REACTIVITY OF THE TYROSYL RADICAL OF ESCHERICHIA-COLI RIBONUCLEOTIDE REDUCTASE - CONTROL BY THE PROTEIN

Ribonucleotide reductase is a key enzyme for DNA synthesis. Its small component, named protein R2, contains a tyrosyl radical essential for activity. Consequently, radical scavengers are potential antiprolifera tive agents. In this study, we show that the reactivity of the tyrosyl radical towards phenols, hydrazines, hydroxyurea, dithionite and asco rbate can be finely tuned by relatively small modifications of its hyd rophobic close environment. For example, in this hydrophobic pecker, L eu77-->Phe mutation resulted in a protein with a much higher susceptib ility to radical scavenging by hydrophobic agents. This might suggest that the protein is flexible enough to allow small molecules to penetr ate in the radical site. When mutations keeping the hydrophobic charac ter are brought further from the radical (for example Ile74-->Phe) the reactivity of the radical is instead very little affected. When a pos itive charge was introduced (for example Ile74-->Arg or Lys) the prote in was more sensitive to negatively charged electron donors such as di thionite. These results allow us to understand how tyrosyl radical sit es have been optimized to provide a good stability for the free radica l.