Prokaryotes, catalase

These are produced by both prokaryotes and eukaryotes, and catalyze a number of important reactions. They are flavoproteins that produce potentially destructive H2O2 that is removed by the activity of catalase or peroxidase. The reactions are formally outlined in Figures 3.31a through c. 

Several catalases, including the type B catalase-peroxidases, seem to show true substrate saturation at much lower levels of peroxide than originally observed for the mammalian enzyme (in the range of a few millimolar). This means that the limiting maximal turnover is less and the lifetime of the putative Michaelis-Menten intermediate (with the redox equivalent of two molecules of peroxide bound) is much longer. The extended scheme for catalase in Fig. 2B shows that relationships between free enzyme and compound I, and the presumed rate-limiting ternary complex with least stability or fastest decay in eukaryotic enzymes of type A and greatest stability or slowest decay in prokaryotic type B enzymes. 

Each subunit of a heme catalase binds a single molecule of heme and some mammalian catalases also possess a second cofactor, NADPH. The binding of NADPH in catalases was at first totally unexpected, but has since been a frequent feature of small-subunit catalases from both prokaryotic and eukaryotic organisms. However, the actual biochemical function of NADPH in these catalases is still not fully understood. One possible role is protection of the enzyme against inactivation by its own substrate, especially under conditions of low-peroxide concentrations. The NADPH binding pocket is located on the molecular surface, just above an entrance chaimel with the nicotinamide active carbon situated approximately 20 A from the closest heme atom (Figure ll(a)). ... 

Catalase, the enzyme which catalyzes the dismutation of hydrogen peroxide to molecular oxygen and water, is contained in both eukaryotes and prokaryotes. Most catalases isolated so far have common properties with respect to subunit composition, absorption spectra, pH dependence of the catalatic activity and inhibition by inhibitors. Recently catalases from a few bacteria, such as Rhodobacter capsulatus [1], Escherichia coli [2] and Comamomas compransoris [5], have shown to have different properties from those of typical catalases. 

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