Ligand Binding Alters the Backbone Mobility of Intestinal Fatty Acid-binding Protein As Monitored by 15N NMR Relaxation and 1H Exchange

The backbone dynamics of the liganded (holo) and unliganded (apo) forms of Escherichia coli-derived rat intestinal fatty acid-binding protein (I-FABP) have been characterized and compared using amide 15N relaxation and 1H exchange NMR measurements. The amide 1H/15N resonances for apo and holo I-FABP were assigned at 25 degrees C, and gradient- and sensitivity-enhanced 2D experiments were employed to measure l5N T1, T2, and [1H]15N NOE values and relative 1H saturation transfer rates. The 15N relaxation parameters were analyzed using five different representations of the spectral density function based on the Lipari and Szabo formalism. A majority of the residues in both apo and holo I-FABP were characterized by relatively slow hydrogen exchange rates, high generalized order parameters, and no conformational exchange terms. However, residues V26-N35, S53-R56, and A73-T76 of apo I-FABP were characterized by rapid hydrogen exchange, low order parameters, and significant conformational exchange. These residues are clustered in a single region of the protein where variability and apparent disorder were previously observed in the chemical shift analyses and in the NOE-derived NMR structures of apo I-FABP. The increased mobility and discrete disorder in the backbone of the apo protein may permit the entry of ligand into the binding cavity. We postulate that the bound fatty acid participates in a series of long-range cooperative interactions that cap and stabilize the C-terminal half of helix II and lead to an ordering of the portal region. This ligand-modulated order-disorder transition has implications for the role of I-FABP in cellular fatty acid transport and targeting.