MICRB311 Lecture Notes - Lecture 18: Carbon Monoxide Dehydrogenase, Methanofuran, Methanogenesis
11 Jan 2017
School
Department
Course
Professor
C1 Metabolism
Topics
1. Methanogenesis
• Reducing CO2 to CH4
2. Methanotrophy
• Oxidizing CH4 to CO2
3. Other C1 conversion
• Carboxydotrophs: oxidizing CO to CO2
• Carbon storage and applications
Methanogens
• Only methanogenic Archaea (no bacteria or eukarya)
➔ So only archaea are methanogenic, no other group. Also, they are strictly
anaerobic
• 2 classes of coenzymes
• 1) C1 carriers from CO2 to CH4
• 2) Redox reactions to supply electrons to reduce CO2 to CH4
• Methanogens are auto-fluorescent due to presence of F420 coenzyme
Enzymes of Methanogenesis (Red = memorise)
• Methanofuran: 5-membered furan ring and amino-N that binds CO2. → First step that grabs CO2
• Methanopterin: resembles folic acid and is C1 carrier in intermediate steps of CO2 reduction to
CH4 → 2nd enzyme in the pathway.
• Coenzyme F420: flavin derivative; it is an electron donor in several steps of CO2 reduction
• Coenzyme F430: also, involved in terminal step of methanogenesis as part of methyl reductase
enzyme complex. → involved in important step.
• Coenzyme M (CoM): small molecule involved in terminal step of methanogenesis; conversion of
CH3 to CH4
• Coenzyme B (CoB): resembles pantothenic acid (part of CoA); electron donor for methyl
reductase enzyme complex
find more resources at oneclass.com
find more resources at oneclass.com
4 Major Steps
1. CO2 activated by methanofuran-containing enzyme (MF), then reduced to formyl group.
• → reduction of CO2 to formyl (MF: methanofuran; makes the formyl group)
2. Formyl group transferred to enzyme containing methanopterin (MP); dehydrated and reduced
in 2 steps to methylene and methyl groups
→ reduction of formyl to methyl. (MP: methanopterin; reduces formyl to methyl)
3. Methyl group transferred to enzyme containing CoM
CoM→ takes methyl and attaches it.
4. Methyl-CoM reduced to methane by methyl reductase system:
• F430: removes CH3 group from CH3-CoM
• Reduced by electrons from CoB: makes CH4 and disulfide complex CoM-S-S-CoB
• Free CoM and CoB regenerated by reduction with H2 for energy conservation
• → The reduction of CoM-S-S-CoB complex is where PMF is generated
The disulfide complex between enzyme CoM and CoB **** IMPORTANT .Take note where the
energy is coming from, in which step. The addition of CoM powers the sodium pump ( for PMF)
• Methyl reductase – F430 complex also important → responsible for PMF.
• 2 energy generations types in methanogenesis
So energy released here generates PMF via sodium pump.
And direct PMF generation
Energy Conservation
• Reduction of CO2 to CH4:
∆G°’ = -131 kJ/mol
• Which is just sufficient for synthesis of 1 ATP.
• Energy from methyl reductase step
• Reduction of CoM-S-S-CoB by heterodisulfide reductase is exergonic and pumps one proton
across membrane
• Electron flow to heterosulfide reductase by methanophenazine carrier
find more resources at oneclass.com
find more resources at oneclass.com
Biosynthesis
• Use reactions of acetyl-CoA pathway (same as homoacetogens)
• Lack THF-driven series of reactions that produce the methyl group Do’t eed THF
• Methanogens derive methyl groups directly from electron donors or from CH4 product
• Carbonyl group derived from CO dehydrogenase as in homoacetogens
Alternate: CH4 from acetate (Can make CH4 from acetate)
• Acetate activated to acetyl-CoA
• Acetyl-CoA interacts with carbon monoxide dehydrogenase of acetyl-CoA pathway
• Methyl group of acetate transferred to corrinoid enzyme
• Then goes through CoM-mediated steps of methanogenesis
What to know about methanogens
• Several steps to reduce CO2 to methyl (CH3)
• Involves enzymes specialized for this process; should be able to recognize the enzyme
names
• DO NOT memorize structures or specific reaction mechanisms (no electron flows etc but
overall steps)
• Carbon dioxide and acetate can enter methanogenesis pathway
• Corrinoid protein enter the scene for transferring CH3 from acetate
• Acetate enters via enzymes from the acetyl-CoA pathway
• All about getting to the methyl group
• Biosynthesis (still the acetyl-CoA pathway – should know this pathway anyway because of
homoacetogens) – still about getting to a methyl and carbonyl group!
• Energy conservation
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Topics: methanogenesis, reducing co2 to ch4, methanotrophy, oxidizing ch4 to co2, other c1 conversion, carboxydotrophs: oxidizing co to co2, carbon storage and applications. Methanogens: only methanogenic archaea (no bacteria or eukarya) So only archaea are methanogenic, no other group. Also, they are strictly anaerobic: 2 classes of coenzymes, 1) c1 carriers from co2 to ch4, 2) redox reactions to supply electrons to reduce co2 to ch4, methanogens are auto-fluorescent due to presence of f420 coenzyme. Enzymes of methanogenesis (red = memorise: methanofuran: 5-membered furan ring and amino-n that binds co2. First step that grabs co2: methanopterin: resembles folic acid and is c1 carrier in intermediate steps of co2 reduction to. Ch4 2nd enzyme in the pathway: coenzyme f420: flavin derivative; it is an electron donor in several steps of co2 reduction, coenzyme f430: also, involved in terminal step of methanogenesis as part of methyl reductase enzyme complex.
