ACYL-COENZYME-A BINDING-PROTEIN - CONFORMATIONAL SENSITIVITY TO LONG-CHAIN FATTY ACYL-COA

Cellular unbound long chain fatty acyl-CoAs (>14 carbon) are potent re gulators of gene transcription and intracellular signaling. Although t he cytosolic acyl-CoA binding protein (ACBP) has high affinity for med ium chain fatty acyl-CoAs, direct interaction of ACBP with >14-carbon fatty acyl-CoAs has not been established. Steady state, photon countin g fluorescence spectroscopy directly established that rat liver ACBP b ound 18-carbon cis-and trans-parinaroyl-CoA, K-d = 7.03 +/- 0.95 and 4 .40 +/- 0.43 nM. Time-resolved fluorometry revealed that ACBP-bound pa rinaroyl-CoAs had high rotational freedom within the single, relativel y hydrophobic (epsilon <32), binding site. Tyr and Trp fluorescence dy namics demonstrated that apo-ACBP was an ellipsoidal protein taxes of 15 and 9 Angstrom) whose conformation was altered by oleoyl-CoA in the holo-ACBP as shown by a 2-Angstrom decrease of ACBP hydrodynamic diam eter and increased Trp segmental motions. Thus, native liver ACBP bind s > 14-carbon fatty acyl-CoAs with nanomolar affinity at a single bind ing site. Acyl-CoA-induced conformational alterations in ACBP may be s ignificant to its putative functions in lipid metabolism and regulatio n of processes sensitive to unbound long chain fatty acyl-CoAs.