Toward a rational design of peptide inhibitors of ribonucleotide reductase: structure-function and modeling studies.

Mammalian ribonucleotide reductase, a chemotherapeutic target, has two subunits, mR1 and mR2, and is inhibited by AcF(1)TLDADF(7), denoted P7. P7 corresponds to the C-terminus of mR2 and competes with mR2 for binding to mR1. We report results of a structure-function analysis of P7, obtained using a new assay measuring peptide ligand binding to mR1, that demonstrate stringent specificity for Phe at F(7), high specificity for Phe at F(1), and little specificity for the N-acyl group. They support a structural model in which the dominant interactions of P7 occur at two mR1 sites, the F(1) and F(7) subsites. The model is constructed from the structure of Escherichia coli R1 (eR1) complexed with the C-terminal peptide from eR2, aligned sequences of mR1 and eR1, and the trNOE-derived structure of mR1-bound P7. Comparison of this model with similar models constructed for mR1 complexed with other inhibitory ligands indicates that increased F(1) subsite interaction can offset lower F(7) subsite interaction and suggests strategies for the design of new, higher affinity inhibitors.