Binding site polarity and ligand affinity of homologous fatty acid-binding proteins from animals with different body temperatures.

Binding affinity and binding-pocket polarity is determined for intracellular fatty acid-binding protein (FABP) from aerobic muscle of Chaenocephalus aceratus, the Antarctic icefish, and from rat heart. FABPs bind fatty acids via weak-bond forces (both ionic and hydrophobic), and these bond forces are temperature sensitive, yet FABPs are present in animals whose body temperatures range over nearly 40 degrees C. To investigate FABP's sensitivity to body temperature, fatty acid binding affinity (Kd) was determined for both rat heart-FABP and icefish heart-FABP at two physiological temperatures (0 degrees C or 37 degrees C). Saturated and unsaturated fatty acids (16:0 and 16:1), delivered in model membranes (liposomes) whose composition is typical of either Antarctic fish (16:0/22:6 phosphatidylcholine) or mammals (bovine-heart phosphatidylcholine) were examined. Incubation at 0 degree C or 37 degrees C dose not significantly affect Kd for rat heart FABP, regardless of liposome composition or fatty acid ligand (Kd = 0.686 +/- 0.127 - 1.129 +/- 0.356 microM at 0 degree C, 0.775 +/- 0.307 - 1.605 +/- 0.427 microM at 37 degrees C). Incubation temperature significantly affects icefish FABP's affinity for 16:1 (0.626 +/- 0.093 microM at 37 degrees C vs. 1.896 +/- 0.343 microM at 0 degree C for fatty acid presented in Antarctic fish liposomes; 0.331 +/- 0.101 microM at 37 degrees C vs. 0.949 +/- 0.121 microM at 0 degree C for bovine heart liposomes) but not 16:0. Kd is not significantly different between FABPs under any set of conditions (with one exception: Kd is significantly lower in rat FABP vs. icefish FABP for 16:0 at 0 degree C for fatty acids delivered in bovine heart liposomes). Although Kd values are largely equivalent between the two FABPs, relative contributions from ionic vs. hydrophobic weak-bond forces are different between the two animals. Rat heart FABP has a binding pocket that is significantly more nonpolar than that of icefish FABP (as measured by quantum yield of the bound fluorescent fatty-acid analogue (PA-DPH); Q = 0.067 +/- 0.008 vs. 0.034 +/- 0.005 at 0 degree C, 0.030 +/- 0.003 vs. 0.019 +/- 0.002 at 37 degrees C). This suggests that rat-heart FABP realizes a micromolar Kd with a greater reliance upon hydrophobic interactions than does icefish FABP.