Involved in the biosynthesis of prenylated phenolics natural products which contribute to the bitter taste of beer and display broad biological activities (Probable). Catalyzes the ligation of CoA on 2-methylpropanoate (isobutyric acid) and 2-methylbutanoate to produce 2-methylpropanoyl-CoA and 2-methylbutanoyl-CoA, respectively (PubMed:23300257). Can also use propanoate and butanoate as substrates with a lower efficiency (PubMed:23300257). 2-methylpropanoate + ATP + CoA = 2-methylpropanoyl-CoA + AMP + diphosphate ATP + CoA + propanoate = AMP + diphosphate + propanoyl-CoA ATP + butanoate + CoA = AMP + butanoyl-CoA + diphosphate 2-methylbutanoate + ATP + CoA = 2-methylbutanoyl-CoA + AMP + diphosphate kcat is 18.3 min(-1) with 2-methylpropanoate as substrate (PubMed:23300257). kcat is 10.5 min(-1) with 2-methylbutanoate as substrate (PubMed:23300257). kcat is 167 min(-1) with CoA as substrate (PubMed:23300257). Secondary metabolite biosynthesis. Mostly expressed in old leaves and in cones and glandular trichomes (lupulin glands) after flowering, and, to a lower extent, in stems, young leaves and flowers. Accumulates progressively in cones and glandular trichomes (lupulin glands) after flowering. Both substrate-binding domains (SBD1 and SBD2) are involved in the substrate recognition, and are sufficient to confer the substrate specificity. Belongs to the ATP-dependent AMP-binding enzyme family.