Cysteines involved in radical generation and catalysis of class III anaerobic ribonucleotide reductase - A protein engineering study of bacteriophageT4 NrdD

Class III ribonucleotide reductase (RNR) is an anaerobic glycyl radical enz yme that catalyzes the reduction of ribonucleotides to deoxyribonucleotides . We have investigated the importance in the reaction mechanism of nine con served cysteine residues in class III RNR from bacteriophage T4, By using s ite-directed mutagenesis, we show that two of the cysteines, Cys-79 and Cys -290, are directly involved in the reaction mechanism. Based on the positio ning of these two residues in the active site region of the known three-dim ensional structure of the phage T4 enzyme, and their structural equivalence to two cysteine residues in the active site region of the aerobic class I RNR, we suggest that Cys-290 participates in the reaction mechanism by form ing a transient thiyl radical and that Cys-79 participates in the actual re duction of the substrate. Our results provide strong experimental evidence for a similar radical-based reaction mechanism in all classes of RNR but al so identify important differences between class III RNR and the other class es of RNR as regards the reduction per se. We also identify a cluster of fo ur cysteines (Cys-543, Cys-546, Cys-561, and Cys-564) in the C-terminal par t of the class III enzyme, which are essential for formation of the glycyl radical. These cysteines make up a CX2C-CX2C motif in the vicinity of the s table radical at Gly-580. We propose that the four cysteines are involved i n radical transfer between Gly-580 and the cofactor S-adenosylmethionine of the activating NrdG enzyme needed for glycyl radical generation.