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 neu ral tube defects. The flavoprotein methylenetetrahydrofolate reductase (MTH FR) is a likely target for these actions of folate. The most common genetic cause of mildly elevated plasma homocysteine in humans is the MTHFR polymo rphism A222V (base change C677-->T). The X-ray analysis of E. coli MTHFR, r eported here, provides a model for the catalytic domain that is shared by a ll MTHFRs. This domain is a beta(8)alpha(8) barrel that binds FAD in a nove l fashion. Ala 177, corresponding to Ala 222 in human MTHFR, is near the bo ttom of the barrel and distant from the FAD. The mutation A177V does not af fect K-m or k(cat) but instead increases the propensity for bacterial RATHE R to lose its essential flavin cofactor. Folate derivatives protect wild-ty pe and mutant E. coli enzymes against flavin loss, and protect human MTHFR and the A222V mutant against thermal inactivation, suggesting a mechanism b y which folate treatment reduces homocysteine levels.