Manganese complexes enzyme model complex

Dinuclear Manganese Complexes as Models for the Manganese Catalase. As discussed previously, the manganese catalase has a dinuclear active site that is thought to function by cycling redox states between Mn(II)2 and Mn(III)2. Although the [Mn(IV)(salpn)(/z2-0)]2 chemistry nicely explains the alternate catalase reactions of the OEC, this system is an inappropriate model for the Mn catalase because the redox cycle in that enzyme is lower and the core structure is believed to be dramatically different. In fact, a [Mn(III/IV)(/z2-0)]2 superoxidized state of the Mn catalase has been identified and shown to be inactive. 

WH Armstrong (1991) Polynuclear manganese complexes as models for the photosystem II water oxidation catalyst In VL Pecararo (ed) Manganese Redox Enzymes, pp 261-286. VCH PubI... 

Manganese complexes as models for manganese-containing pseudocatalase enzymes Synthesis, structural and catalytic activity studies... 

An early model for the active site was found in the manganese salen class of compounds. The active sites of the enzyme and model are shown in Fig. 4.13. The Schiff base model complexes are unusual in that they are able to maintain manganese in the highly reactive +3 oxidation state, ready to catalyse the oxidation of superoxide, yet still be flexible enough to bind the larger manganese(II) ion. 

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

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 following segment on the reactivity of manganese complexes will be broken into two sections. The first will cover some of the areas that compose research into the reactivity of manganese-based systems as applied to a specific synthetic organic transformation. The second segment will concentrate on systems designed specifically to model the reactivity of particular enzyme systems. 

A series of multinuclear manganese complexes have been studied as models for the enzyme Photosystem 11 (PSII) which converts H2O to O2 (equation 21.53). 

T. thermophilus MnSOD, and the data showed that there were three phases to this reaction a fast burst phase of quick HO2 dismutation, a slower second phase, and a final fast phase (78). A dead-end form of the enzyme was implicated to account for the slow phase (78). This has been formulated as a side-on-bound Mn -peroxo species based on spectroscopic similarities to manganese model complexes with side-on-bound peroxy groups (83). Such phases are not observed for FeSOD (84). 

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