Reaktionsschritte der Methionin‐Synthese bei Escherichia coli

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Abstract
Methionine synthetase has been purified 860 fold from Escherichia coli. The homogeneous red protein has a particle weight of 135,000 and is a vitamin B12 protein, containing one mole of dimethylbenzimidazole‐cobalamin per mole of protein. Several of its properties: Michaelis constants, spectrum, pH‐optimum, and stability have been determined, and studies on its mechanism of action performed by exchange and isolation experiments. In the presence of S‐adenosylmethionine and a reducing agent, the enzyme‐bound B12 is methylated by methyltetrahydrofolate. Its methyl group then is transferred to homocysteine to yield methionine. The one reaction product, tetrahydrofolate, is only loosely bound to the enzyme, whereas S‐adenosylmethionine is tightly attached to the protein, though not to its B12‐site. The other product, enzyme‐bound methyl‐cobalamin, was isolated after enzymic hydrolysis, and identified. Its labelling corresponded to the labelling of the methionine in the overall reaction. Relative to the methionine formed, S‐adenosylmethionine and the reducing system—in this instance a NADH/FAD‐reductase—are required in catalytic amounts. The former can act as methylating agent of the enzyme‐bound B12 if used in stoichiometric concentration. However, under physiological conditions, it is not used as methyl donor for methionine. On the other hand, the enzyme also catalyzes the transfer of a methyl group from S‐adenosyl methionine to tetrahydrofolate, yielding 5‐methyl tetrahydrofolic acid, thus accounting for the observed dilution of label. The ratio of synthesis to transfer is about 1200. By following the stoichiometry of reduction and methionine synthesis together with the lack of exchange of methyl groups from methyl‐B12 to tetrahydrofolate on irradiation, it is concluded that the enzyme reaction proceeds by ionic steps. A plausible reaction scheme is given in which, however, the rôle of S‐adenosylmethionine is still a matter of conjecture. It seems possible that the enzyme participates in the quaternization of the methyl donor, 5‐methyltetrahydrofolate, to facilitate the dissociation of its methyl group.

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