Monacolin J acid methylbutanoyltransferase; part of the gene cluster that mediates the biosynthesis of lovastatin (also known as mevinolin, mevacor or monacolin K), a hypolipidemic inhibitor of (3S)-hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR) (PubMed:10334994, PubMed:12929390, PubMed:21495633). The first step in the biosynthesis of lovastatin is the production of dihydromonacolin L acid by the lovastatin nonaketide synthase lovB and the trans-acting enoyl reductase lovC via condensation of one acetyl-CoA unit and 8 malonyl-CoA units (PubMed:10334994, PubMed:10381407, PubMed:19900898, PubMed:22733743). Dihydromonacolin L acid is released from lovB by the thioesterase lovG (PubMed:23653178). Next, dihydromonacolin L acid is oxidized by the dihydromonacolin L monooxygenase lovA twice to form monacolin J acid (PubMed:12929390, PubMed:21495633). The 2-methylbutyrate moiety of lovastatin is synthesized by the lovastatin diketide synthase lovF via condensation of one acetyl-CoA unit and one malonyl-CoA unit (PubMed:19530726, PubMed:21069965). Finally, the covalent attachment of this moiety to monacolin J acid is catalyzed by the transesterase lovD to yield lovastatin (PubMed:10334994, PubMed:17113998, PubMed:18988191, PubMed:19875080, PubMed:24727900). LovD has broad substrate specificity and can also convert monacolin J to simvastatin using alpha-dimethylbutanoyl-S-methyl-3-mercaptopropionate (DMB-S-MMP) as the thioester acyl donor, and can also catalyze the reverse reaction and function as hydrolase in vitro (PubMed:19875080). LovD has much higher activity with LovF-bound 2-methylbutanoate than with free diketide substrates (PubMed:21069965). (S)-2-methylbutanoyl-[2-methylbutanoate polyketide synthase] + monacolin J carboxylate = holo-[2-methylbutanoate polyketide synthase] + lovastatin carboxylate kcat is 0.62 min(-1) for simvastatin synthesis. Polyketide biosynthesis; lovastatin biosynthesis. Interacts with LovF. Loss of lovastatin biosynthesis. Lovastatin acts as a hypolipidemic agent that works as inhibitor of (3S)-hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR) which reduces HMG-CoA to mevalonate and is the key step in cholesterol biosynthesis (PubMed:6933445). Lovastatin, simvastatin and related compounds are widely used to treat hypercholesteremia and reduce the risk of cardiovascular disease (PubMed:6933445). Furthermore, statins such as lovastatin were found to be anticancer agents (PubMed:29236027, PubMed:29932104). Directed evolution toward higher catalytic activity with free diketides led to an enzyme with 1000-fold higher activity in simvastatin synthesis, due to numerous mutations that affect protein folding and promote optimal alignment of the residues that are important for substrate binding and catalysis. Belongs to the class-A beta-lactamase family.