In enzymology, a 3-methyl-2-oxobutanoate hydroxymethyltransferase (EC 2.1.2.11) is an enzyme that catalyzes the chemical reaction

5,10-methylenetetrahydrofolate + 3-methyl-2-oxobutanoate + H₂O ${\displaystyle \rightleftharpoons }$ tetrahydrofolate + 2-dehydropantoate

The 3 substrates of this enzyme are 5,10-methylenetetrahydrofolate, 3-methyl-2-oxobutanoate, and H₂O, whereas its two products are tetrahydrofolate and 2-dehydropantoate.

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 1M3U, 1O66, 1O68, and 1OY0.

• Powers SG, Snell EE (1976). "Ketopantoate hydroxymethyltransferase. II. Physical, catalytic, and regulatory properties". J. Biol. Chem. 251 (12): 3786–93. doi:10.1016/S0021-9258(17)33413-0. PMID 6463.
• Teller JH, Powers SG, Snell EE (1976). "Ketopantoate hydroxymethyltransferase. I. Purification and role in pantothenate biosynthesis". J. Biol. Chem. 251 (12): 3780–5. doi:10.1016/S0021-9258(17)33412-9. PMID 776976.