Catalase azide complex

Crystal structures of manganese catalases (in the (111)2 oxidation state) from Lactobacillus plantarum,its azide-inhibited complex, " and from Thermus thermophilus have been determined. There are differences between the structures that may reflect distinct biological functions for the two enzymes, the L. plantarum enzyme functions only as a catalase, while the T. thermo-philus enzyme may function as a catalase/peroxidase. The active sites are conserved in the two enzymes and are shown schematically in Figure 32. Each subunit contains an Mu2 active site,... 

The inhibition of catalase by cyanide shows none of the characteristics of the azide or hydroxylamine inhibition as is to be expected if cyanide combines with the ferric form. At low peroxide concentrations about 10-3 M the equilibrium constant for the formation of the cyan-catalase complex (Ki) determined from kinetic data using the expression... 

Catalase was found to form an intermediate compound in the presence of hydrogen peroxide (Chance, 69). The spectrum was measured from 380-430 nqi and is slightly shifted toward the visible as compared with free catalase. The complex shows no similarities to cyan-catalase or the compound formed when peroxide is added to azide catalase. Its formation is very rapid, the bimolecular velocity constant having a value of about 3 X 107 M.-1 sec.-1. In the absence of added hydrogen donors, the complex decomposes slowly according to a first order reaction with a velocity constant of about 0.02 sec.-1. This catalase complex thus resembles the green primary hydrogen peroxide complex of peroxidase. 

Complex 52 is similar to Mn-catalase in that it is azide insensitive and forms a MnnMnm species on addition of hydroxylamine and a MnIIIMnIV catalyti-cally inactive form with an EPR spectrum closely resembling that of the enzyme. Importantly, the complex maintains its dinuclear structure in solution, while cycling between the MnnMnn, MnmMnm oxidation states and shows a good catalytic rate (kcat = 13 1 sec-1) and stability (>1000 turnovers). Catalases, however, are approximately 3000 times more efficient. 

The bacterium Lactobacillus plantarum and its closest allies are unusual in that they are aerobic organisms but do not produce a superoxide dismutase. This bacterium instead accumulates Mn(II) to an intramolecular level on the order of 25 mM (150-152). In vitro studies indicated that Mn(II) formed a complex with lactate which possessed significant superoxide activity (153). These bacteria are additionally unable to produce heme and, consequently, when grown in the absence of heme, produce a hemeless catalase, or pseudocatalase (154-158). Unlike heme-containing catalases, the enzyme is not inhibited by cyanide or azide, and the addition of either Mn or Fe into the growth medium increased the amount of the pseudocatalase present. However, neither of the metals could be detected in partially purified enzyme assays (157). 

Catalase reacts reversibly with some weak acids forming spectroscopically and magnetically distinct noncovalent derivatives. Of these, catalase-cyanide, -azide, -fluoride, -formate, and -acetate complexes have been extensively studied (37, 135, 136) and reviewed in some detail (16-18). Briefly, there is a consensus that such reactions do not involve heme-heme interaction and, with the possible exception of carboxylate ligands (102), all presumably result in replacement of the proximal aquo ligand at Ls in a stoichiometric reaction shown in Eq. (11) ... 

Inhibition of Catalase Activity of Azide, Hydroxylamine, and Cyanide, the Relative Affinity of These Substances for the Enzyme and the Corresponding Dissociation Constants for the Complexes... 

In a more general context of hemoproteins some further studies appear worth mentioning. A coral allene oxide synthase has been characterized which employs a heme in the conversion of 8R-hydroperoxyeicosatetraenoic acid into the corresponding allene oxide. EPR of the ferric enzyme and its cyanide and azide complexes strongly suggested tyrosinate ligation, as in catalase, but the access of small molecules to the heme as well as the interaction with the protein environ-... 

Reactions of hemoglobin with cyanide and the magnetic properties of the cyanide complex. Equilibrium constant of the reaction. Reaction of cyanide with catalase and peroxidase. Magnetic properties of products. Reaction of hemoglobin with fluoride and its relation to the reaction with hydroxyl ion. Equilibrium constants for reactions of fluoride with cytochrome c, catalase, and peroxidase. Magnetic effects associated with these reactions. Velocity of reaction of ferrihemoglobin with hydro-sulfide. Magnetic properties of product. Reactions with azide. Reactions of hydroxyl ion with various heme proteins. Reconsideration of all the results in connection with possibility of interactions between hemes. 

Tryptophan pyrrolase is claimed to be an iron-porphyrin protein which is reduced to its active ferrous form by peroxide. This activation may be analogous to the reduction of methemoglobin and of catalase-azide complex of the oxygenated ferrous forms by peroxide (Keilin and Hartree, 1950 and 1954). 

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