Very stable ribonucleotide substrate radical relevant for class I ribonucleotide reductase

In recent experimental studies on the E441Q mutant of ribonucleotide reduct ase, a new substrate radical was discovered on the minute time scale. This radical is kinetically coupled to a cysteine-based radical appearing on the 10 s time scale. In the present study, density functional calculations hav e been performed to investigate the nature of these radicals. The most inte resting result is that a very stable substrate radical was found, which lie s outside the normal substrate pathway. This radical is so stable that its creation has to be avoided by the enzyme, or the substrate reactions would be slowed by several orders of magnitude. It is suggested that the enzyme a ccomplishes this task by considerably straining the mobility of the Cys225 residue. A previously suggested reaction mechanism is modified to take thes e recent findings into account. The modification does not significantly cha nge the energetics of the model reactions.