ISSN:
1432-072X
Keywords:
Methyltransferase
;
Membrane protein
;
Corrinoids
;
Methanogenic bacteria
;
Archaebacteria
;
Energy conservation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Abstract An assay is described that allows the direct measurement of the enzyme activity catalyzing the transfer of the methyl group from N 5-methyltetrahydromethanopterin (CH3−H4MPT) to coenzyme M (H−S−CoM) in methanogenic archaebacteria. With this method the topology, the partial purification, and the catalytic properties of the methyltransferase in methanol- and acetate-grown Methanosarcina barkeri and in H2/CO2-grown Methanobacterium thermoautotrophicum were studied. The enzyme activity was found to be associated almost completely with the membrane fraction and to require detergents for solubilization. The transferase activity in methanol-grown M. barkeri was studied in detail. The membrane fraction exhibited a specific activity of CH3−S−CoM formation from CH3−H4MPT (apparent K m=50 μM) and H−S−CoM (apparent K m=250 μM) of approximately 0.6 μmol·min-1·mg protein-1. For activity the presence of Ti(III) citrate (apparent K m=15 μM) and of ATP (apparent K m=30 μM) were required in catalytic amounts. Ti(III) could be substituted by reduced ferredoxin. ATP could not be substituted by AMP, CTP, GTP, S-adenosylmethionine, or by ATP analogues. The membrane fraction was methylated by CH3−H4MPT in the absence of H−S−CoM. This methylation was dependent on Ti(III) and ATP. The methylated membrane fraction catalyzed the methyltransfer from CH3−H4MPT to H−S−CoM in the absence of ATP and Ti(III). Demethylation in the presence of H−S−CoM also did not require Ti(III) or ATP. Based on these findings a mechanism for the methyltransfer reaction and for the activation of the enzyme is proposed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00290817
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