3.C.1 The Na⁺ Transporting Methyltetrahydromethanopterin:Coenzyme M Methyltransferase (NaT-MMM) Family

The NaT-MMM (EC 2.1.1.86) of Methanobacterium thermoautotrophicumhas been purified and characterized, and the Na⁺ transport reaction has been studied in inverted membrane vesicles. Sequencing of the encoding genes revealed eight contiguous genes organized in an operon. The majority of the encoded subunits are probably integral membrane proteins. Thus, MtrA, B, F and G have 1 TMS; MtrC has 7; MtrD has 6; MtrE has 6; and MtrH has none (Gottschalk and Thauer, 2001). MtrE is believed to be the Na⁺-transporting subunit. MtrA bears a cob(I)amide prosthetic group which is methylated and demethylated in the catalytic cycle. Demethylation is the Na⁺-dependent step. Thus, methyl transfer from methylated MtrA to coenzyme M drives the electrogenic translocation of Na⁺. The overall methyl transfer reaction is strongly exergonic (ΔG°= -30 kJ/mol) (Gottschalk and Thauer, 2001).

The NaT-MMM complex probably contains iron/sulfur clusters. As noted above, the enzyme catalyzes two chemical reactions as follows:

(1) CH₃-H₄MPT + E:Co(I) → H₄MPT + E:CH₃-Co(III).

(2) E:CH₃-Co(III) + HS-CoM → E:Co(I) + CH₃-S-CoM.

Reaction 1 is reversible, but reaction 2 is essentially irreversible in the direction shown. Na⁺ efflux is probably coupled to the second of these two reactions.

The generalized transport reaction is therefore probably:

Na⁺ (in) + E:CH₃-Co(III) + HS-CoM → Na⁺ (out) + E:Co(I) + CH₃-S-CoM.