(6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + 3-methyl-2-oxobutanoate + H2O = (6S)-5,6,7,8-tetrahydrofolate + 2-dehydropantoate Cofactor biosynthesis; (R)-pantothenate biosynthesis; (R)-pantoate from 3-methyl-2-oxobutanoate: step 1/2. Belongs to the PanB family.