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You are watching: Very few gram-positive bacteria utilize the entner-doudoroff glycolytic pathway.

Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): college of Texas medical Branch in ~ Galveston; 1996.


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Heterotrophic Metabolism

Heterotrophic management is the biology oxidation of essential compounds, such together glucose, to productivity ATP and also simpler necessary (or inorganic) compounds, i beg your pardon are required by the bacterial cell for biosynthetic or assimilatory reactions.


Respiration

Respiration is a kind of heterotrophic line that provides oxygen and in which 38 moles of ATP are derived from the oxidation that 1 mole of glucose, yielding 380,000 cal. (An added 308,000 cal is lost as heat.)


Fermentation

In fermentation, another form of heterotrophic metabolism, an essential compound quite than oxygen is the terminal electron (or hydrogen) acceptor. Less energy is generated from this incomplete kind of glucose oxidation, however the process supports anaerobic growth.


Krebs Cycle

The Krebs cycle is the oxidative procedure in respiration through which pyruvate (via acetyl coenzyme A) is totally decarboxylated come CO2. The pathway returns 15 mole of ATP (150,000 calories).


Glyoxylate Cycle

The glyoxylate cycle, which occurs in some bacteria, is a change of the Krebs cycle. Acetyl coenzyme A is generated straight from oxidation of fat acids or various other lipid compounds.


Electron Transport and also Oxidative Phosphorylation

In the final stage that respiration, ATP is developed through a collection of electron carry reactions in ~ the cytoplasmic membrane that drive the oxidative phosphorylation that ADP to ATP. Bacteria use miscellaneous flavins, cytochrome, and non-heme steel components and multiple cytochrome oxidases for this process.


Mitchell or Proton Extrusion Hypothesis

The Mitchell hypothesis explains the power conservation in every cells on the basis of the selective extrusion that H+ ions across a proton-impermeable membrane, i beg your pardon generates a proton motive force. This energy allows for ATP synthetic both in respiration and also photosynthesis.


Bacterial Photosynthesis

Bacterial photosynthesis is a light-dependent, anaerobic setting of metabolism. Carbon dioxide is lessened to glucose, which is offered for both biosynthesis and energy production. Relying on the hydrogen resource used to alleviate CO2, both photolithotrophic and photoorganotrophic reactions exist in bacteria.


Autotrophy

Autotrophy is a unique kind of metabolism discovered only in bacteria. Inorganic compounds space oxidized directly (without making use of sunlight) come yield power (e.g., NH3, NO2–, S2, and Fe2+). This metabolic mode additionally requires power for CO2 reduction, favor photosynthesis, but no lipid-mediated procedures are involved. This metabolic setting has also been referred to as chemotrophy, chemoautotrophy, or chemolithotrophy.


Anaerobic Respiration

Anaerobic respiration is another heterotrophic mode of metabolism in i beg your pardon a specific compound various other than O2 serves together a terminal electron acceptor. Together acceptor compounds include NO3–, SO42–, fumarate, and even CO2 because that methane-producing bacteria.


The Nitrogen Cycle

The nitrogen cycle consists of a recycling procedure by i m sorry organic and inorganic nitrogen compounds are used metabolically and recycled among bacteria, plants, and animals. Important processes, consisting of ammonification, mineralization, nitrification, denitrification, and also nitrogen fixation, are lugged out generally by bacteria.


Introduction

Metabolism describes all the biochemical reactions that happen in a cabinet or organism. The examine of bacterial metabolism concentrates on the chemical diversity the substrate oxidations and also dissimilation reactions (reactions through which substrate molecule are damaged down), which normally function in bacteria to create energy. Additionally within the scope of bacter metabolism is the study of the uptake and also utilization of the not natural or essential compounds compelled for growth and maintenance that a cellular stable state (assimilation reactions). These respective exergonic (energy-yielding) and endergonic (energy-requiring) reactions room catalyzed in ~ the living bacterial cabinet by incorporated enzyme systems, the end an outcome being self-replication of the cell. The ability of microbial cells to live, function, and also replicate in an appropriate chemical milieu (such together a bacterial society medium) and also the chemical transforms that an outcome during this transformation constitute the border of bacterial metabolism.

The bacterial cabinet is a highly devoted energy transformer. Chemical energy generated by substrate oxidations is conserved by development of high-energy compound such together adenosine diphosphate (ADP) and also adenosine tree phosphate (ATP) or compound containing the thioester bond


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(acetyl ~ SCoA) or succinyl ~ SCoA. ADP and ATP represent adenosine monophosphate (AMP) add to one and two high-energy phosphates (AMP ~ P and also AMP ~ P~ P, respectively); the power is save in this compounds as high-energy phosphate bonds. In the visibility of appropriate enzyme systems, these compounds deserve to be used as power sources come synthesize the new complicated organic compounds needed by the cell. Every living cells must preserve steady-state biochemical reactions for the formation and also use of such high-energy compounds.

Kluyver and Donker (1924 to 1926) known that bacterial cells, regardless of species, to be in plenty of respects comparable chemically come all various other living cells. For example, this investigators known that hydrogen move is a typical and basic feature of all metabolic processes. Bacteria, favor mammalian and also plant cells, usage ATP or the high-energy phosphate shortcut (~ P) as the main chemical power source. Bacteria additionally require the B-complex vitamin as useful coenzymes for numerous oxidation-reduction reactions essential for growth and energy transformation. An organism such as Thiobacillus thiooxidans, get an impression in a tool containing just sulfur and also inorganic salts, synthesizes huge amounts the thiamine, riboflavine, nicotinic acid, pantothenic acid, pyridoxine, and biotin. Therefore, Kluyver suggest the unity concept of biochemistry (Die Einheit in der Biochemie), which says that all straightforward enzymatic reactions which support and maintain life processes within cell of organisms, had more similarities than differences. This principle of biochemical unified stimulated plenty of investigators to use bacteria as version systems because that studying connected eukaryotic, plant and animal biochemical reactions that are basically "identical" at the molecule level.

From a nutritional, or metabolic, viewpoint, three significant physiologic types of bacteria exist: the heterotrophs (or chemoorganotrophs), the autotrophs (or chemolithotrophs), and also the photosynthetic bacteria (or phototrophs) (Table 4-1). This are discussed below.


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Heterotrophic Metabolism

Heterotrophic bacteria, which encompass all pathogens, obtain energy native oxidation of necessary compounds. Carbohydrates (particularly glucose), lipids, and protein are the most typically oxidized compounds. Biology oxidation of this organic compounds by bacteria results in synthesis of ATP as the chemical power source. This process also patent generation of easier organic link (precursor molecules) necessary by the bacteria cell for biosynthetic or assimilatory reactions.

The Krebs cycle intermediary compounds offer as precursor molecules (building blocks) because that the energy-requiring biosynthesis of facility organic link in bacteria. Deterioration reactions the simultaneously create energy and also generate precursor molecules for the biosynthesis of new cellular constituents are referred to as amphibolic.

All heterotrophic bacteria require preformed organic compounds. This carbon- and nitrogen-containing link are expansion substrates, i m sorry are provided aerobically or anaerobically to generate reducing equivalents (e.g., diminished nicotinamide adenin dinucleotide; NADH + H+); this reducing equivalents subsequently are chemical energy sources for all biologic oxidative and also fermentative systems. Heterotrophs room the most typically studied bacteria; they thrive readily in media comprise carbohydrates, proteins, or other facility nutrients such as blood. Also, development media may be enriched through the addition of various other naturally emerging compounds such together milk (to examine lactic acid bacteria) or hydrocarbons (to examine hydrocarbon-oxidizing organisms).


Respiration

Glucose is the most usual substrate supplied for researching heterotrophic metabolism. Most aerobic organisms oxidize glucose fully by the following reaction equation:


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This equation expresses the to move oxidation process called respiration. Respiration occurs within the cell of plants and animals, generally generating 38 ATP molecules (as energy) native the oxidation of 1 molecule that glucose. This yields roughly 380,000 calorie (cal) per setting of glucose (ATP ~ 10,000 cal/mole). Thermodynamically, the complete oxidation the one mole that glucose have to yield around 688,000 cal; the energy that is not conserved biologically together chemical energy (or ATP formation) is liberated as heat (308,000 cal). Thus, the to move respiratory process is in ~ best about 55% efficient.

Glucose oxidation is the most commonly studied dissimilatory reaction top to energy production or ATP synthesis. The finish oxidation of glucose may involve three basic biochemical pathways. The first is the glycolytic or Embden- Meyerhof-Parnas pathway (Fig. 4-1), the 2nd is the Krebs bike (also called the citric mountain cycle or tricarboxylic acid cycle), and the 3rd is the series of membrane-bound electron transfer oxidations coupled to oxidative phosphorylation.


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Respiration takes ar when any type of organic compound (usually carbohydrate) is oxidized completely to CO2 and also H2O. In aerobic respiration, molecular O2serves together the terminal acceptor of electrons. For anaerobic respiration, NO3–, SO42–, CO2, or fumarate can serve together terminal electron acceptors (rather 보다 O2), depending on the bacter studied. The end an outcome of the respiratory procedure is the complete oxidation that the organic substrate molecule, and also the end assets formed are mostly CO2 and also H2O. Ammonia is formed additionally if protein (or amino acid) is the substrate oxidized. The biochemical pathways normally associated in oxidation of assorted naturally occurring organic compounds space summarized in figure 4-2.


Metabolically, bacteria room unlike cyanobacteria (blue-green algae) and also eukaryotes in that glucose oxidation may happen by more than one pathway. In bacteria, glycolysis represents one of several pathways whereby bacteria have the right to catabolically assault glucose. The glycolytic pathway is most commonly associated with anaerobic or fermentative line in bacteria and also yeasts. In bacteria, other minor heterofermentative pathways, such as the phosphoketolase pathway, additionally exist.

In addition, two various other glucose-catabolizing pathways are discovered in bacteria: the oxidative pentose phosphate pathway (hexose monophosphate shunt), (Fig. 4-3) and also the Entner-Doudoroff pathway, which is almost exclusively found in obligate aerobic bacteria (Fig. 4-4). The extremely oxidative Azotobacter and also most Pseudomonas species, because that example, make use of the Entner-Doudoroff pathway because that glucose catabolism, due to the fact that these organisms absence the enzyme phosphofructokinase and hence cannot synthesize furustos 1,6-diphosphate, a an essential intermediate link in the glycolytic pathway. (Phospho-fructokinase is also sensitive to molecule O2 and also does not duty in obligate aerobes). Various other bacteria, which absence aldolase (which splits fructose-1,6-diphosphate into two triose phosphate compounds), likewise cannot have actually a functional glycolytic pathway. Return the Entner-Doudoroff pathway is usually linked with obligate aerobic bacteria, the is existing in the facultative anaerobe Zymomonas mobilis (formerly Pseudomonas lindneri). This biology dissimilates glucose to ethanol and also represents a major alcoholic fermentation reaction in a bacterium.


Glucose dissimilation also occurs through the hexose monophosphate shunt (Fig. 4-3). This oxidative pathway was discovered in organization that proactively metabolize glucose in the presence of two glycolytic pathway inhibitors (iodoacetate and fluoride). No inhibitor had an result on glucose dissimilation, and NADPH + H+ generation emerged directly native the oxidation the glucose-6-phosphate (to 6-phosphoglucono-δ-lactone) by glucose-6phosphate dehydrogenase. The pentose phosphate pathway subsequently permits the direct oxidative decarboxylation that glucose come pentoses. The capability of this oxidative metabolic device to bypass glycolysis explains the term shunt.

The biochemical reactions of the Entner-Doudoroff pathway room a alteration of the hexose monophosphate shunt, except that pentose sugars space not directly formed. The 2 pathways are the same up to the formation of 6-phosphogluconate (see Fig. 4-4) and also then diverge. In the Entner-Doudoroff pathway, no oxidative decarboxylation of 6-phosphogluconate occurs and also no pentose link is formed. Because that this pathway, a new 6 carbon link intermediate (2-keto-3-deoxy6-phosphogluconate) is created by the activity of 6-phosphogluconate dehydratase (an Fe2+– and glutathione-stimulated enzyme); this intermediate link is then directly cleaved into the triose (pyruvate) and a triose-phosphate compound (glyceraldehyde-3-phosphate) by the 2-keto-3-deoxy6-phosphogluconate aldolase. The glyceraldehyde-3-phosphate is more oxidized to an additional pyruvate molecule by the same enzyme solution that catalyze the terminal glycolytic pathway (see Fig. 4-4).

The glycolytic pathway may be the significant one currently concomitantly with the minor oxidative pentose phosphate - hexose monophosphate shunt pathway; the Entner-Doudoroff pathway likewise may duty as a major pathway with a minor hexose monophosphate shunt. A few bacteria possess just one pathway. All cyanobacteria, Acetobacter suboxydans, and also A. Xylinum possess just the hexose monophosphate shunt pathway; Pseudomonas saccharophilia and Z. Mobilis possess specifically the Entner-Doudoroff pathway. Thus, the end commodities of glucose dissimilatory pathways are as follows:


All significant pathways of glucose or hexose catabolism have several metabolic attributes in common. First, there room the preparatory actions by which key intermediate compounds such together the triose-PO4, glyceraldehyde-3-phosphate, and/or pyruvate space generated. The last two compound are nearly universally forced for additional assimilatory or dissimilatory reactions in ~ the cell. Second, the significant source the phosphate for every reactions including phosphorylation of glucose or various other hexoses is ATP, not not natural phosphate (Pi). Actually, chemistry energy consisted of in ATP must be at first spent in the an initial step of glucose management (via kinase-type enzymes) to create glucose-6-phosphate, i m sorry initiates the reactions involving hexose catabolism. Third, NADH + H+or NADPH + H+is produced as reduce equivalents (potential energy) directly by one or an ext of the enzymatic reactions associated in each of this pathways.


Fermentation

Fermentation, an additional example of heterotrophic metabolism, requires an essential compound together a terminal electron (or hydrogen) acceptor. In fermentations, simple organic end commodities are formed from the anaerobic dissimilation the glucose (or some other compound). Power (ATP) is produced through the dehydrogenation reaction that take place as glucose is damaged down enzymatically. The simple organic end commodities formed from this incomplete biology oxidation process also serve as last electron and hydrogen acceptors. Top top reduction, these organic end assets are secreted right into the medium as rubbish metabolites (usually alcohol or acid). The organic substrate compounds room incompletely oxidized by bacteria, however yield sufficient power for microbial growth. Glucose is the most typical hexose provided to examine fermentation reactions.

In the so late 1850s, Pasteur demonstrated the fermentation is a crucial process connected with the growth of specific microorganisms, and also that each form of fermentation deserve to be characterized by the major organic end product developed (lactic acid, ethanol, acetic acid, or butyric acid). His researches on butyric mountain fermentation led directly to the discovery of anaerobic microorganisms. Pasteur concluded the oxygen inhibited the microorganisms responsible because that butyric acid fermentation because both bacterial mobility and butyric acid development ceased as soon as air to be bubbled into the fermentation mixture. Pasteur also introduced the terms aerobic and anaerobic. His views on fermentation space made clear from his microbiologic researches on the production of beer (from Etudes sur la Biere, 1876):

In the experiment which we have described, fermentation by yeast is watched to be the direct consequence of the procedures of nutrition, assimilation and life, as soon as these are brought on without the firm of cost-free oxygen. The heat compelled in the achievement of that work have to necessarily have actually been borrowed from the decomposition that the fermentation matter…. Fermentation by yeast appears, therefore, to be essentially linked with the building possessed through this minute cellular plant of performing its respiratory tract functions, somehow or other, through the oxygen existing combined in sugar.

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For many microbial fermentations, glucose dissimilation occurs v the glycolytic pathway (Fig. 4-1). The an easy organic link most generally generated is pyruvate, or a compound acquired enzymatically indigenous pyruvate, such as acetaldehyde, α-acetolactate, acetyl ~ SCoA, or lactyl ~ SCoA (Fig. 4-5). Acetaldehyde have the right to then be reduced by NADH + H+ to ethanol, which is excreted by the cell. The end product of lactic mountain fermentation, which occurs in streptococci (e.g., Streptococcus lactis) and also many lactobacilli (e.g., Lactobacillus casei, L. Pentosus), is a single organic acid, lactic acid. Organisms that develop only lactic acid from glucose fermentation room homofermenters. Homofermentative lactic acid bacteria dissimilate glucose exclusively through the glycolytic pathway. Organisms that ferment glucose to multiple end products, such together acetic acid, ethanol, formic acid, and also CO2, are described as heterofermenters. Instances of heterofermentative bacteria encompass Lactobacillus, Leuconostoc, and Microbacterium species. Heterofermentative fermentations are more common amongst bacteria, together in the mixed-acid fermentations lugged out by bacteria that the family members Enterobacteriaceae (e.g., Escherichia coli, Salmonella, Shigella, and Proteus species). Many of these glucose fermenters usually develop CO2 and also H2 with various combinations of mountain end products (formate, acetate, lactate, and also succinate). Other bacteria such as Enterobacter aerogenes, Aeromonas, Serratia, Erwinia, and Bacillus types also form CO2 and H2 and also other neutral end assets (ethanol, acetylmethylcarbinol , and 2,3-butylene glycol). Plenty of obligately anaerobic clostridia (e.g., Clostridium saccharobutyricum, C. Thermosaccharolyticum) and Butyribacterium types ferment glucose with the manufacturing of butyrate, acetate, CO2, and H2, whereas other Clostridum types (C. Acetobutylicum and C. Butyricum) also form these fermentation end assets plus others (butanol, acetone, isopropanol, formate, and also ethanol). Similarly, the anaerobic propionic acid bacteria (Propionibacterium species) and the associated Veillonella types ferment glucose to type CO2, propionate, acetate, and succinate. In these bacteria, propionate is formed by the partial reversal that the Krebs bicycle reactions and involves a CO2fixation by pyruvate (the Wood-Werkman reaction) that develops oxaloacetate (a four-carbon intermediate). Oxaloacetate is then decreased to malate, fumarate, and also succinate, i beg your pardon is decarboxylated come propionate. Propionate is also formed by an additional three-carbon pathway in C. Propionicum, Bacteroides ruminicola, and Peptostreptococcus species, entailing a lactyl ~ SCoA intermediate. The obligately aerobic acetic acid bacteria (Acetobacter and also the associated Gluconobacter species) can also ferment glucose, producing acetate and gluconate. Number 4-5 summarizes the pathways by which the various significant fermentation finish products form from the dissimilation the glucose v the common intermediate pyruvate.