CHAPTER 10 - The Molybdenum Cofactor Common to Nitrate Reductase, Xanthine Dehydrogenase and Sulfite Oxidase

Abstract

This chapter discusses the molybdenum cofactor common to nitrate reductase, xanthine dehydrogenase, and sulfite oxidase. Both nitrate reductase and xanthine dehydrogenase share a common molybdenum cofactor, the synthesis of which is controlled by five independent genetic loci. The molybdenum cofactor present in nitrogenase is unique, while the molybdenum containing component of other molybdoenzymes is of a more universal nature as originally hypothesized. Although considerable effort has been applied to the determination of the structural elements of the nitrogenase molybdenum cofactor and of the molybdenum-containing cofactor of other molybdoenzymes, the information presently available is limited. Cofactor sources can be divided into two major categories: those that supply cofactor in a form that can reconstitute demolybdoenzymes directly and those that have latent cofactor activity, as part of an intact molybdenum enzyme. In the latter case, rather drastic pretreatment such as acidification is required to release the cofactor from the donor molybdoenzyme and make it available to the acceptor molecule. One major disadvantage to the use of the various inactive nitrate reductases as substrates for assay of molybdenum cofactor is that the apoenzyme molecules are particularly labile and are rapidly inactivated under many in vitro conditions.