Test of the Contribution of an Amino-Aromatic Hydrogen Bond to Protein Function

Robert S. Jamison, Vanderbilt University
Bharati Kakkad, Vanderbilt University
Daniel H. Ebert, Vanderbilt University
Marcia E. Newcomer, Vanderbilt University
David E. Ong, Vanderbilt University

Abstract

Hydrogen bonds which form between a hydrogen bond donor and an aromatic ring as acceptor are thought to contribute to the stability and function of proteins. We have tested the function of such an interaction in a highly homologous pair of proteins, cellular retinol-binding protein (CRBP) and cellular retinol-binding protein, type II [CRBP(II)]. Both proteins bind the ligand all-trans-retinal with comparable affinities, but CRBP has an approximately 100-fold higher affinity for all-trans-retinol. The greater affinity of CRBP for all-trans-retinol has been attributed to the presence of an amino-aromatic hydrogen bond, which is absent in CRBP(II). We have generated a pair of mutant proteins, in which the amino-aromatic interaction was removed from CRBP and introduced into CRBP(II). Spectral analyses of retinol when bound to the wild-type and mutant CRBP suggested that it adopted an identical conformation within both proteins, a conformation that was distinct from that of retinol bound to CRBP(II), both wild-type and mutant. Unexpectedly, the affinities of the mutant binding proteins for all-trans-retinol were indistinguishable from those of their corresponding wild-type proteins. Further, in ligand competition experiments, there were no observable differences between mutant and wild-type CRBP, or between mutant and wildtype CRBP(II), in their preferences for binding all-trans-retinol versus all-trans-retinal. The results of this direct test of the proposed function of an amino role for such bonds, at least in this system. © 1995, American Chemical Society. All rights reserved.