron transfer reaction is the primary manner of generating energy; electrons (iie-) from a depression-redox-potential compound such as NADH + H⁺ are sequentially transferred to a specific flavoprotein dehydrogenase or oxidoreductase (flavin mononucleotide [FMN] type for NADH or flavin adenine dinucleotide [FAD] type for succinate); this electron pair is and so transferred to a nonheme iron middle (FeS) and finally to a specific ubiquinone or a naphthoquinone derivative. This transfer of electrons causes a differential chemic redox potential modify so that within the membrane plenty chemical energy is conserved to be transferred by a coupling mechanism to a high-energy compound (e.g., ADP + Pi → ATP). ATP molecules represent the final stable high-free energy intermediate compound formed.

A similar serial of redox changes too occurs between ubiquinone and cytochrome c, simply with a greater differential in the oxidation-reduction potential level, which allows for some other ATP synthesis stride. The final electron transfer reaction occurs at the cytochrome oxidase level between reduced cyotchrome c and molecular O₂; this reaction is the terminal ATP synthesis stride.

Mitchell or Proton Extrusion Hypothesis

A highly complex but attractive theory to explain energy conservation in biologic systems is the chemiosmotic coupling of oxidative and photosynthetic phosphorylations, ordinarily chosen the Mitchell hypothesis. This theory attempts to explain the conservation of free energy in this procedure on the basis of an osmotic potential acquired past a proton concentration differential (or proton slope) beyond a proton-impermeable membrane. Energy is generated past a proton extrusion reaction during membrane-bound electron transport, which in essence serve as a proton pump; free energy conservation and coupling follow. This represents an obligatory "intact" membrane phenomenon. The energy thus conserved (once more inside the confines of the membrane and is coupled to ATP synthesis. This would occur in all biologic cells, fifty-fifty in the lactic acrid leaner that lack a cytochrome-dependent electron ship chain but still possesses a cytoplasmic membrane. In this hypothesis, the membrane allows for charge separation, thus forming a proton slope that drives all bioenergization reactions. By such ways, electromotive forces can be generated by oxidation-reduction reactions that tin be directly coupled to ion translocations, as in the separation of H⁺ and OH^– ions in electrochemical systems. Thus, an enzyme or an electron transfer carrier on a membrane that undergoes an oxidation-reduction reaction serves as a specific usher for OH^– (or 0^ii–), and "hydrodehydration" provides electromotive ability, as it does in electrochemical cells.

The concept underlying Mitchell's hypothesis is circuitous, and many modifications have been proposed, just the theory's well-nigh attractive characteristic is that information technology unifies all bioenergetic conservation principles into a single concept requiring an intact membrane vesicle to function properly. Figure iv-9 shows how the Mitchell hypothesis might be used to explain energy generation, conservation, and transfer past a coupling process. The to the lowest degree satisfying aspect of the chemiosmotic hypothesis is the lack of understanding of how chemic energy is actually conserved within the membrane and how it is transmitted by coupling for ATP synthesis.

Bacterial Photosynthesis

Many prokaryotes (bacteria and cyanobacteria) possess phototrophic modes of metabolism (Table 4-1) . The types of photosynthesis in the ii groups of prokaryotes differ mainly in the type of compound that serves as the hydrogen donor in the reduction of CO_ii to glucose (Table iv-1). Phototrophic organisms differ from heterotrophic organisms in that they utilise the glucose synthesized intracellularly for biosynthetic purposes (every bit in starch synthesis) or for energy production, which usually occurs through cellular respiration.

Dissimilar phototrophs, heterotrophs require glucose (or another preformed organic compound) that is direct supplied as a substrate from an exogenous source. Heterotrophs cannot synthesize large concentrations of glucose from CO_twoby specifically using H_iiO or (H₂Due south) as a hydrogen source and sunlight as energy. Plant metabolism is a archetype example of photolithotrophic metabolism: plants need CO₂ and sunlight; H₂O must exist provided as a hydrogen source and usually NO₃ ^– is the nitrogen source for protein synthesis. Organic nitrogen, supplied as fertilizer, is converted to NO_iii ^– in all soils by bacteria via the procedure of ammonification and nitrification. Although plant cells are phototrophic, they besides showroom a heterotrophic style of metabolism in that they respire. For example, plants utilize classic respiration to catabolize glucose that is generated photosynthetically. Mitochondria too as the soluble enzymes of the glycolytic pathway are required for glucose dissimilation, and these enzymes are likewise plant in all plant cells. The soluble Calvin bike enzymes, which are required for glucose synthesis during photosynthesis, are also constitute in plant cells. It is non possible to feed a plant past pouring a glucose solution on it, just water supplied to a found will exist "photolysed" past chloroplasts in the presence of lite; the hydrogen(s) generated from H₂O is used by Photosystems I and 2 (PSI and PSII) to reduce NADP⁺ to NADPH + H⁺. With the ATP generated by PSI and PSII, these reduced pyridine nucleotides, CO₂ is reduced intracellularly to glucose. This metabolic procedure is carried out in an integrated manner by Photosystems I and 2 ("Z" scheme) and by the Calvin cycle pathway. A new photosynthetic, and nitrogen fixing bacterium, Heliobacterium chlorum, staining Gram positive was isolated, characterized, and establish to incorporate a new type of chlorophyll, i.e., bacteriochlorophyll 'g'. 16S r-RNA sequence analyses showed this organism to be phylogenetically related to members of the family unit Bacillaceae, although all currently known phototrophes are Gram negative (meet Table iv.iv). A few Heliobacteriium strains did show the presence of endospores. Another unusual phototrophe is the Gram negative Halobacterium halobium (now named Halobacterium salinarium), an archaebacterium growing best at 30°C in 4.0–5.0 Chiliad (or 25%, westward/5) NaCl. This bacterium is a facultative phototrophe having a respiratory way; it also possesses a purple membrane within which bacteriorhodopsin serves as the active photosynthetic pigment. This purple membranae possesses a light driven proton translocation pump which mediates photosynthetic ATP synthesis via a proton extrusion reaction (come across Mitchell Hypothesis). Table four-iv summarizes the characteristics of known photosynthetic bacteria.

Table 4-four

Characteristics Commonly Exhibited by Phototrophic Bacteria^a.

Autotrophy

Bacteria that grow solely at the expense of inorganic compounds (mineral ions), without using sunlight as an energy source, are called autotrophs, chemotrophs, chemoautotrophs, or chemolithotrophs. Similar photosynthetic organisms, all autotrophs use CO₂ as a carbon source for growth; their nitrogen comes from inorganic compounds such every bit NH₃, NO_iii ^–, or N_two (Table 4-1). Interestingly, the free energy source for such organisms is the oxidation of specific inorganic compounds. Which inorganic chemical compound is oxidized depends on the bacteria in question (Table iv-5). Many autotrophs will not grow on media that contain organic matter, even agar.

Tabular array 4-5

Inorganic Oxidation Reactions Used past Autotrophic Bacteria as Energy Sources.

Also found among the autotrophic microorganisms are the sulfur-oxidizing or sulfur-chemical compound-oxidizing leaner, which seldom exhibit a strictly autotrophic mode of metabolism like the obligate nitrifying bacteria (run into discussion of nitrogen cycle beneath). The representative sulfur compounds oxidized by such bacteria are H₂Southward, S₂, and S₂O₃. Among the sulfur leaner are 2 very interesting organisms; Thiobacillus ferrooxidans, which gets its energy for autotrophic growth by oxidizing elemental sulfur or ferrous atomic number 26, and T. denitrificans, which gets its energy past oxidizing Due south₂O₃ anaerobically, using NO_iii ^– as the sole terminal electron acceptor. T denitrificans reduces NO_three to molecular N₂, which is liberated every bit a gas; this biologic process is called denitrification.

All autotrophic leaner must assimilate CO_two, which is reduced to glucose from which organic cellular matter is synthesized. The energy for this biosynthetic process is derived from the oxidation of inorganic compounds discussed in the previous paragraph. Note that all autotrophic and phototrophic bacteria possess substantially the same organic cellular constituents found in heterotrophic bacteria; from a nutritional viewpoint, however, the autotrophic style of metabolism is unique, occurring merely in bacteria.

Anerobic Respiration

Some leaner exhibit a unique fashion of respiration chosen anaerobic respiration. These heterotrophic leaner that will not grow anaerobically unless a specific chemical component, which serves every bit a final electron acceptor, is added to the medium. Among these electron acceptors are NO₃ ^–, Then₄ ^ii–, the organic compound fumarate, and CO₂. Bacteria requiring one of these compounds for anaerobic growth are said to exist anaerobic respirers.

A large group of anaerobic respirers are the nitrate reducers (Table iv-6). The nitrate reducers are predominantly heterotrophic bacteria that possess a circuitous electron transport system(south) assuasive the NO_iii ^– ion to serve anaerobically as a terminal acceptor of electrons

. The organic compounds that serve every bit specific electron donors for these three known nitrate reduction processes are shown in Table 4-6. The nitrate reductase activity is common in bacteria and is routinely used in the uncomplicated nitrate reductase examination to place leaner (see Bergey's Manual of Deterininative Bacteriology, 8th ed.).

The methanogens are among the well-nigh anaerobic bacteria known, being very sensitive to small concentrations of molecular O₂. They are as well archaebacteria, which typically alive in unusual and deleterious environments.

All of the in a higher place anaerobic respirers obtain chemical free energy for growth past using these anaerobic energy-yielding oxidation reactions.

The Nitrogen Cycle

Nowhere can the total metabolic potential of bacteria and their diverse chemical-transforming capabilities be more fully appreciated than in the geochemical cycling of the element nitrogen. All the basic chemical elements (Southward, O, P, C, and H) required to sustain living organisms have geochemical cycles similar to the nitrogen wheel.

The nitrogen cycle is an ideal demonstration of the ecologic interdependence of bacteria, plants, and animals. Nitrogen is recycled when organisms utilise one course of nitrogen for growth and excrete another nitrogenous compound as a waste. This waste product is in turn utilized by another type of organism as a growth or energy substrate. Effigy 4-10 shows the nitrogen cycle.

When the specific breakdown of organic nitrogenous compounds occurs, that is, when proteins are degraded to amino acids (proteolysis) and then to inorganic NH_three, by heterotrophic leaner, the process is called ammonification. This is an essential step in the nitrogen cycle. At death, the organic constituents of the tissues and cells decompose biologically to inorganic constituents by a process called mineralization; these inorganic finish products tin then serve every bit nutrients for other life forms. The NH₃ liberated in plow serves as a utilizable nitrogen source for many other bacteria. The breakup of feces and urine also occurs past ammonification.

The other important biologic processes in the nitrogen cycle include nitrification (the conversion of NH₃ to NO₃by autotrophes in the soil; denitrification (the anaerobic conversion of NO_iii to N₂ gas) carried out by many heterotrophs); and nitrogen fixation (North₂to NH₃, and cell protein). The latter is a very specialized prokaryotic process called diazotrophy, carried out past both gratuitous-living bacteria (such as Azotobacter, Derxia, Beijeringeia, and Azomona species) and symbionts (such as Rhizobium species) in conjunction with legume plants (such as soybeans, peas, clover, and bluebonnets). All constitute life relies heavily on NO_iii ^– as a nitrogen source, and most fauna life relies on plant life for nutrients.

References

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7. Jurtshuk P Jr, Liu JK. Cytochrome oxidase and analyses of Bacillus strains: existence of oxidase-positive species. Int J Syst Bacterol. 1983;33:887.

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10. Jurtshuk P, Jr, Yang TY: Oxygen reactive hemoprotein components in bacterial respiratory systems. In Knowles CJ (ed): Diversity of Bacterial Respiratory Systems. Vol. 1. CRC Press, Boca Raton, FL, 1980 .

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13. Kornberg HL: The role and maintenance of the tricarboxylic acrid cycle in Escherichia coli. In Goodwin TW (ed): British Biochemistry Past and Present. Biochemistry Society Symposium no. thirty. Bookish Press, London, 1970 . [PubMed: 4322317]

14. Lemberg R, Barrett J: Bacterial cytochromes and cytochrome oxidases. In Lemberg R, Barrett J: Cytochromes. Academic Press, New York, 1973 .

15. Mandelstam J, McQuillen Thousand, Dawes I (eds): Biochemistry of Bacterial Growth. 3rd Ed. Blackwell, Oxford, 1982 .

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Source: https://www.ncbi.nlm.nih.gov/books/NBK7919/

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