Superoxide dismutase biological function

Superoxide dismutase enzymes are functional dimers of molecular weight (Mr) of approximately 32 kDa. The enzymes contain one copper ion and one zinc ion per subunit. Superoxide dismutase (SOD) metalloenzymes function to disproportionate the biologically harmful superoxide ion-radical according to the following reaction ... 

Metals in biological systems function in a number of different ways. Group 1 and 2 metals operate as structural elements or in the maintenance of charge and osmotic balance (Table 1.2). Transition metal ions that exist in single oxidation states, such as zinc(II), function as structural elements in superoxide dismutase and zinc fingers, or, as an example from main group +2 ions, as triggers for protein activity—that is, calcium ions in calmodulin or troponin C... 

Fee, J. A. On the question of superoxide toxicity and the biological function of superoxide dismutases. In Oxidases and Related Redox Systems (King, T. E., Mason, H. S., Morrison, M., eds.), Oxford-New York, Pergamon Press, 1981... 

The most common metal encountered in electron transfer systems is iron, although copper and manganese play vital functions. Merely to emphasise the complexity of the catalysts that are used in biology, the structures of the active sites of ascorbate oxidase (Fig. 10-11) and superoxide dismutase (Fig. 10-12) are presented. It is clear that we have only just begun to understand the exact ways in which metal ions are used to control the reactivity of small molecules in biological systems. 

Cells have substantial chemical defenses against the UV photoproducts produced in seawater and intracellular fluids. Many organisms have antioxidants (e.g., carotenoids, ascorbate, tocopherols, anthocyanins, and tridentatols) that quench photo-oxidative reactions.64-67 Cells also have enzymes (e.g., catalase and superoxide dismutase) that can counteract the oxidative nature of peroxides and other radicals.26 Some compounds, such as the UV-absorbing pigment melanin, can act as both optical filter and antioxidant.68 The MAA mycosporine-glycine (Figure 15.3) functions in a similar dual capacity.69 The role of UV-mediated reactions in seawater relative to biological effects is an important current area of study. 

The biological transformations of -NO described above are all oxidative. Although chemical studies have shown that reduction of -NO to form nitroxyl (HNO) can occur via some biochemical mechanisms (e.g. superoxide dismutase (SOD), NO synthase, mitochondrial metabolism, and reaction of nitrosothiols with thiols ), httle evidence has been presented to suggest that functionally relevant quantities of nitroxyl are produced in vivo. ... 

Metalloporphyrins, characterized by a redox-active transitional metal coordinated to a cyclic porphyrin core ligand, mitigate oxidative/nitrosative stress in biological systems. Side-chain substitutions tune redox properties of metalloporphyrins to act as potent superoxide dismutase mimetics, peroxynitrite decomposition catalysts, and redox regulators of transcription factor function. Metalloporphyrins are efficacious in AD models [538],... 

Manganese is tlie third most abundant transition element 1]. It is present in a number of industrial, biological, and environmental systems, representative examples of which include manganese oxide batteries [2] the oxygen-evolving center of photosystem II (PSII) [3] manganese catalase, peroxidase, superoxide dismutase (SOD), and other enzymes [4, 5] chiral epoxidation catalysts [6] and deep ocean nodules [7]. Oxidation-reduction chemistry plays a central role in the function of most, if not all, of these examples. 

A likely biological function for the superoxide dismutase proteins (SOD) is to remove Oa -, and thereby preclude formation of HOO- [Eq. (5-18)] and prevent initiation of lipid peroxidation and autoxidation (Scheme 5-2). An SOD model... 

This mechanistic proposal in turn prompts the suggestion that the function of the superoxide dismutase proteins is to prevent free HOO- from coming into contact with allylic C-H bonds in the biological matrix. One approach is to minimize the lifetime of O2 -/HOO-, which is in addition to the radical-radical coupling proposition to deactivate HOO-. For a steady-state flux of 30 X 10 M O2 -/HOO- at pH 5 (1) without superoxide dismutase (SOD) the approximate half-life of O2 -/HOO- is about 30 ms... 

A second problem arises for the protection to the reactive intermediates of the oxygen reduction pathway. A perfect protection should be achieved if oxidations with 02 in biological systems would be activated by biocatalysts which completely avoid any formation of free intermediate species. Though many enzymes catalyzing 02-reduction, like cytochromeoxidase (see 6.4.), seem to act in this manner, a complete suppression of free superoxide radical, Oj, and peroxide formation could not be attained. Thus, nature has evolved special enzyme systems acting as scavengers which catalyze rapid disproportion. There exist two types of enzymes superoxide-dismutases 183,184) an(j catalases185,186 Their catalytic function is described by ... 

McCord, J. M. and Fridovich, 1. Superoxide dismutase. An enzymic function for erythrocuprein (hemocuprein). Journal of Biological Chemistry 244 6049-6055 1969. 

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