Structure-based optimization of peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR), a potential target for cancer int ervention, is composed of two subunits, mR1 and mR2, whose association is c ritical for enzyme activity. Tn this article we describe the structural fea tures of the mRR-inhibitor Ac-F-[ELAK]-DF (Peptide 3) while bound to the mR 1 subunit as determined by transferred NOEs. Peptide 3 is a cyclic analogue of the N-acetylated form of the heptapeptide C-terminus of the mR2 subunit (Ac-FTLDADF), which is the link between the two subunits and previously sh own to be the minimal sequence inhibitor mRR by competing with mR2 for bind ing to mR1. Structural refinement employing an ensemble-based, full-relaxat ion matrix approach resulted in two structures varying in the conformations of F-1 and the cyclic lactam side chains of E-2 and K-5. The remainder of the molecule, both backbone and side chains, is extremely well-defined, wit h an RMSD of 0.54 Angstrom. The structural features of this conformationall y constrained analogue provide unique insight into the requirements for bin ding to mR1, critical for further inhibitor development.