Methanogenesis

SUBMITTED BY-SURENDER RAWAT

• Methanogenesis is the biological production of CH4 from either CO2 plus H2 or from methylated organic compounds.

• Process of methanogensis was first demonstrated over 200 years ago by Alessandro Volta.

• The process is strictly anaerobic.

• All known methanogenic bacteria belong to Archaebacteria.

• They are also called methanogens.• A variety of unique coenzymes are involved in methanogenesis.

Methanogens belong to the Archaea group of Prokaryotes.

They can be classified as Chemolithotrophic methanogens and Methylotrophic methanogens.

Methanogenic bacteria are extremely oxygen sensitive.

Methanogens require anaerobic conditions .

They are found in the digestive systems of herbivores, marshes or lake bottoms, all sorts of mud sediments and in man made anaerobic digestors in sewage treatment plants.

Complex organic compounds cannot be utilised by methanogens.

They use only C1 compounds and the only C2 compound: Acetate.

There are two nutritional groups of methanogens.

• Obligate chemolithotrophic methanogens that grow with CO2 plus H2according to the equation:-

CO2 + 4H2 CH4 + 2H2O

• Some of these organisms live with the Quasi-chemolithotrophicsubstrates HCOOH and CO

4HCOOH 4CO2 + 4H2

CO2 + 4H2 CH4 + 2H2O

Methylotrophic methanogens that grow with methyl group containing substrates for eg. Methanol, methylamines, acetate

The reaction for acetate is:CH3COOH CH4 + CH2

Organisms such as Methanosarcina barkeri grow on methanol or methylamines. Here, one fourth of the substrate has to be oxidised to CO2 for reducing power generation.

CH3OH + H2O CO2 + 6H3CH3OH + 6H 3CH4 + 3 H2O4CH3OH 3CH4 + CO2 + 2 H2O

Group 2 organisms produce methane directly from methyl groups and not via CO2.

Obligate chemolithotrophic methanogens do not contain cytochromes which are present in methylotrophic methanogens

• The first two novel coenzymes discovered in methanogens are coenzyme M and coenzyme F420

• Coenzyme M is a simple chemical compound . Its reactive group is the mercepto group which can be methylated and methyl coenzyme M is the ultimate precursor of mehane.

• Coenzyme F420 is a deazaflavin, it is a redox carrier and its role is analogous to that of ferrodoxin in other anaerobes.

• It functions as electron acceptor of hydrogenase and as electron donor in several reduction reactions.

• Tetrahydromethanopterin and Methanofuran are involved in CO2 reduction. Methanofuran is the primary CO2 aceeptor.

Methane formation from CO2 and H2 by Barker’s scheme.

• The first carrier molecule is Methanofuran.

• In a reaction that requires CO2 and reducing equivalents it is converted to formylmethanofuran with the formyl group residing at the aminomethyl group of the furan ring.

• Transfer of the C1 moiety to tetrahydromethanopterin and reduction of the formyl to methyl group follows.

• It is finally transferred to the Coenzyme M and is reduced to CH3 by the enzyme methyl co enzyme M methylreductase.

Formation of methane from bicarbonate

• The four steps leading to formation of methane from bicarbonate are as followsHCO3- + H2 → HCOO- + H2O

HCOO- + H2 + H+ → CH2O + H2O

CH2O + H2 → CH3OH

CH3OH + H2 → CH4 + H2O

HCO3 + H+ + 4H2 → CH4 + 3H2O

• The fourth step is coupled to the generation of proton motive force at the membrane which in turn is used by an ATP synthase for the phosphorylation of ADP to produce ATP.

• Eg. Methanosarcina barkeri

Formation of methane from Acetate• Methanosarcina barkeri is a classic and representative species for

those methanogenic bacteria that utilize acetate, methanol and methylamine as substrates.

• Formation of methane from acetate is a oxidoreduction process.

• First CO and methyl coenzyme M are produced which is the precursor for methane. CO2 is also produced.

Formation of methane from Methanol and Methylamine

Methanogenesis of methanol and methylamine can be subdivided into two processes:

• Oxidation of one fourth of the methyl groups to CO2

• Reduction of three fourth methyl groups to CH4.

• A methanol:coenzyme M and a trimethylamine:coenzyme M methyltransferase have been characterized.

• The methyl group is first transferred to the protein bound 5 hydroxy benzimidazolylcobamide.

• Methyl groups transfer further proceeds to coenzyme M.