mcrA Gene Review Notes

Colleague Question

Contact: sgtringe@lbl.gov
Key Issue: Difficulty distinguishing methanogens vs methanotrophs by sequence alone

Key Findings

Core Function

• Methyl-coenzyme M reductase catalyzes the terminal step of methanogenesis
• Converts methyl-CoM + CoB → methane + CoM-S-S-CoB
• Contains unique F430 nickel cofactor essential for catalysis

Methanogen vs Methanotroph Distinction

CRITICAL INSIGHT: The same enzyme works in REVERSE in methanotrophs!
- Methanogens: Use mcrA for methane production (forward direction)
- Methanotrophs (ANME): Use mcrA for methane oxidation (reverse direction)
- Sequence alone is insufficient - need ecological/metabolic context

Distinguishing Features

1. Phylogenetic context: ANME lineages cluster separately
2. Syntrophic partners: ANME often with sulfate-reducing bacteria
3. Environmental context: Deep sea vs terrestrial/freshwater
4. Gene neighborhood: Look for associated electron transfer proteins
5. Isotope signatures: In environmental samples, δ13C values differ

Evolutionary Significance

• Ancient enzyme, possibly predating oxygenic photosynthesis
• Horizontal transfer between archaea documented
• Key player in global carbon cycle and climate regulation

GO Annotation Review

• Accepted core methanogenesis annotations
• Added note about bidirectional activity
• Emphasized nickel cofactor binding specificity

References

• PMID:9367957 - Crystal structure revealing F430 cofactor
• PMID:21593753 - Reverse methanogenesis in ANME archaea
• PMID:23539639 - Global importance in methane cycling

Remaining Questions

• How do post-translational modifications affect directionality?
• Can mcrA be engineered for industrial methane conversion?
• What determines forward vs reverse operation at molecular level?

Revision, 2026-10-15 (ARCHAEA pilot follow-up)

Deep research providers were unavailable; the existing mcrA-falcon-research.md
was left untouched. It contains an organism-level error: it says M. acetivorans
uses hydrogenotrophic methanogenesis, but the species description says
PMID:16346552.
It is cited in the review only for the complex statement.

GOA refresh

just fetch-gene METAC mcrA --force (UniProt entry version 112 -> 114). Current GOA
has 5 rows: cytoplasm (IEA GO_REF:0000044; ISS from P11558), methanogenesis (IEA
InterPro2GO GO_REF:0000002), coenzyme-B sulfoethylthiotransferase activity (IEA
GO_REF:0000120 now including ARBA:ARBA00098318; ISS from P11558). The keyword-mapped
transferase activity (KW-0808) and metal ion binding (KW-0479) rows, the
GO_REF:0000120 methanogenesis row, and the pre-ARBA GO_REF:0000120 activity row
are gone. Per docs/goa_source_refresh.md these four historical records are kept
with retired: true (a source change, not a biological REMOVE), with corrected reviews.

Physiology

• Methanogenic substrates: acetate, methanol, methylamines; not H2/CO2 (PMID:16346552 above).
• CO: PMID:15550538;
  the abstract describes M. acetivorans as "a metabolically versatile methanogen devoid of significant hydrogen metabolism".
• Lab growth substrates in the structural study: PMID:37307484.
• Hence GO:0015948 methanogenesis (generic) is kept. GO children exist only for CO2
  (GO:0019386), acetate (GO:0019385) and methanol (GO:0019387) routes (QuickGO children
  query); methylamine and CO routes have no term, so narrowing would drop routes. This
  differs deliberately from METTH mcrA, an obligate hydrogenotroph narrowed to GO:0019386.

Structure / cofactor (M. acetivorans MCR, cryo-EM, PDB 8GF5/8GF6)

• PMID:37307484
• PMID:37307484
• PMID:37307484
• ChEBI: coenzyme F430 (CHEBI:28265) is_a nickel tetrapyrrole / tetrapyrrole (OLS ancestors).
  GO:0046906 tetrapyrrole binding has children heme, chlorophyll, cobalamin binding; no F430 term.

PTM

• PMID:29743535
  This paper is about the MA4551 methyltransferase; it does not assay MCR catalysis, so
  its earlier use as support for GO:0050524 was a miscitation.

Decisions on flagged problems (from the METTH reviewer)

1. Transferase activity REMOVE -> TOO_GENERAL (confirmed: GO:0050524 is_a ... GO:0016740 in QuickGO). Row now retired from GOA.
2. Cytoplasm support "See deep research file..." was non-verbatim (confirmed); replaced by UniProt and PMID:30951290 quotes.
3. PMID:29743535 for GO:0050524 (confirmed miscited; reference_review MISCITED); replaced by PMID:37307484.
4. NEW GO:0051291 protein heterooligomerization (confirmed problem): assembly of the
   subunit into the hexamer is not a function the subunit performs for the cell;
   complex membership is captured by GO:0044674. NEW row and core_function entry deleted.
5. F430 binding as GO:0016151 nickel cation binding (confirmed problem): NEW row
   changed to GO:0046906 tetrapyrrole binding (IDA, PMID:37307484), retired
   metal ion binding row MODIFY -> GO:0046906, and "coenzyme F430 binding" proposed
   as a new term plus a suggested question.
6. Qualifiers backfilled by fetch-gene; status COMPLETE.
7. GO:0044674 NEW kept; evidence changed IPI -> IDA (purified complex structure).
8. Core function rewritten to match METTH structure (GO:0050524, GO:0044674, cytoplasm),
   with generic methanogenesis as the process.
9. Removed a non-verbatim core_functions quote attributed to PMID:11932238
   ("MCR catalyzes the final step in methanogenesis" is not in the cached abstract).