Manganese catalase enzymes

Figure 10.1). EPR and UV-Vis spectroscopic investigations revealed that under conditions of H2O2 decomposition, both Mn "-Mn" and Mn -Mn" oxidation states are present, similar to those observed for the natural manganese catalase enzymes [28]. 

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,... 

Manganese is the third most abundant transition element [1]. It is present in a number of industrial, hiological, and environmental systems, representative examples of which include manganese oxide batteries [2] the oxygen-evolving center of photosystem II (PSII) [3] manganese catalase, peroxidase, superoxide dismutase (SOD), and other enzymes [4, 5] chiral epoxidation catalysts [6] and deep ocean nodules [7]. Oxidation-reduction chemistry plays a central role in the function of most, if not all, of these examples. 

This process is catalyzed by a variety of catalase enzymes, the most common being the heme catalases, which accomplish the two-electron chemistry of Eq. (12) at a mononuclear heme center. Here, both the iron and its surrounding porphyrin ligand participate to the extent of one electron each in the redox process. Manganese catalases contain a binuclear Mn center and cycle between Mn2(II,II) and Mn2(III,III) oxidation states while carrying out the disproportionation of H2O2. The enzyme can... 

In general, as can be seen from the actual parameters for dihydrogen peroxide decomposition (Table 3), manganese catalases are less active than their heme iron counterparts. Catalytic rates comparable to these are a target for functional models of the Mn enzyme. 

The importance of reactions 1-3 in the biosphere is clear. However, relatively little is known about the catalytic mechanisms of these reactions, particularly reactions 2 and 3. In order to better understand the catalytic mechanisms of these enzymes, it is important to establish the correlation between metal site structure and enzymatic function. X-ray absorption spectroscopy is one of the premier tools for determining the local structural environment of metalloprotein metal sites. In the following, we summarize our results using X-ray absorption spectroscopy to characterize the structure of the Mn active site environments in manganese catalase and in the OEC and show how these structural results can be used to deduce details of the catalytic mechanism of these enzymes. 

Considering all of the manganese catalases together, there have been four cluster oxidation levels that are established Mn(II,II), Mn(II,III), Mn(III,III), and Mn(III,IV). The as-isolated enzyme contains a mixture of these states. The Mn(II,II) enzyme can be prepared by the addition of hydroxylamine to the isolated enzyme. If hydrogen peroxide is added to this sample, without removing the hydroxylamine, the enzyme is converted to the Mn(III,IV) form however, if the hydroxylamine is first... 

Although manganese catalases have often been referred to as azide insensitive/ these enzymes actually are inhibited by azide and related molecules albeit at higher concentrations than are necessary for the heme enzymes. Penner-Hahn and co-workers (22) have shown that HN3 is the likely protonation state of the inhibitor and have calculated an apparent of 80 mM. Slope replots of the pH dependence of azide inhibition are linear with a slope of 1. These data can be used to calculate a true K of 300 mM. Because azide is a competitive inhibitor with respect to peroxide, it is likely that azide is bound directly to the manganese center. Recent EPR and lH paramagnetic relaxation enhancement studies support this viewpoint. Other inhibitors include fluoride and thiocyanide. All of the reported inhibition studies are consistent with the catalase cycle and hydroxylamine inhibition of the catalase cycle. 

Dinuclear Manganese Complexes as Models for the Manganese Catalase. As discussed previously, the manganese catalase has a dinuclear active site that is thought to function by cycling redox states between Mn(II)2 and Mn(III)2. Although the [Mn(IV)(salpn)(/z2-0)]2 chemistry nicely explains the alternate catalase reactions of the OEC, this system is an inappropriate model for the Mn catalase because the redox cycle in that enzyme is lower and the core structure is believed to be dramatically different. In fact, a [Mn(III/IV)(/z2-0)]2 superoxidized state of the Mn catalase has been identified and shown to be inactive. 

Improved purification procedures were subsequently developed for the manganese catalase (164). While the enzyme from this preparation... 

Manganese is an element that is essential for life. It is present at the active site of many enzymes [4, 5]. Those enzymes in which the metal center is involved in a redox process are manganese catalase [101], peroxidase [102], and SOD [103]. In addition, a cluster containing four Mn and one Ca atoms in the water-oxidizing center (WOC) of PSII is the site at which dioxygen is produced photosynthehcally on Earth [3,104]. 

Manganese catalases, sometimes referred to as pseudocatalases, are found in lactic acid bacteria and in thermophihc bacteria. The molecular weight of these enzymes ranges from 170 to 210 kDa. They may form unusual ohgomeric structures like homopentamers and homohexamers. Unhke heme catalases, they are not inhibited by CN or Nj [ 197 ]. 

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