Catalyzes the reversible reaction in which hydroxymethyl group from 5,10-methylenetetrahydrofolate is transferred onto alpha-ketoisovalerate to form ketopantoate. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + 3-methyl-2-oxobutanoate + H2O = (6S)-5,6,7,8-tetrahydrofolate + 2-dehydropantoate Binds 1 Mg(2+) ion per subunit. Can also use Co(2+), Zn(2+), Ni(2+) and Ca(2+) to a lesser extent. Optimum pH is 7.0-7.5. Cofactor biosynthesis; (R)-pantothenate biosynthesis; (R)-pantoate from 3-methyl-2-oxobutanoate: step 1/2. Homodecamer; pentamer of dimers. Pupylated at an undetermined lysine residue by the prokaryotic ubiquitin-like protein Pup with the help of the ligase PafA, which leads to its degradation by the proteasome. The cross-link involves the side-chain carboxylate of the C-terminal glutamate of Pup and the side-chain amino group of a lysine in PanB. Was identified as a natural substrate of the M.tuberculosis proteasome. Was identified as a high-confidence drug target. Belongs to the PanB family.