Molybdenum metalloenzymes containing

VII. References to Metalloenzymes Containing Copper, Iron and Molybdenum. 332... 

The first indication of an essential metabolic role for molybdenum was obtained in 1953, when it was discovered that xanthine oxidase, important in purine metabolism, was a metalloenzyme containing molybdenum. Subsequently the element was shown to be a component of two other enzymes, aldehyde oxidase and sulphite oxidase. The biological functions of molybdenum, apart from its reactions with copper (see p. 123), are concerned with the formation and activities of these three enzymes. In addition to being a component of xanthine oxidase, molybdenum participates in the reaction of the enzyme with cytochrome C and also facilitates the reduction of cytochrome C by aldehyde oxidase. 

Metalloenzymes, 33 40, see also Enzymes crystal structure, 44 230-258 DNA repair, 45 251 inhibitors, 36 40-42 molybdenum, 45 2, 53, 60-63 nuleic acid hydrolysis, 45 251-252 superoxide dismustases and, 45 130 zinc-containing, hard and soft acid-base behavior, 42 103-109 Metalloids... 

A wide range of metal ions is present in metalloenzymes as cofactors. Copper zinc snperoxide dismntase is a metalloenzyme that nses copper and zinc to help catalyze the conversion of snperoxide anion to molecnlar oxygen and hydrogen peroxide. Thermolysin is a protease that nses a tightly bonnd zinc ion to activate a water atom, which then attacks a peptide bond. Aconitase is one of the enzymes of the citric acid cycle it contains several iron atoms bonnd in the form of iron-sulfur clusters, which participate directly in the isomerization of citrate to isocitrate. Other metal ions fonnd as cofactors in metalloenzymes include molybdenum (in nitrate rednctase), seleninm (in glutathione peroxidase), nickel (in urease), and vanadinm (in fungal chloroperoxidase). see also Catalysis and Catalysts Coenzymes Denaturation Enzymes Krebs Cycle. 

At cellular level, the overall reduction process may be represented as (11.72), where the nitroge-nase system is an enzyme complex which includes phosphate energy carriers ATP and NADPH and three proteins. Two of these proteins are metalloenzymes, one of which contains iron, and the other both iron and molybdenum. The third protein, ferredoxin, contains iron and sulphur and acts as an electron provider through NADPH. 

Many enzymes incorporate one or more metal ions as essential parts of their structure. Different metalloenzymes make use of ions of magnesium, calcium, manganese, iron, cobalt, copper, zinc, or molybdenum. For example, the molecule of alcohol dehydrogenase (molecular mass 87000 d), which catalyzes the oxidation of ethanol to acetic acid in the human liver, contains two atoms of zinc, and the amylase in human saliva contains an atom of calcium (Ca" " ). Some enzyme molecules contain several metal atoms, which may be of different kinds. An example is cysteamine oxidase, which catalyzes the oxidation of cysteamine, HSCH2CH2NH2 this enzyme contains an atom of iron, an atom of copper, and an atom of zinc. 

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