Me Than o Genesis

8/13/2019 Me Than o Genesis

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The Methanogens


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The major genera are listed in

Table 12-1.


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Methanogens are obligate anaerobic Archaea thatproduce energy from the biosynthesis of methane.These lithotrophic microorganisms are widely distributedin oxygen-free environments and participate actively inthe carbon cycle. Indeed, methanogenesis plays a majorrole in the last step of the anoxic degradation of organicsubstances, transforming acetate, CO(2), and H(2) tomethane. The vast majority of the known methanogensare classified as hydrogenotrophic because they use

principally H(2) as the electron donor to drive thereduction of CO(2). Unlike many other cultured Archaea,many methanogens thrive in neutral pH, low salinity, andtemperate environments.


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Methanogens can be used to producemethane (aka natural gas, biogas) frombiomass and degrade and detoxify

agricultural, municipal, and industrialwastes. Methane is also a big contributorto global warming, which is why a betterunderstanding of how methanogenesis

works is needed so that we can usemethane as a renewable source of energyand limit its greenhouse gas effect


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Although most

methanogens have a pH

optima near neutral, there

are some methanogens

that live in extreme pHenvironments.

Methanogenesis has

been shown to occur at

low pH's (pH=3.0), butthe optimal pH value is

near 6.0.


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Methanogens thrive in anaerobic environments rich in organicmatter: the rumen and intestinal system of animals, freshwater and

marine sediments, swamps and marshes, hot springs, anaerobic

sludge digesters, and even within anaerobic protozoa. Methanogens

often are of ecological significance. The rate of methane production

can be so great that bubbles of methane will sometimes rise to thesurface of a lake or pond. Rumen methanogens are so active that a

cow can belch 200 to 400 liters of methane a day


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Methanogens are autotrophic archaebacteriathat use anaerobic respiration for ATP synthesis.

Methanogens use CO2 taken up from theirgrowth environment as the carbon substrate forgrowth. They use some CO2 as the ultimateoxidizing agent of an electron transport chain

which, by a chemiosmotic mechanism, maintainsa transmembrane electrochemical ion gradientwhich powers ATP production.


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The reducing agent that drives the electron transportchain is hydrogen also taken up from the growthenvironment. This hydrogen is the waste end product ofthe metabolism of other, heterotrophic microorganisms.

Methanogens use this hydrogen and this processmaintains a lowered hydrogen partial pressure in thereticulo-rumen. Some of the hydrogen producingheterotrophic microorganisms show altered patterns ofmetabolism because of methanogen usage of the

hydrogen they produce. This process lies at the heart ofsome of the best characterised syntrophic relations seenin the reticulo-rumen and and is referred toas "interspecies hydrogen transfer"
http://www.vet.ed.ac.uk/clive/cal/rumencal/Glossary/Syntrophy.htmlhttp://www.vet.ed.ac.uk/clive/cal/rumencal/Glossary/Syntrophy.html


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This phenomenon is important because hydrogen utilization by themethanogens reduces the hydrogen partial pressure of the reticulo-rumen and this alters the pattern of metabolism of syntrophichydrogen "donor" partner species. This is due to these organismshaving hydrogen-sensitive hydrogenases. Raising or lowering thepartial pressure of hydrogen in their growth environment determines

their fermentation pathways and thus affects their ATP productionwhich affects their growth. As we shall see further on in this sectionwhen studying the fermentation patterns of Ruminococcusflavifaciensa lowering of the partial pressure of hydrogen byinterspecies hydrogen transfer allows enhanced growth of thesyntrophic hydrogen "donor" species.

The equation shows the reduction of CO2 by H2 to producemethane. This redox reaction sustains anaerobic respiration whichallows the production of ATP.


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The methane produced by reduction of the

carbon dioxide is lost from the reticulo-

rumen by eructation. It is a waste of feed

carbon because the rumen does not havemethanotrophic bacteria and the host

ruminant can not utilize this gas. Methane

lost in this way is one reason whymethanogens contribute to lowered food

conversion efficiency of the host ruminant.


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Methanogens are strict anaerobes that obtain energy by converting

CO2, H2, formate, methanol, acetate, and other compounds to

either methane or methane and CO2.

They are autotrophic when growing on H2 and CO2.

This is the largest group of archaea.

There are five orders (Methanobacteriales, Methanococcales,Methanomicrobiales, Methanosarcinales, and Methanopyrales) and

26 genera, which differ greatly in overall shape, 16S rRNAsequence,cell wall chemistry and structure, membrane lipids, andother features.


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Metabolism

Most methanogens can grow on CO2 andH2 as their sole energy source.

coenzyme bound C1-intermediates

Methanofuran (MFR),tetrahydromethanopterin (H4MPT), andcoenzyme M (H-S-CoM). Other keycoenzymes are F420 and N-7-mercaptoheptanoyl-O-phospho-L-threonine (H-S-HTP).
http://microbewiki.kenyon.edu/index.php/File:F420.gif


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their metabolism is unusual.

These procaryotes contain several unique cofactors:

tetrahydromethanopterin (H4MPT),

coenzyme M

methanofuran (MFR),

(2-mercaptoethanesulfonic acid),

coenzyme F420, andcoenzyme F430

The first three cofactors bear the C1 unit when CO2 is reduced to CH4.F420 carries electrons and hydrogens, and F430 is a nickel tetrapyrroleserving as a cofactor for the enzyme methyl-CoM methylreductase


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These procaryotes contain severalunique cofactors:

tetrahydromethanopterin (H4MPT),

coenzyme M

methanofuran (MFR),

(2-mercaptoethanesulfonic acid),

coenzyme F420, and

coenzyme F430

The first three cofactors bear the C1unit when CO2 is reduced to CH4.F420 carries electrons and hydrogens,and F430 is a nickel tetrapyrroleserving as a cofactor for the enzyme

methyl-CoM methylreductase


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G

1. 4 CH3OH 3 CH4 + CO2 + 2 H20 -106

2. CH3OH +H2 CH4 + H2O -112.5

3. 4 CH3NH2 + 2 H2O 3 CH4 + CO2 + 4 NH3 -76.7

4. 2 (CH3)2NH + 2 H2O 3 CH4 + CO2 + 2 NH3 -74.8

5. 4 (CH3)3N + 6 H2O 9 CH4 + 3 CO2 + 4 NH3 -75.8

6. 2 (CH3)2S + 2 H2O 3 CH4 + CO2 + 2 H2S -52.1

7. 4 (CH3)SH + 2 H20 3 CH4 + CO2 + 4 H2S -51 8.

8.(CH3)SH + H2 CH4 +H2S -69.3

9. 4 H2 + CO2 CH4 + 2 H2O -130.4

10. CH3COO- + H+ CH4 + CO2 -36 11. 4 CO + 2 H20 CH4 + 3 CO2 -211

Energy-yielding reactions used bymethylotrophic methanogens. Oneof the more common reactions,and the reaction showed above, isthe 9th one down. While the lastreaction involving methane is themost favorable, methane is not as

readily available as carbondioxide.


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Methanogenic archaea have an unusual type of metabolismbecause they use H2 + CO2, formate, methylated C1 compounds,or acetate as energy and carbon sources for growth. Themethanogens produce methane as the major end product of theirmetabolism in a unique energy-generating process. The organismsreceived much attention because they catalyze the terminal step in

the anaerobic breakdown of organic matter under sulfate-limitingconditions and are essential for both the recycling of carboncompounds and the maintenance of the global carbon flux on Earth.Furthermore, methane is an important greenhouse gas that directlycontributes to climate changes and global warming. Hence, theunderstanding of the biochemical processes leading to methaneformation are of major interest. This review focuses on the metabolicpathways of methanogenesis that are rather unique and involve anumber of unusual enzymes and coenzymes. It will be shown howthe previously mentioned substrates are converted to CH4 via theCO2-reducing, methylotrophic, or aceticlastic pathway.


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All catabolic processes finally lead to the formation of amixed disulfide from coenzyme M and coenzyme B thatfunctions as an electron acceptor of certain anaerobicrespiratory chains. Molecular hydrogen, reduced

coenzyme F420, or reduced ferredoxin are used aselectron donors. The redox reactions as catalyzed by themembrane-bound electron transport chains are coupledto proton translocation across the cytoplasmicmembrane. The resulting electrochemical proton

gradient is the driving force for ATP synthesis ascatalyzed by an A1A0-type ATP synthase.


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Some examples of rod shaped cells includeMethanobacteriumspp. and Methanopyrus kandleri.Examples of the coccoid methanogens include speciesfrom Methanococcusand Methanosphaerato name a

few. Methanoculleusand Methanogeniumare coccoid aswell but are irregularly shaped, possibly due to S-layersnot being so strongly bonded like other wall structures.Methanogens are not just limited to these shapes, butinclude a plate shaped genus Methanoplanus,

Methanospirillumthat are long thin spirals, andMethanosarcinathat are cluster of round cells.


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Ref Prescott pg 612

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