N(6)-[(R)-lipoyl]-L-lysyl-[lipoyl-carrier protein] + 4-methyl-2-oxopentanoate + malonyl-[ACP] + 2 H(+) = 5-methyl-3-oxohexanoyl-[ACP] + N(6)-[(R)-dihydrolipoyl]-L-lysyl-[lipoyl-carrier protein] + 2 CO2 — reaction