Properties of the methylcobalamin:H4folate methyltransferase involved in chloromethane utilization by Methylobacterium sp strain CM4

Methylobacterium sp. strain CM4 is a strictly aerobic methylotrophic proteobacterium growing with chloromethane as the sole carbon and energy sourer. Genetic evidence and measurements of enzyme activity in cell-free extracts have suggested a multistep pathway for the conversion of chloromethane to formate. The postulated pathway is initiated by a corrinoid-dependent methyltransferase system involving methyltransferase I (CmuA) and methyltransferase II (CmuB), which transfer the methyl group of chloromethane onto tetrahydrofolate (H(4)folate) [Vannelli et nl. (1999) Proc. Natl Acad. Sci. USA 96, 4615-4620]. We report the overexpression in Escherichia coli and the purification to apparent homogeneity of methyltransferase II. This homodimeric enzyme, with a subunit molecular mass of 33 kDa, catalyzed the conversion of methylcobalamin and H(4)folate to cob(I)alamin and methyl-H(4)folate with a specific activity of 22 nmol.min(-1)(mg protein)(-1). The apparent kinetic constants for H(4)folate were: K-m = 240 mu M, V-max = 28.5 nmol.min(-1)(mg protein)(-1). The reaction appeared to be first order with respect to methylcobalamin at concentrations up to 2 mM, presumably reflecting the fact that methylcobalamin is an artificial substitute for the methylated methyltransferase I, the natural substrate of the enzyme. Tetrahydromethanopterin, a coenzyme also present in Methylobacterium, did not serve as a methyl group acceptor for methyltransferase II. Purified methyltransferase II restored chloromethane dehalogenation by a cell free extract of a strain CM4 mutant defective in methyltransferase II.