Is there a Class IV ribonucleotide reductase?

描述（由申请人提供）：高等生物中的核糖核苷酸还原酶（RNR）利用二铁簇与氧的反应性将酪氨酸残基氧化为酪氨酰自由基和二铁簇辅因子。然后，这种所谓的I类辅因子参与核糖核苷酸向脱氧核糖核苷酸的转化，这是DNA合成中的限速步骤。靶向RNR的化学性质是治疗几种类型癌症的有效方法。原核生物有替代的辅因子来催化相同的反应。 一些具有真核生物的I类辅因子的变体（Ib类），一些利用钴胺素（II类），并且一些在厌氧条件下利用Fe-S簇（III类）。第四类还原酶需要锰也报道在棒状细菌。然而，C.产氨酶（CA）的研究表明，这种酶可能是一种Ib类铁酶，类似于在一些人类病原体中发现的酶。尽管如此，文献中仍有大量证据表明CA-RNR是锰酶。考虑到基因序列与氧活化Fe酶的相似性，出现了一种假设，即活性位点在基因序列中是保守的，以产生Mn可以活化氧以产生酪氨酰自由基的环境。虽然锰被用于生物反应中，使O-O键，如在光合水氧化，这将是第一次观察到的氧激活的锰酶。因此，该RNR反应可以是利用金属位点通过控制反应性中间体来促进困难氧化的又一个实例。下面概述的实验将建立C的金属特异性。产氨剂RNR.观察锰对氧的活化作用将是一种新的化学反应，其反应中间体将被研究。Fe辅因子的观察将使C.产氨菌RNR反应是病原菌产氨菌的反应模型。在任一情况下，将研究控制辅因子组装和最佳活性的因素。

DESCRIPTION (provided by applicant): 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.