Anaerobic conditions are maintained in some ecosystems where the rate of oxygen supply - PowerPoint Presentation

1. Anaerobic conditions are maintained in some ecosystems where the rate of oxygen supply is lower than that of consumption. Organic compounds are removed from anaerobic ecosystems through the concerted action of fermentative and anaerobic respiratory microorganisms.In microbiology, the term ‘fermentation’ can be used to describe either microbial processes that produce useful products or a form of anaerobic microbial growth using internally supplied electron acceptors and generating ATP mainly through substrate-level phosphorylation (SLP).fermentation

2. Fermentation and anaerobic respirationRespiration refers to the reduction of oxygen by electrons from theelectron transport chains coupled to the generation of a proton motiveforce through electron transport phosphorylation .Under anaerobic conditions, some microorganisms grow using an ETP process with externally supplied oxidized compounds other than oxygen as the terminal electron acceptor. This type of growth is referred to as anaerobic respiration. In a fermentative process, ATP is generated through SLP with the oxidation of electron donors coupled to the reduction of electron carriers such as NAD(P)+ or Flavin adenine dinucleotide (FAD). The reduced electron carriers are reoxidized reducing the metabolic intermediate

3. In fermentation, ATP is generated not only through SLP but also by other mechanisms such as the reactions catalyzed by fumarate reductase and Na+ dependent decarboxylase, and lactate Hydrogen in fermentationThe product formed/substrate consumed ratio is constant in somefermentations such as the ethanol and homolactate fermentations while it is variable in others such as the clostridial fermentation .Fermentation processes with a constant ratio are referred to as linear pathways, whilethe others are referred to as branched fermentative pathways.

4. A branched pathway yields more ATP and more oxidized products than a linear pathway. To produce more oxidized products, a proportion of the reduced electron carriers such as NAD(P)H should be oxidized coupled to the reduction of H+ to H2.The formation of products in a branched pathway is dictated by them environmental growth conditions, especially the hydrogen partial pressure.

5. microbes are classified according to their response to molecular oxygen (O2) into aerobes, facultative anaerobes and obligate anaerobes. Aerotolerant obligate anaerobes are distinguished from strict anaerobes among the obligateanaerobes. O2 comprises about 20% of air, and air-saturated liquid media contains about 7–8 mg/l O2 at ambient temperature.Molecular oxygen and anaerobesMicroaerophiles use O2 as their electron acceptor at a low dissolved O2 concentration of 0.1–3 mg/l O2, but they cannot grow above this concentration. Strict anaerobes and microaerophiles are inhibited by molecular oxygen or its metabolites. Several hypotheses have beenproposed to explain the inhibitory mechanism.

6. Molecular oxygen reacts with reduced flavoproteins, Fe-S proteins and cytochromes to be reduced to hydrogen peroxide (H2O2) or superoxide (O2). These are very strong oxidants with high redox potentials and destroy cellular polymers such as DNA, RNA, proteins and other essential components. anaerobes therefore possess enzymes which detoxify them. These enzymes are superoxide dismutase (SOD) and catalasePeroxidase is another enzyme that removes hydrogen peroxide. Thereactions are:

7. Strict anaerobes have been thought to be sensitive against O2 because they do not possess SOD and catalase. This is true in some cases, but these enzyme activities have been identified in some strict anaerobes, and genes for these enzymes and related proteins have been found in their genomes

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9. Though some strict anaerobes have SOD and catalase activities ,they do not grow under aerobic conditions in the laboratory.This may be due to the nature of the molecular oxygen.Dissolved oxygen increases the redox potential of the solution and a high redox potential inhibits the growth of some strict anaerobes.Methanogens grow at a redox potential lower than 0.3 V. Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to form a disulfide. The organisms might not be able to grow with such inactivated enzymes. One hypothesis proposes that growth is impossible due to a lack of reducing equivalents for biosynthesis since electrons are exhausted to reduce oxygen. It is most likely that multiple mechanisms are responsible for growthinhibition by oxygen.

10. Ethanol fermentationSaccharomyces cerevisiae ferments carbohydrates through the EMP pathway to ethanol, and the ED pathway is used by Zymomonas mobilis . Pyruvate is decarboxylated to acetaldehyde, which is used as the electron acceptor. Acetaldehyde is reduced to ethanol, which consumes the electrons generated during the glycolytic process where ATP is generated through SLP .Saccharomyces cerevisiae generates 2 ATP from 1 hexose molecule but a single ATP results from 1 hexose molecule in Zymomonas mobilis.

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12. Pyruvate decarboxylase has thiamine pyrophosphate as a prosthetic group as in pyruvate dehydrogenase. Pyruvate decarboxylase is known mainly in eukaryotes. In addition to Zymomonas mobilis, this enzyme is found in a facultative anaerobe, Erwinia amylovora, and in a strictly anaerobic acidophile, Sarcina ventriculi. Pyruvate decarboxylase is a key enzyme of ethanol fermentation.It should be noted that ethanol is produced through different reactions in saccharolytic clostridia, hetero fermentative lactic acid bacteria and enteric bacteria. These bacteria oxidize pyruvate to acetyl-CoA before reducing it to ethanol. They do not possess pyruvate decarboxylase. Ethanol production in clostridia is catalyzed by the following reactions:

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14. Ethanol fermentation through pyruvate decarboxylase in a linear fermentative pathway does not produce any by-products except CO2 and water, while ethanol fermentation through acetyl-CoA is a branched fermentative pathway and produces various fermentation products such as lactate, acetate and H2. Thermophilic anaerobes ferment various carbohydrates including cellulose and pentoses through acetyl-CoA to ethanol. Among them are Thermo anaerobium brockii, Thermoan aerobacter ethanolicus and Clostridium thermos cellum

15. Lactate fermentationLactate is a common fermentation product in many facultative andobligate anaerobes. Some bacteria produce lactate as a major fermentationproduct and these are referred to as lactic acid bacteria (LAB).Most LAB have a limited ability to synthesize monomers for biosynthesisand vitamins which are needed as growth factors.LAB are regarded as obligate anaerobes but they can use oxygen, synthesizing cytochromes when hemin is provided in the medium. Some LAB produce only lactate from sugars while others produce acetate and ethanol in addition to lactate .The former are referred to as homo fermentative and the latter hetero fermentative LAB. Homo fermentative LAB ferment sugars through the EMP pathway and hetero fermentative LAB ferment sugars through the phosphoketolase pathway

16. Homolactate fermentationHomo fermentative LAB include most species of Lactobacillus,Sporolactobacillus, Pediococcus, Enterococcus and Lactococcus. They usehexoses through the EMP pathway to generate ATP. Lactate dehydrogenasethe NADH reduced during the EMP pathway reoxidizes and the pyruvate used as the electron acceptor .As fermentation proceeds, lactate is accumulated lowering the intracellular pH. Lactate dehydrogenase is active in acidic conditions producing lactate as the major product.

17. Hetero lactate fermentationSpecies of Leuconostoc and Bifidobacterium produce ethanol and acetate in addition to lactate. They employ a unique glycolytic pathwayknown as the phosphoketolase pathway . Hetero fermentative LAB like Leuconostoc mesenteroides oxidize glucose-6-phosphate to ribulose-5-phosphate. Epimerase converts ribulose-5-phosphate to xylulose-5-phosphate, which then cleavage to glyceraldehyde-3-phosphate and acetyl-phosphate by the action of phosphoketolase. Glyceraldehyde-3-phosphate is metabolized to lactate as in the homolactate fermentation generating ATP. Acetyl-phosphate is reduced to ethanol acting as the electron acceptor to oxidize the NADH reduced in the glucose-6-phosphate oxidation process. One ATP per hexose is available from this fermentation.

18. Pentoses are converted to xylulose-5-phosphate without reducing NAD+ . In this case, acetyl-phosphate is not used as the electron acceptor but is used to synthesize ATP. Leuconostoc mesenteroides synthesizes 1 ATP from a molecule of hexose and 2 ATP from amolecule of pentose.Bifidobacterium bifidum ferments 2 molecules of hexose to 2 molecules of lactate and 3 molecules of acetate employing two separate phosphoketolases, one active on fructose-6-phosphate and the other on xylulose-5-phosphate .This bacterium synthesizes 5 ATP from 2 molecules of glucose. Since hexose-6-phosphate is not metabolized through a reductive process, acetyl-phosphate is used to synthesize ATP as in pentose metabolism by Leuconostoc mesenteroides.

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