John B. Vincent

Higher Oxidation State Manganese Biomolecules

Publisher Summary This chapter discusses the nature of higher oxidation manganese (Mn) biomolecules and the efforts directed toward the synthesis of satisfactory inorganic models. With the exception of acid phosphatase and transferrin, all the biological systems discussed are involved in the same basic function, viz interconversion of oxygen among its various oxidation states. Thus, these Mn enzymes are involved in superoxide dismutation to O2 and O22− (a one-electron process), peroxide disproportionation to O2 and H2O (a two-electron process), and water oxidation to O2 (a four-electron process). These systems employ mononuclear, dinuclear, and tetranuclear sites, respectively. The chapter describes the structural interrelationship between the superoxide dismutase (SOD) and proposed catalase Mn sites, and the catalase and proposed WOC Mn sites. The hemerythrin-like Mn2O(O2CR)2 site bears striking resemblance to the result of fusing two mononuclear SOD Mn sites. The chapter argues that the need for a system readily capable of sustaining two-electron processes have resulted in the evolutionary merging of two one-electron systems. A similar argument can be made for the mononuclear site of Fe SOD (isostructural to Mn SOD) and hemerythrin.

A molecular ‘double-pivot’ mechanism for water oxidation

the arrangement of the metal atoms, their ligation or their precise mode of action. A number of mechanistic proposals have been presented e.g. [l-4], but none have been able to satisfactorily account for all the available EXAFS [5,6], EPR [7,8] and W-Vis data [9, lo]. Recent reviews of this area are available [ 11, 121. The photosynthetic Mn center is capable of cycling between five distinct oxidation levels, labelled So to Sq, with oxygen evolution occurring during the S4 + S,-, transition (eqn. (2)) [ 131. It is

The molecular ‘double-pivot’ mechanism for water oxidation

the arrangement of the metal atoms, their ligation or their precise mode of action. A number of mechanistic proposals have been presented e.g. [l-4], but none have been able to satisfactorily account for all the available EXAFS [5,6], EPR [7,8] and W-Vis data [9, lo]. Recent reviews of this area are available [ 11, 121. The photosynthetic Mn center is capable of cycling between five distinct oxidation levels, labelled So to Sq, with oxygen evolution occurring during the S4 + S,-, transition (eqn. (2)) [ 131. It is

Model complexes suggest certain S-state changes of the photosynthetic water-oxidation enzyme may involve an Mn(II)–Mn(III) transition

The ultraviolet‐visible absorbance differences spectra of Mn(II,III) and Mn(III,III) oxo‐bridged carboxylate complexes are reported. The difference spectra are remarkably similar to those of the photosynthetic water‐oxidation enzyme complex reported by Dekker et al. [(1984) Biochim. Biophys. Acta 764, 301–309] which were interpreted as being due exclusively to Mn(II→IV) transitions. This result indicates that certain S‐state changes of the enzyme complex may instead involve Mn(II→III) transitions, and that difference spectra alone cannot be used with confidence to assign the Mn oxidation state changes during water oxidation.