In enzymology, a 3-methyl-2-oxobutanoate hydroxymethyltransferase () is an enzyme that catalyzes the chemical reaction
The enzyme converts ñ-ketoisovaleric acid into ketopantoic acid by hydroxymethylation. The is transferred from the cofactor 5,10-methylenetetrahydrofolate (5,10-CH<sub>2</sub>-THF), which is converted to tetrahydrofolate (THF).
This enzyme belongs to the family of transferases that transfer one-carbon groups, specifically the hydroxymethyl-, formyl- and related transferases. The systematic name of this enzyme class is 5,10-methylenetetrahydrofolate:3-methyl-2-oxobutanoate hydroxymethyltransferase. Other names in common use include alpha-ketoisovalerate hydroxymethyltransferase, dehydropantoate hydroxymethyltransferase, ketopantoate hydroxymethyltransferase, oxopantoate hydroxymethyltransferase, 5,10-methylene tetrahydrofolate:alpha-ketoisovalerate, and hydroxymethyltransferase. This enzyme participates in pantothenate and coa biosynthesis.
As of late 2007, 4 structures have been solved for this class of enzymes, with PDB accession codes , , , and .