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Abstract

Methanol:coenzyme M methyltransferase from methanogenic archaea is a
cobalamin-dependent enzyme composed of three different subunits: MtaA,
MtaB and MtaC. MtaA is a zinc protein that catalyzes the methylation of
coenzyme M (HS-CoM) with methylcob(III)alamin. We report zinc XAFS
(X-ray absorption fine structure) results indicating that, in the
absence of coenzyme M, zinc is probably coordinated by a single sulfur
ligand and three oxygen or nitrogen ligands. In the presence of coenzyme
M, one (N/O)-ligand was replaced by sulfur, most likely due to ligation
of the thiol group of coenzyme M. Mutations in His237 or Cys239, which
are proposed to be involved in ligating zinc, resulted in an over 90%
loss in enzyme activity and in distinct changes in the zinc ligands. In
the His237 --> Ala and Cys239 --> Ala mutants, coenzyme M also seemed to
bind efficiently by ligation to zinc indicating that some aspects of the
zinc ligand environment are surprisingly uncritical for coenzyme M
binding.