Serine hydroxymethyltransferase catalyzes the hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate.
The combined activities of rabbit liver cytosolic serine hydroxymethyltransferase and C1-tetrahydrofolate synthase convert tetrahydrofolate and formate to 5-formyltetrahydrofolate. In this reaction C1-tetrahydrofolate synthase converts tetrahydrofolate and formate to 5,10-methenyltetrahydrofolate, which is hydrolyzed to 5-formyltetrahydrofolate by a serine hydroxymethyltransferase-glycine complex. Serine hydroxymethyltransferase, in the presence of glycine, catalyzes the conversion of chemically synthesized 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate with biphasic kinetics. There is a rapid burst of product that has a half-life of formation of 0.4 s followed by a slower phase with a completion time of about 1 h. The substrate for the burst phase of the reaction was shown not to be 5,10-methenyltetrahydrofolate but rather a one-carbon derivative of tetrahydrofolate which exists in the presence of 5,10-methenyltetrahydrofolate. This derivative is stable at pH 7 and is not an intermediate in the hydrolysis of 5,10-methenyltetrahydrofolate to 10-formyltetrahydrofolate by C1-tetrahydrofolate synthase. Cytosolic serine hydroxymethyltransferase catalyzes the hydrolysis of 5,10-methenyltetrahydrofolate pentaglutamate to 5-formyltetrahydrofolate pentaglutamate 15-fold faster than the hydrolysis of the monoglutamate derivative. The pentaglutamate derivative of 5-formyltetrahydrofolate binds tightly to serine hydroxymethyltransferase and dissociates slowly with a half-life of 16 s. Both rabbit liver mitochondrial and Escherichia coli serine hydroxymethyltransferase catalyze the conversion of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate at rates similar to those observed for the cytosolic enzyme. Evidence that this reaction accounts for the in vivo presence of 5-formyltetrahydrofolate is suggested by the observation that mutant strains of E. coli, which lack serine hydroxymethyltransferase activity, do not contain 5-formyltetrahydrofolate, but both these cells, containing an overproducing plasmid of serine hydroxymethyltransferase, and wild-type cells do have measurable amounts of this form of the coenzyme.
Department of Biochemistry and Molecular Biophysics, Virginia Commonwealth University, Medical College of Virginia, Richmond 23298.
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D000818:Animals; D003600:Cytosol / Q000201:enzymology; D004926:Escherichia coli / Q000201:enzymology; D012696:Glycine Hydroxymethyltransferase / Q000378:metabolism*; D006868:Hydrolysis; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D002955:Leucovorin / Q000096:biosynthesis*; D008099:Liver / Q000201:enzymology; D008930:Mitochondria, Liver / Q000201:enzymology; D008954:Models, Biological; D011817:Rabbits; D013053:Spectrophotometry; D013763:Tetrahydrofolates / Q000378:metabolism*; D014166:Transferases / Q000378:metabolism*
D007527:Isoenzymes; D013763:Tetrahydrofolates; C018092:5,10-methenyltetrahydrofolate; D014166:Transferases; D012696:Glycine Hydroxymethyltransferase; D002955:Leucovorin