Hille R
Department of Medical Biochemistry, Ohio State University, Columbus 43210-1218, USA.
Essays Biochem. 1999;34:125-37. doi: 10.1042/bse0340125.
There are many molybdenum-containing enzymes distributed throughout the biosphere. The availability of molybdenum to biological systems is due to the high water solubility of oxidized forms of the metal. Molybdenum enzymes can be grouped on the basis of the structure of the metal centre. Three principal families of enzyme exist, with active sites consisting of (ppt)MoOS(OH) (the molybdenum hydroxylases), (ppt)MoO2(S-Cys) (the eukaryotic oxotransferases) and (ppt)2MoOX (the bacterial oxotransferases). Here, ppt represents a unique ppt cofactor (pyranopterin) that co-ordinates to the metal, and X is a metalliganded serine, cysteine or selenocysteine. The molybdenum hydroxylases catalyse their reactions differently to other hydroxylase enzymes, with water rather than molecular oxygen as the ultimate source of the oxygen atom incorporated into product, and with the generation rather than consumption of reducing equivalents. The active sites possess a catalytically labile Mo-OH (or possibly Mo-OH2) group that is transferred to substrate in the course of the hydroxylation reaction. These enzymes invariably have other redoxactive centres. The eukaryotic oxotransferases consist of the sulphite oxidases and plant nitrate reductases. They catalyse the transfer of an oxygen atom to or from their substrate (to and from nitrate) in a manner that involves formal oxidation-state changes of the molybdenum. As with the molybdenum hydroxylases, the ultimate source of oxygen is water rather than molecular oxygen. The bacterial oxotransferases and related enzymes differ from the other two groups of molybdenum enzymes in having two equivalents of the ppt cofactor co-ordinated to the metal. This family is quite diverse, as reflected in the fact that serine, cysteine or selenocysteine may be found co-ordinated to the molybdenum, depending on the enzyme. As in the case of the molybdenum hydroxylases, both eukaryotic and bacterial oxotransferases utilize water (rather than molecular oxygen) as the source of the oxygen atom incorporated into product, although for these enzymes, the catalytically labile oxygen in the active site is an Mo = O group rather than an Mo-OH.
生物圈中分布着许多含钼酶。钼在生物系统中的可利用性归因于该金属氧化态的高水溶性。钼酶可根据金属中心的结构进行分类。存在三个主要的酶家族，其活性位点分别由（ppt）MoOS（OH）（钼羟化酶）、（ppt）MoO2（S-半胱氨酸）（真核生物氧转移酶）和（ppt）2MoOX（细菌氧转移酶）组成。此处，ppt代表与金属配位的独特的ppt辅因子（吡喃蝶呤），X是与金属配位的丝氨酸、半胱氨酸或硒代半胱氨酸。钼羟化酶催化反应的方式与其他羟化酶不同，其将水而非分子氧作为掺入产物中的氧原子的最终来源，并且产生而非消耗还原当量。活性位点具有一个在催化过程中不稳定的Mo-OH（或可能是Mo-OH2）基团，该基团在羟化反应过程中转移到底物上。这些酶总是具有其他氧化还原活性中心。真核生物氧转移酶包括亚硫酸盐氧化酶和植物硝酸还原酶。它们以涉及钼的形式氧化态变化的方式催化氧原子向底物的转移或从底物的转移（向硝酸盐和从硝酸盐的转移）。与钼羟化酶一样，氧的最终来源是水而非分子氧。细菌氧转移酶及相关酶与其他两类钼酶的不同之处在于有两个当量的ppt辅因子与金属配位。这个家族非常多样，这体现在根据酶的不同，丝氨酸、半胱氨酸或硒代半胱氨酸可能与钼配位。与钼羟化酶的情况一样，真核生物和细菌氧转移酶都利用水（而非分子氧）作为掺入产物中的氧原子的来源，尽管对于这些酶来说，活性位点中在催化过程中不稳定的氧是一个Mo = O基团而非Mo-OH。
