Ketopantoate hydroxymethyltransferase. I. Purification and role in pantothenate biosynthesis.

A new enzyme, ketopantoate hydroxymethyltransferase (5,10-methylene tetrahydrofolate: alpha-ketoisovalerate hydroxymethyltransferase) has been purified 2400-fold to apparent homogeneity from Escherichia coli K12. It catalyzes the first committed step in pantothenate biosynthesis, the reversible formation of ketopantoate (2-keto-3,3-dimethyl-4-hydroxybutyrate) according to Equation 1, has low Km values for its substrates, and is abent (1) Methylenetetrahydrofolate + alpha-ketoisovalerate in equilibrium tetrahydrofolate + ketopantoate from a mutant of E. coli auxotrophic for ketopantoate. It thus appears to be the enzyme responsible for catalysis of ketopantoate formation in vivo. A previously described enzyme that catalyzes reaction 2 irreversible (McIntosh, E.N., Purko, M., and Wood, W.A. (1957) J. Biol. Chem. 228, 499-509) and does not require (2) HCHO + alpha-ketoisovalerate leads to ketopantoate tetrahydrofolate can be obtained free of ketopantoate hydroxymethyltransferase and is present in equal amounts in ketopantoate auxotrophs and wild type E. coli. We conclude that the latter enzyme is not involved in the normal biosynthetic pathway leading to pantothenate; its function is unknown.