A comparative study of the conformational properties of Escherichia coli-derived rat intestinal and liver fatty acid binding proteins.

Fourier transform infrared spectroscopy has been used to examine the conformation in aqueous solution of Escherichia coli-expressed rat intestinal and liver fatty-acid binding proteins (I-FABP and L-FABP, respectively). While I-FABP is known from X-ray analysis to have a predominantly beta-structure with 10 antiparallel beta-strands forming two orthogonal sheets that surround the ligand binding pocket, no structural data are available for L-FABP. As expected for homologous proteins with related functions, the secondary structures of I-FABP and L-FABP are very similar. In both proteins, the conformation-sensitive amide-I band shows the maximum absorption at around 1630 cm-1, proving that beta-sheet is the major structural element. However, there are three critical differences between I-FABP and L-FABP; (i), a different solvent accessibility of the protein backbone; (ii), a different pH sensitivity and (iii), a different thermostability, with L-FABP being thermally more stable than I-FABP. These results suggest that, in spite of having a similar overall conformation, the architecture of these proteins is stabilized by slightly different interactions. Such dissimilarities, well-paralleled by fatty-acid binding studies, may provide a structural basis for their functional diversification.