Peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. In common with other class Ia RRs, the enzyme is composed of two subunits (mR1 and mR2), with mR1 containing both the active site and allosteric effector sites and mR2 containing a stable tyrosyl radical that is essential for enzymatic activity. mRR is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which competes with mR2 for binding to mR1. The enzyme has two physiologically important active forms, mR12mR22 and mR16(mR22)j (j=1-3), with high ATP concentrations favoring the latter. Here, we report on our progress in using structural and functional studies in conjunction with library screening to identify derivatives of tri-, tetra- and hexapeptides, and cyclic heptapeptides, having equal or significantly higher activities than P7 toward inhibition of one or both active forms. These identifications were made by screening candidate peptides both for their abilities to bind to mR1 competitively with P7 and to inhibit ribonucleotide reductase activity. A significant feature of both P7 and the newly identified derivatives is that they are stronger inhibitors of mR12mR22 than of mR16(mR22)j. For the tetrapeptides, this is due in part to their preferential binding to mR1 monomer. The possible application of these peptide derivatives in cancer chemotherapy, exploiting their preferential inhibition of mR12mR22, is considered.