The common homocystinuria-associated P1173L variant of human methionine synthase impairs reductive methylation

Human methionine synthase (MTR) is a cobalamin-dependent enzyme that catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine to form methionine and tetrahydrofolate. The enzyme is susceptible to oxidative inactivation and is repaired by methionine synthase reductase (MTRR) in the presence of NADPH and S-adenosylmethionine (AdoMet). The P1173L missense mutation is the most common clinical variant of MTR and causes homocystinuria, an inborn error of metabolism that is associated with aggressive occlusive cardiovascular diseases. In this study, we report that co-expression of MTR with the B₁₂ chaperone MMADHC helps overcome prior challenges with soluble expression of full-length human MTR. Kinetic analysis reveals that wild-type and P1173L MTR exhibit comparable specific activities in the in vitro assay with an artificial reducing system although the K_M for AdoMet is ∼40-fold higher for the variant. In contrast, the P1173L variant is ∼30-fold less active in the presence of the physiologically relevant MTRR/NADPH-reducing system. EPR and kinetic analyses reveal that complex formation with MTRR, which limits the reactivation reaction in wild-type MTR, is unaffected in P1173L MTR, pointing to a switch in the rate-limiting step. Pre-steady state kinetic studies reveal pleiotropic impacts of the P1173L mutation, with electron transfer from MTRR to cob(II)alamin being rate-liming. Our study predicts that physiologically relevant small-molecule electron donors, some of which have been tested in this study, might have therapeutic potential to circumvent the penalties associated with P1173L MTR.