Methylenetetrahydrofolate Reductase: Comparison of theEnzyme from Mammalian and Bacterial Sources

Methylenetetrahydrofolate reductase catalyzes the reduction of methylenetetrahydrofolate to methyltetrahydrofolate, 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. Properties of Mammalian Methylenetetrahydrofolate Reductase The conversion of homocysteine to methionine requires a sequence of three enzymatic reactions, which are catalyzed by serine hydroxymethyltransferase (41), methylenetetrahydrofolate reductase (MTHFR [4-2]), and cobalamin-dependent methionine synthase (4-3), respectively. Serine + H4folate → Glycine + CH2 –H4folate (1) CH2 –H4folate + NADPH → CH3-H4folate + NADP+ (2) CH3-H4folate + Homocysteine → H4folate + Methionine (3) The combined action of these three enzymes results in the methylation of homocysteine by transfer of a methyl group derived from the β-carbon of serine. Methionine can be converted to adenosylmethionine (AdoMet) by adenosylmethionine synthetase, as shown in (4). AdoMet is an activated methyl donor, and serves to provide methyl groups in a wide variety of biological methylation reactions. Methionine + ATP → AdoMet + tripolyphosphate (4) Methylenetetrahydrofolate reductase has been purified from a variety of bacterial and mammalian sources and has been shown to contain enzyme-bound FAD as a prosthetic group [1—6]. The flavin plays an essential role in the catalytic reaction. In the porcine enzyme, which has been purified to homogeneity [6] and extensively studied, NADPH stoichiometrically reduces the flavin as indicated in (5). FADPH + E ∙ FADox → NADP+ + E ∙ FADred (5) This reaction is irreversible both in vivo and in vitro [7], and imparts irreversibility to the entire reaction shown in (2). In the physiologic reaction, the reduced Published (as Chapter 4) in Homocysteine Metabolism: From Basic Science to Clinical Medicine, Ed. Ian Graham, Helga Refsum, Irwin H. Rosenberg, Per Magne Ueland, & Jill M. Shuman (Dordrecht: Kluwer Academic Publishers, 1997), pp. 31–35. Copyright © 1997 Kluwer Academic Publishers. Used by permission. Methylenetetrahydrofolate Reductase: Comparison of the Enzyme from Mammalian and Bacterial Sources Christal A. Sheppard,1 James S. Sumner,2 Phillipe Goyette,3 Phyllis Frosst,3 Rima Rozen,3 and Rowena G. Matthews1 1. University of Michigan, Ann Arbor, MI 2. Merck & Co., Inc., West Point, NY 3. McGill University, Montreal, QC