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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.

References associated with 3.C.1 family:

Becher, B., V. Müller and G. Gottschalk (1992a). The methyl-tetrahydromethanopterin-coenzyme-M methyltransferase of Methanosarcina strain GO1 is a primary sodium pump. FEMS Microbiol. Lett. 91: 239-244.
Becher, B., V. Müller and G. Gottschalk (1992b). N-5-methyl-tetrahydromethanopterin-coenzyme-M methyltransferase of Methanosarcina strain GO1 is an Na+-translocating membrane protein. J. Bacteriol. 174: 7656-7660 1447136
Dimroth, P. (1997). Primary sodium ion translocating enzymes. Biochim. Biophys. Acta 1318: 11-51. 9030254
Gottschalk, G. and R.K. Thauer (2001). The Na+-translocating methyltransferase complex from methanogenic archaea. Biochim. Biophys. Acta. 1505: 28-36. 11248186
Harms, U., D.S. Weiss, P. Gärtner, D. Linder and R.K. Thauer (1995). The energy conserving N-5-methyltetrahydromethanopterin-coenzyme M methyltransferase complex from Methanobacterium thermoautotrophicumis composed of eight different subunits. Eur. J. Biochem. 228: 640-648. 7737157
Stupperich, E., A. Juza, M. Hoppert and F. Mayer (1993). Cloning, sequencing and immunological characterization of the corrinoid-containing subunit of the N5-methyltetrahydromethanopterin:coenzyme-M methyltransferase from Methanobacterium thermoautotrophicum. Eur. J. Biochem. 217: 115-121. 8223548