Hydrogen atom transfer in ribonucleotide reductase (RNR)

The communication between the cysteine, Cys439, at the substrate site and the tyrosyl radical, Tyr122, in ribonucleotide reductase is studied by quantum chemical models at the DFT-B3LYP level. Recent theoretical and experimental studies have indicated that an electron transfer between these sites is highly unlikely. Instead, a model based on the hydrogen atom transfer (HAT) mechanism is investigated. In this mechanism both the proton and electron are moved in each step to avoid a costly charge separation. It is found that the hydrogen atom transfer steps required for communication between Cys439 in R1 and Trp48 in the region of the iron dimer in R2 all have quite low energy barriers. The radical transfer between Tyr731 and Tyr730 has a barrier of 4.9 kcal/mol, while the one between Tyr730 to Cys439 has a barrier of 8.1 kcal/mol. An interesting aspect of these transfers is that the dielectric contribution from the protein is very small, indicating very small charge separations. The radical transfer from Tyr122 to Trp48 over the iron dimer is considerably more complicated. A model is suggested where this transfer occurs in essentially one step by a hydrogen atom transfer from a water ligand of the iron dimer to Tyr122. In this process an electron is transferred between Trp48 to the hydroxyl ligand of iron over the Trp48-Asp237-His241 chain leading to a cationic tryptophan radical.