TEST OF THE CONTRIBUTION OF AN AMINO-AROMATIC HYDROGEN-BOND TO PROTEIN FUNCTION

Hydrogen bonds which form between a hydrogen bond donor and an aromati c ring as acceptor an thought to contribute to the stability and funct ion of proteins. We have tested the function of such an interaction in a highly homologous pair of proteins, cellular retinol-binding protei n (CRBP) and cellular retinol-binding protein, type II [CRBP(II)]. Bot h proteins bind the ligand all-trans-retinal with comparable affinitie s, but CRBP has an approximately 100-fold higher affinity for all-tran s-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 introd uced into CRBP(LI). Spectral analyses of retinol when bound to the wil d-type and mutant CRBP suggested that it adopted an identical conforma tion 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 w ere indistinguishable from those of their corresponding wild-type prot eins. Further, in ligand competition experiments, there were no observ able differences between mutant and wild-type CRBP, or between mutant and wildtype CRBP(IT), in their preferences for binding all-trans-reti nol versus all-trans-retinal. The results of this direct test of the p roposed function of an amino-aromatic hydrogen bond did not support a functional role for such bonds, at least in this system.