Christal A. Sheppard

The Structure and Properties of Methylenetetrahydrofolate Reductase from Escherichia Coli Suggest How Folate Ameliorates Human Hyperhomocysteinemia

Elevated plasma homocysteine levels are associated with increased risk for cardiovascular disease and neural tube defects in humans. Folate treatment decreases homocysteine levels and dramatically reduces the incidence of neural tube defects. The flavoprotein methylenetetrahydrofolate reductase (MTHFR) is a likely target for these actions of folate. The most common genetic cause of mildly elevated plasma homocysteine in humans is the MTHFR polymorphism A222V (base change C677→T). The X-ray analysis of E. coli MTHFR, reported here, provides a model for the catalytic domain that is shared by all MTHFRs. This domain is a β8α8 barrel that binds FAD in a novel fashion. Ala 177, corresponding to Ala 222 in human MTHFR, is near the bottom of the barrel and distant from the FAD. The mutation A177V does not affect Km or kcat but instead increases the propensity for bacterial MTHFR to lose its essential flavin cofactor. Folate derivatives protect wild-type and mutant E. coli enzymes against flavin loss, and protect human MTHFR and the A222V mutant against thermal inactivation, suggesting a mechanism by which folate treatment reduces homocysteine levels.

Methylenetetrahydrofolate Reductase and Methionine Synthase: Biochemistry and Molecular Biology

Abstract Methylenetetrahydrofolate reductase and cobalamin-dependent methionine synthase catalyze the penultimate and ultimate steps in the biosynthesis of methionine in prokaryotes, and are required for the regeneration of the methyl group of methionine in mammals. Defects in either of these enzymes can lead to hyperhomocysteinemia. The sequences of the human methylenetetrahydrofolate reductase and methionine synthase are now known, and show clear homology with their bacterial analogues. Mutations in both enzymes that are known to occur in humans and to be associated with hyperhomocysteinemia affect residues that are conserved in the bacterial enzymes. Structure/function studies on the bacterial proteins, summarized in this review, are therefore relevant to the function of the human enzymes; in particular studies on the effects of bacterial mutations analogous to those causing hyperhomocysteinemia in human may shed light on the defects associated with these mutations.

Methylenetetrahydrofolate Reductase: Comparison of the Enzyme from Mammalian and Bacterial Sources

Methylenetetrahydrofolate reductase catalyzes the reduction of methylenetetrahydrofolate to methyl-tetrahydrofolate, which serves as the methyl donor for the conversion of homocysteine to methionine in the reaction catalyzed by methionine synthase. Recent studies have provided evidence for association of a common polymorphism of methylenetetrahydrofolate reductase with elevated levels of blood homocysteine, a metabolite correlated with increased cardiovascular risk in humans. In this chapter, we review earlier studies on the properties of the mammalian enzyme, interpreting these observations in the light of the deduced amino acid sequence for the human enzyme. The catalytic portion of the human enzyme shows significant sequence homologies with methylenetetrahydrofolate reductase from bacterial sources, and in particular mutations that are known to cause elevated blood homocysteine levels in humans affect residues that are conserved in the bacterial enzyme. Thus we believe that studies of mutant E coli proteins will provide valuable information on the consequences of human mutations of equivalent residues.