Protection from Radicals Catalytic Pro- and Antioxidants

It is ironic that while most organisms rely on oxygen for respiration, its mere presence can have damaging effects. Oxidation occurs slowly in an oxygen rich atmosphere but is accelerated by many transition metals such as those regularly encountered in biological systems. When oxidation of lipids, DNA, RNA, proteins and other biomolecules occurs a major problem is encountered as it affects their functions. There are, of course, many necessary enzymes that catalyse oxidation 

Superoxide dismutase turns out to be the key mammalian antioxidant enzyme. SOD is essential genetically modified mice lacking the enzyme die shortly after birth. It exists in several forms but the most closely studied version is SOD1, a protein containing zinc and copper. The catalytic site, shown in Fig. 4.11, contains a copper cation held by three neutral histidines and one histidine anion which in turn is bound to a zinc atom. 

The enzyme removes the toxic superoxide free radical by the following pathway [CuZn]n+ + 02 - [CuZn](n 1)+ + 02 

A closer look at the catalytic cycle, as shown in Fig. 4.12, shows how zinc plays a largely structural role while copper continually changes oxidation state and coordination environment. It should be immediately apparent that one of the by-products is the equally toxic reactive species, hydrogen peroxide. Fortunately a second enzyme, catalase, is present to scavenge the peroxide and convert it to oxygen and water. 

Catalase is a haem-type enzyme containing an iron atom in a porphyrin ring. A two step reaction removes the dangerous reactive oxygen species through the oxidation and subsequent reduction of the central iron atom  

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