Catalyzes the ligation of CoA on isovalerate to produce 3-methylbutanoyl-CoA (PubMed:23300257). Can also use butanoate, pentanoate, hexanoate, 3-methylpentanoate and 4-methylpentanoate as substrates with a lower efficiency (PubMed:23300257). ATP + CoA + isovalerate = 3-methylbutanoyl-CoA + AMP + diphosphate ATP + CoA + hexanoate = AMP + diphosphate + hexanoyl-CoA ATP + butanoate + CoA = AMP + butanoyl-CoA + diphosphate ATP + CoA + pentanoate = AMP + diphosphate + pentanoyl-CoA 3-methylpentanoate + ATP + CoA = 3-methylpentanoyl-CoA + AMP + diphosphate 4-methylpentanoate + ATP + CoA = 4-methylpentanoyl-CoA + AMP + diphosphate Expressed at low levels in leaves, flowers and developing seeds. Belongs to the ATP-dependent AMP-binding enzyme family.