Manganese histidine complexes

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

S Padhye, T Kambara, DN Hendrickson and Govindjee (1986) Manganese-histidine clusters as the functional center of the water oxidation complex in photosynthesis. Photosynthesis Res 9 103-112RD... 

Figure 6-6 Logarithms of formation constants of metal complexes of histidine, glycine, and ATP plotted against the atomic number of elements from manganese to zinc.

Fig. 7. (A) The oxidation states of Mn in the various S-states. The model incorporates a histidine radical formation in the Sj-state with no oxidation of Mn in the Sj- Sa transition the model also accommodates the 1 0 1 2 proton-release pattern in the Kok cycle (B) a proposed topological model for the photosynthetic water-oxidizing Mn-complex based on XAS and EPR studies. Figure source (A) [adapted] and (B) Sauer, Yachandra, Britt and Klein (1992) The photosynthetic water oxidation complex studied by EPR and X-ray absorption spectroscopy. In VL Pecararo (ed) Manganese Redox Enzymes, pp 141-175. VCH Publ.

The chemical shift of the phosphorus resonance of various nucleotides has been studied as a function of pH in the presence and absence of RNase A. The signal shifts upheld on protonation of the phosphate and the apparent pATa of the phosphate group in 2 -CMP complex with RNase is the same as the pATa of histidine-119 in this enzyme as determined by n.m.r. From n.m.r. relaxation rates for the ternary complex manganese(n)-phosphate-E. coli alkaline phosphatase, it has been concluded that an outer-sphere complex is formed which has a shorter lifetime than the enzyme turnover rate. The latter conclusion is consistent with the participation of the complex in the enzymic reaction. 

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