The enzyme ribonucleotide reductase (RNR) in higher organisms utilizes the reactivity of a diferrous cluster with oxygen for the oxidation of a tyrosine residue to a tyrosyl radical and diferric cluster cofactor. This so called Class I cofactor then participates in the conversion of ribonucleotides to deoxyribonucleotides, the rate limiting step in DNA synthesis. Targeting the chemistry of RNR has been an effective approach for treatment of several types of cancer. Prokaryotes have alternate cofactors to catalyze the same reaction. Some have a variation of the Class I cofactor of the eukaryotes (Class Ib), some utilize cobalamine (Class II), and some utilize Fe-S clusters under anaerobic conditions (Class Ill). A fourth class of reductase requiring manganese was also reported in Coryneform bacteria. However, the gene sequence of the RNR from C. ammoniagenes (CA) suggests that the enzyme may be a Class Ib iron enzyme, similar to those found in some human pathogens. Nonetheless, there remains substantial evidence in the literature suggesting that the CA-RNR is a manganese enzyme. Given the similarity of the gene sequence to oxygen activating Fe enzymes, a hypothesis emerges that the active site is conserved in the gene sequence to create an environment where Mn can activate oxygen for the generation of the tyrosyl radical. Whereas manganese is utilized in biological reactions that make the O-O bond such as in photosynthetic water oxidation, this would be the first observation of oxygen activation by a manganous enzyme. Thus, this RNR reaction may be yet another example of utilizing metal sites to facilitate difficult oxidations by controlling the reactive intermediates. The experiments outlined below will establish the metal specificity of the C. ammoniagenes RNR. Observation of oxygen activation with Mn will be novel chemistry whose reaction intermediates will be studied. Observation of an Fe cofactor would make the C. ammoniagenes RNR reaction a model for those in pathogenic bacteria. In either case, the factors that control the cofactor assembly and optimal activity will be investigated. ? ? ?
