Redox systems biochemical, table

Table 12 shows redox properties of some redox systems of biochemical nature. Generally, the redox potentials are modest, cytochrome P450 possibly being an exception. If cytochrome P450 functions as an electron transfer oxidant towards xenobiotic molecules, it is necessary to postulate a considerably higher potential (1.3-1.8V) from considerations of the Marcus theory (Eberson, 1990). 

Another property that distinguishes various cytochromes is the redox potential E ° (Table 6-8), which in this discussion is given for pH 7.0. Cytochromes carry electrons between other oxidoreductase proteins of widely varying values of E°. Because of the various heme environments cytochromes have greatly differing values of E°, allowing them to function in many different biochemical systems. 97a/97b For mitochondrial cytochrome c the value of E ° is + 0.265 V but for the closely related cytochrome/of chloroplasts it is +0.365 V and for cytochrome c3 of Desulfovibrio about -0.330 V. There is more than an 0.6-volt difference between E ° ... 

Some quantities valid for reactions modelling different chemical and biochemical processes in waters are shown in Table 3.14. Redox potentials of some biologically important systems are presented in Table 3.15. 

The third section of the periodic table comprises transition metals or the d-group elements, which are involved in many biological defence mechanisms against toxic radicals. As in chemical catalysis the same metals are active in biochemical systems. Surprisingly these redox mediators are involved in both, the generation and the removal of reactive oxygen species. 

From table 3, it can be seen that for the actinides in the first half of the transition series a multiplicity of valence states are possible, whilst those in the second half have a more restricted number of valence states available and have more in common with the lanthanide elements. This multiplicity of oxidation states can lead to some extremely complicated solution chemistry, but, fortunately, all of the actinides have one oxidation state which is dominant under fairly well-defined solution conditions. As a result, actinide redox behaviour is understood to a reasonable extent under physiological conditions although there are exceptions as will be discussed. It is, however, worthwhile discussing, briefly, general actinide reduction-oxidation behaviour because valence states which dominate in the environment and which impinge on biochemical/biologi-cal systems may not dominate under physiological conditions. 

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