Oxygen biological function

FIGURE 9.18 The variation of the extent of saturation of myoglobin (Mb) and hemoglobin (Hbl with the partial pressure of oxygen. The different shapes of the curves account for the different biological functions of the two proteins. 

But there is more to water than its role as solvent, since, like oxygen, it is also a reagent (as well as a product, but this aspect is irrelevant here) in innumerable biochemical reactions. This is the biochemical role of water, a double one, in fact. First and foremost, water is a reagent in innumerable lifesupporting reactions. Secondly, and like for oxygen, evolution has recruited water as a major player in the detoxification and elimination of xenobiotics. This biological function of water, we believe, is presented here for the first time in its full depth and breadth. 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

Clearly, the short half-life of nitric oxide is an important determinant for its biological function, but the chemical basis for this short half-life is still unknown. It cannot be due to the most commonly accepted mechanism reaction with oxygen to form nitrogen dioxide. 

The H202 formed may be reduced by catalase, peroxidase or suitable reducing agents. SOD has been identified in many animal and bacterial cells its biological function is to protect tissue against oxygen free radicals in anaerobic systems (reviewed by Farkye, 1992). 

The target of singlet oxygen in vivo has been the subject of a huge number of studies as it has the potential to change chemical reactivity, and hence biological function, at many sites. Unsaturated fatty acids, for example, are excellent targets [316] and peroxidation of cell membranes is probably... 

Fig. 2. Periodic table indicating relative abundance and biological importance of elements. Highlighted elements are the 30 most abundant crustal elements with concentrations ranging from 46 % for oxygen to 0.1 yg/g for selenium. Bold faced elements have known biological functions with indicating limited data and controversy. Data taken from Wood and Wang, 1983.

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