Question

In anaerobic respiration in some microbes, chemicals other than oxygen are used as the final electron acceptor in an electron-transport chain. In terms of how much ATP generated through these alternative pathways, what is expected?

Answer 1

Fermentation, another example of heterotrophic metabolism, requires an organic 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 compound). Energy (ATP) is generated through the dehydrogenation reactions that occur as glucose is broken down enzymatically. The simple organic end products formed from this incomplete biologic oxidation process also serve as final electron and hydrogen acceptors. On reduction, these organic end products are secreted into the medium as waste metabolites (usually alcohol or acid). The organic substrate compounds are incompletely oxidized by bacteria, yet yield sufficient energy for microbial growth. Glucose is the most common hexose used to study fermentation reactions.
In the late 1850s, Pasteur demonstrated that fermentation is a vital process associated with the growth of specific microorganisms, and that each type of fermentation can be defined by the principal organic end product formed (lactic acid, ethanol, acetic acid, or butyric acid). His studies on butyric acid fermentation led directly to the discovery of anaerobic microorganisms. Pasteur concluded that oxygen inhibited the microorganisms responsible for butyric acid fermentation because both bacterial mobility and butyric acid formation ceased when air was bubbled into the fermentation mixture. Pasteur also introduced the terms aerobic and anaerobic.
For most microbial fermentations, glucose dissimilation occurs through the glycolytic pathway. 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. Acetaldehyde can then be reduced by NADH + H+ to ethanol, which is excreted by the cell. The end product of lactic acid fermentation, which occurs in streptococci (e.g., Streptococcus lactis) and many lactobacilli (e.g., Lactobacillus casei, L. pentosus), is a single organic acid, lactic acid. Organisms that produce only lactic acid from glucose fermentation are homofermenters. Homofermentative lactic acid bacteria dissimilate glucose exclusively through the glycolytic pathway. Organisms that ferment glucose to multiple end products, such as acetic acid, ethanol, formic acid, and CO2, are referred to as heterofermenters. Examples of heterofermentative bacteria include Lactobacillus, Leuconostoc, and Microbacterium species. Heterofermentative fermentations are more common 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 acid 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 production of butyrate, acetate, CO2, and H2, whereas other Clostridum species (C. acetobutylicum and C. butyricum) also form these fermentation end 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 partial 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 also formed by another three-carbon pathway in C. propionicum, Bacteroides ruminicola, and Peptostreptococcus species, involving a lactyl ~ SCoA intermediate. The obligately aerobic acetic acid bacteria (Acetobacter and the related Gluconobacter species) can also ferment glucose, producing acetate and gluconate.
For thermodynamic reasons, bacteria that rely on fermentative process for growth cannot generate as much energy as respiring cells. In respiration, 38 ATP molecules (or approximately 380,000 cal/mole) can be generated as biologically useful energy from the complete oxidation of 1 molecule of glucose (assuming 1 NAD(P)H = 3 ATP and 1 ATP → ADP + Pi = 10,000 cal/mole).Although only 2 ATP molecules are generated by this glycolytic pathway, this is apparently enough energy to permit anaerobic growth of lactic acid bacteria and the ethanolic fermenting yeast, Saccharomyces cerevisiae. The ATP-synthesizing reactions in the glycolytic pathway specifically involve the substrate phosphorylation reactions catalyzed by 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 also generated by proton extrusion reactions (acid liberation), which occur with intact membrane systems and involve the proton extrusion reactions of energy conservation as it applies to fermentative metabolism.

Energy Obtained from Bacterial fermentations by substrate Phosphorylations. Fermentation Actual Theoritical Efficiency Homolactic H (Calsad) (carsmole) (%) CHI206 - →2CH2-C-COOH + 20000 57000 35 (Glucose) Glycolysis OH Energy energy a (0%) Alcoholic cele I - Cohi206. - → 2CH-C-04+ 2002 20000 58000 Glycolysis 34 I

lactate pyruvate 2Hf inside FAD . > OH cytb мо. 1 Fes membrane Outside 24tk NO NO + H₂O The electron transport chain for E coli during anaerobic respiration (based on smith and wood. Dymet at and Bertero et al.). FAD is flavin, Fes are iron and sulfive clusters, & is menaquinone, cyfb-btype cytochrome and mo is molybdopterin - guanosine - dinucleotide. Glucose Glycolysis 2 NAD+ ADP 2ATPAN 2 Pyruvate oate NADT regeneration - 2NADHIK > 2 NAD + 2 Lactate Glucose Glycolysis Pyruvate > Acrobic respiration. Anaerobic respiration Fermentation Lactic acid. Ethand Alcohol

Glycolysis. Reactants. 1 One glucose. 2) NART 3) 2ATP End products: 1) 2 pyruvate 2) NADH 3 4ATPK Net gain of ATP = 2ATP. used 2ATP In the process. We Krebs cycle yields - 2ATP Electron Transport chain yields. Reactants End products. 1 NADH 1) NADT 2) FADH₂ 2 H2O 3) 02 3) 32-34 ATP "Aerobic respiration - - RATP & RATP & 32 B 4 ATP (Glycolysis (krebs (ETC) - 36-38ATP. Cycle) (ETC) Anaerobic despiration – No krebs cycle. • Two steps. - Gucci sis — Poduco ATP . - Lactate formation - regeneration of A Only Glycolysis produce ATP so total ATP produced 2ATP. Lactate accumullation leads to & pH. NADT.