Figure 3

Electron relay in the class I ribonucleotide reductase (RNR). Each enzymatic turnover of RNR is accompanied by the transfer of an electron from the active site cysteine (Cys439) to a long-lived tyrosyl radical (Tyr122) stabilized by a diiron center. Cys439 and Tyr122 reside in different subunits of the enzyme and are separated by a formidable distance of approximately 3.5 nm. Intervening residues (Tyr730, Tyr731, Tyr356, and Trp48) relay the electron by forming transient amino acid radicals and thus function as "stepping stones" for a tunneling electron. The electron relay chain greatly outperforms unistep tunneling in terms of the rate of electron flow it can support. If unistep tunneling alone were responsible for electron transfer from Cys439 to Tyr122, the estimated waiting time for a single ET event would be hours or years. However, a single turnover actually occurs in approximately 200 ms [270].