Catalyzes the reduction of crotonobetainyl-CoA to gamma-butyrobetainyl-CoA (PubMed:8060125, PubMed:10209289, PubMed:10978161). The electron donor could be the FixA/FixB complex (PubMed:12081978). gamma-butyrobetainyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = crotonobetainyl-CoA + reduced [electron-transfer flavoprotein] Binds 1 FAD per subunit. Optimum pH is 7.8. Optimum temperature is 40-45 degrees Celsius. Amine and polyamine metabolism; carnitine metabolism. Homotetramer. Transcribed during anaerobic growth in the presence of L-carnitine or crotonobetaine. Deletion of the gene abolishes gamma-butyrobetaine formation and increases L-carnitine production under anaerobic and aerobic conditions. Belongs to the acyl-CoA dehydrogenase family.