Expression, purification, and crystal structure determination of recombinant human epidermal-type fatty acid binding protein

We describe the crystal structure of human epidermal-type fatty acid binding protein (E-FABP) that was recently found to be highly upregulated in human psoriatic keratinocytes, To characterize E-FABP with respect to ligand-binding properties and tertiary structure, we cloned the respective cDNA, overexpressed the protein in Escherichia coli and purified it to homogeneity by a combination of ion-exchange and size-exclusion chromatographic steps with a yield of 30 mg/L broth. The purified protein revealed a 5-fold higher affinity for stearic acid than for oleic and arachidonic acids. The crystal structure of recombinant human E-FABP was determined to 2.05 Angstrom and refined to an R-factor of 20.7%, The initial residual electron density maps clearly showed the presence of a ligand, which was identified as endogenous bacterial fatty acid. Within a central cavity of 252 Angstrom(3), this ligand is bound in a U-shaped conformation, its carboxyl group interacting with tyrosine 131 and arginines 129 and 109, the latter via an ordered water molecule. The E-FABP crystal structure is unique in the FABP family because of the presence of a disulfide bridge between cysteines 120 and 127 that may be physiologically as well as pathophysiologically relevant. Cysteines 67 and 87 are also in close vicinity but in contrast do not form a disulfide bridge. We postulate that this protein belongs to a particular FABP subfamily whose members share common structural as well as functional features.