Catalase coordination positions

As in myoglobin, hemoglobin (Fig. 7-23), and cytochrome c (see Fig 16-8), one axial coordination position on the iron of most heme proteins (customarily called the proximal position) is occupied by an imidazole group of a histidine side chain. However, in cytochrome P450 and chloroperoxidase a thiolate (-S ) group from a cysteinyl side chain, and in catalase a phenolate anion from a tyrosyl side chain, occupies the proximal position. The sixth or distal coordination position is occupied by the sulfur atom of methionine in cytochrome c and most other cytochromes with low-spin iron but cytochromes b5 and c3 have histidine. The high-spin heme proteins, such as cytochromes c, ... 

Catalase simulator PPFe0H/Al203 possesses hydroxide in the fifth coordination position of the ion. In this form, the catalyst displays high catalase and peroxidase activity (see below). 

Tests on synthesized biomimics indicated their high catalase activity (specifically for these applied on A1202) in H202 dissociation. It is the author s opinion that these studies gave essentially important results for explaining the Chance complex formation. It consists of bonding of Fe3+ ion in the sixth coordinate position in the biomimic to hydroxide anion. In this very form it manifests catalase activity. 

The question of whether the sixth coordinate position of Fe3+ ion in native catalase is open (i.e. free) or is occupied by hydroxide is partly resolved in these studies. On all occasions, the possibility of Fe3+ hydroxo-form realization in the active site of catalase is confirmed, and this fact may not be neglected. 

Catalase clearly remains high-spin to a more marked degree than any other haem-protein, for example the cyanide is about 50% high-spin, and this observation too is in keeping with a weak ligand field group in the fifth coordination position. 

It is well know that copper(II) labile complexes are most widely spread in nature as catalysts of oxidation-reduction reactions and in this sense probably they take second place after iron complexes. On the other hand, however, the copper chelate complexes do not act as catalysts of oxidation, but very often are among the most effective inhibitors of the oxidation reactions. The differences in their catalytic properties may be explained by the fact that all four coordination positions of the central ion are occupied by the chelate ligands. This was demonstrated by Siegel(l) by studying the catalase and peroxidase action of some copper chelate complexes (see scheme 1 in the next page). It was established that the above rate constants sharply decrease in the order I>>II>III of the following scheme and almost reach zero on coordination of the four ligand atoms. 

However it cannot be assumed that formation of a chelated metal complex is an essential feature of biological catalysis involving trace metals for such important substances as vitamin B12, hemoglobin, catalases, and the cytochromes have a 6-coordinate metal (Fe or Co) bound with four links to a porphyrin system, with one more to a proteinlike structure (e.g., the imidazole in Bw) and have only one valence position... 

The copolymer-bounded Fe(III)-Pc dispersed in water is the five-coordinate high-spin type similar to catalase and peroxidase, as evidenced by the ESR signals with g = 4.27. One coordination site at the axial position of Fe(III)-Pc ring is occupied by the nitrogen of the pyridine groups. High-... 

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