Human methionine synthase reductase is a molecular chaperone for human methionine synthase

Abstract

Sustained activity of mammalian methionine synthase (MS) requires MS reductase (MSR), but there have been few studies of the interactions between these two proteins. In this study, recombinant human MS (hMS) and MSR (hMSR) were expressed in baculovirus-infected insect cells and purified to homogeneity. hMSR maintained hMS activity at a 1:1 stoichiometric ratio with a K act value of 71 nM. Escherichia coli MS, however, was not activated by hMSR. Moreover, hMS was not significantly active in the presence of E. coli flavodoxin and flavodoxin reductase, which maintain the activity of E. coli MS. These results indicate that recognition of MS by their reductive partners is very strict, despite the high homology between MS from different species. The effects of hMSR on the formation of hMS holoenzyme also were examined by using crude extracts of baculovirus-infected insect cells containing hMS apoenzyme (apoMS). In the presence of MSR and NADPH, holoenzyme formation from apoMS and methylcobalamin was significantly enhanced. The observed stimulation is shown to be due to stabilization of human apoMS in the presence of MSR. Apoenzyme alone is quite unstable at 37°C. MSR also is able to reduce aquacobalamin to cob(II)alamin in the presence of NADPH, and this reduction leads to stimulation of the conversion of apoMS and aquacobalamin to MS holoenzyme. Based on these findings, we propose that MSR serves as a special chaperone for hMS and as an aquacobalamin reductase, rather than acting solely in the reductive activation of MS.aquacobalamin reductase ͉ cobalamin ͉ holoenzyme formation

Protection of aquo/hydroxocobalamin from reduced glutathione by a B12 trafficking chaperone

“…In addition, the reductive elimination of the cyanide ligand from CNCbl by bCblC might form a base-off cob(II)alamin (9), which should be rapidly converted to OH2/ OHCbl under aerobic conditions. The cob(I)-and cob(II)alamin may be substrates for enzyme cofactor synthesis (18,19), however, there is currently no evidence of direct transfer of the oxygen sensitive cobalamins from CblC to the proteins for catalysis of MeCbl and AdoCbl synthesis. In particular, the synthesis of AdoCbl is catalyzed in the mitochondria by adenosyltranferase and delivered to methylmalonyl-CoA mutase (20).…”

Population-level diversity in the association of genetic polymorphisms of one-carbon metabolism with breast cancer risk

“…The MTRR A66G showed a deleterious effect on MTRR protein. Since MTR and MTRR act together in 1:1 stoichiometric ratio to form holoenzyme complex (Yamada et al 2006), it is likely that MTR and MTRR variant alleles act in synergy in modulating breast cancer risk.…”

Metabolic derangement of methionine and folate metabolism in mice deficient in methionine synthase reductase

“…Polyclonal antibodies were generated to highly purified recombinant human methionine synthase reductase expressed in SF9 insect cells infected with baculovirus containing the cDNA for human methionine synthase reductase fused in frame to an amino-terminal hexahistidine tag [28]. Antibodies were applied as serum.…”

“…We monitored the weights of Mtrr +/+ , Mtrr +/gt , and Mtrr gt/gt mice of both sexes beginning at 2 weeks of age and weekly thereafter until mice reached 10 weeks of age (Fig. 4) 28.0 g for wild type males at week 10. The trend was evident at the first weighing, and became statistically significant by 7 weeks of age.…”