Manganese-Proteins and -Enzymes and Relevant Trinuclear Synthetic Complexes

Abstract

Manganese is an essential element in many biological processes. Two functional values can be distinguished; the Mn²⁺ as a Lewis acid, like divalent ions, Magnesium, Calcium, Zinc and in higher oxidation states(Mn³⁺, Mn⁴⁺) as an oxidation catalyst, like Copper, Iron, Cobalt. The most well known Mn(II) proteins are: Arginase, containing 4 Mn(II) ions per enzyme; Concanavalin A, a manganese-calcium metalloprotein; Glutamine-Synthatase, requiring two Mn(II) ions; Phosphoenolpyruvate Carboxykinase, converting irreversible cytoplasmic oxaloacetate to phosphoenolpyruvate; a manganese ribonucleotide reductase isolated from B. ammoniagemes; Mn Thiosulfate Oxidase containing a binuclear Mn(II) site; Isopropylmalate Synthase, with the Mn(II) bound to the S-H group near site; Pyruvate carboxylase, the first metalloenzyme shown to contain manganese. Manganese redox enzymes with manganese in oxidation states 3+ and 4+ are: Manganese SOD catalysing the dismutasion of superoxide radicals to oxygen and hydrogen peroxide with a single Mn(III) center; The Manganese Peroxidase(MnP) is one of the two known enzymes capable for the oxidative degradation of lignin containing protoporphyrin IX heme prosthetic group; non heme manganese catalase containing two manganese per subunit and the Oxygen Evolving Complex, catalysing one of the most important reactions occurring in the plants, the light driven oxidation of water to oxygen and protons, containing four manganese atoms while the presence of calcium and chloride ions is required for proper functioning. Open structure mixed valence trinuclear complexes with the formula Mn(II)/Mn(III)₂(Schiff-base)₂(OOCR)₄(L)₂ were prepared and structurally characterised in an attempt to mimic the active site of OEC.