Comparison of three methyl-coenzyme M reductases from phylogenetically distant organisms: Unusual amino acid modification, conservation and adaptation

The nickel enzyme methyl-coenzyme M reductase (MCR) catalyzes the terminal step of methane formation in the energy metabolism of all methanogenic arch aea. In this reaction methyl-coenzyme M and coenzyme B are converted to met hane and the heterodisulfide of coenzyme M and coenzyme B. The crystal stru ctures of methyl-coenzyme M reductase from Methanosarcina barkeri (growth t emperature optimum, 37 degreesC) and Methanopyrus kandleri (growth temperat ure optimum, 98 degreesC) were determined and compared with the known struc ture of MCR from Methanobacterium thermoautotuophicum (growth temperature o ptimum, 65 degreesC). The active sites of MCR from M. barkeri and hi. kandl eri were almost identical to that of M. thermoautotuophicum and predominant ly occupied by coenzyme M and coenzyme B. The electron density at 1.6 Angst rom resolution of the M. barkeri enzyme revealed that four of the five modi fied amino acid residues of MCR from M. thermoautotrophicum, namely a thiop eptide, an S-methylcysteine, a 1-N-methylhistidine and a 5-methylarginine w ere also present. Analysis of the environment of the unusual amino acid res idues near the active site indicates that some of the modifications may be required for the enzyme to be catalytically effective. In M. thermoautotrop hicum and M. kandleri high temperature adaptation is coupled with increasin g intracellular concentrations of lyotropic salts. This was reflected in a higher fraction of glutamate residues at the protein surface of the thermop hilic enzymes adapted to high intracellular salt concentrations. (C) 2000 A cademic Press.