Energy enzymatic system

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

The reactions of polyhydric alcohols with the hydroxyl radical in aqueous solution have been extensively studied (e.g. in radiolytic and biomimetic systems), mainly because of their suitability as models for more complicated carbohydrate substrates [55] or enzymatic systems involving glycol-type radicals [56, 57]. Because there are no double bonds to which OH could add, only H-abstraction reactions are possible. Because the C-H bond energy is significantly lower than the 0-H bond energy, it is the carbons from which H are abstracted and not the alcohol function. In this type of reaction, a,yS-dihydroxyalkyl radicals are formed. The same radicals could, in principle, be produced by addition of "OH to enols, see Scheme 2, lower part. This shows the complementarity of H-abstraction and OH-addition and thereby the relevance of the former to one-electron oxidation of olefinic bonds (Scheme 2). 

The ammonia-oxidizing bacteria biosynthesizes the cellular materials from carbon dioxide. For this purpose, they need NAD(P)H. Electrons to reduce NAD(P)+ seem to come from ferrocytochrome c-552 by the supply of energy, because Aleem (1966) reported that he had demonstrated that NAD(P)+ was anaerobically reduced with horse ferrocytochrome c on addition of ATP using the cell-free extracts of N. europaea, though the enzymatic system participating in the reduction of NAD(P)+ has not been known. However, every attempt by the author and his colleagues to reproduce his results has been unsuccessful to date. 

An important technical issue is the large-scale applicability of co-factor-dependent enzymatic systems. It is generally accepted that, e.g., NADH-requiring oxidoreductases can easily be used in whole-cell biocatalysis such as baker s yeast-mediated reductions, where the cofactor recycling step is simultaneously performed within the intact cell, driven by the reduction equivalents introduced via the external carbon and energy source (glucose). 

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