Nonheme catalases

The diversity among catalases, evident in the variety of subunit sizes, the number of quaternary structures, the different heme prosthetic groups, and the variety of sequence groups, enables them to be organized in four main groups the classic monofunctional enzymes (type A), the catalase-peroxidases (type B), the nonheme catalases (type C), and miscellaneous proteins with minor catalatic activities (type D). 

The nonheme catalases are sometimes referred to as pseudocatalases. However, this is inappropriate since there is nothing pseudo about their catalase activity. They are better described as Mn catalases. Bacteria that lack Mn catalase (either genetically or through inactivation of the enzyme) show decreased viability, suggesting that Mn catalase does, in fact, play a physiological role in the detoxification of H2O2. 

Rochat, T, Gratadoux, J.J., Gruss, A.,et al. (2006). Production of a heterologous nonheme catalase by Lactobacillus casei an efficient tool for removal of H202 and protection of Lactobacillus bulgaricus from oxidative stress in milk. Appl Environ Microbiol 72, 5143-5149. 

Bacterial SODs typically contain either nonheme iron (FeSODs) or manganese (MnSODs) at their active sites, although bacterial copper/zinc and nickel SODs are also known (Imlay and Imlay 1996 Chung et al. 1999). Catalases are usually heme-containing enzymes that catalyze disproportionation of hydrogen peroxide to water and molecular oxygen (Eq. 10.2) (Zamocky and Koller 1999 Loewen et al. 2000). 

On the basis of the structure of their active site, catalases may be classified as heme or nonheme enzymes. Those that contain heme iron are efficient catalysts, operating close to the diffusion limit, 108M 1sec 1. In iron catalases the metal is coordinated by four heme nitrogens and a proximal Tyr residue, which occupies the fifth coordination site. A catalytically required His is found on the distal side of the heme. In addition, a water molecule has also been observed close to the iron sixth coordination site. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Nitrones, formation of, 153 p-Nitrophenylacetate, glyceraldehyde-3-phosphate dehydrogenase and, 21,45 Nitrous acid, catalase and, 388, 398 Nitrous oxide, catalase and, 400 Nonheme iron, see also Iron adenylyl sulfate reductase and, 282,... 

The catalases catalyze the disproportionation of hydrogen peroxide (equations). Most catalases contain the iron-protoporphyrin IX prosthetic group see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, some bacteria are able to synthesize catalases that are not inhibited even by millimolar concentrations of azide and cyanide, suggesting that some catalases are nonheme enzymes it is now known that these enzymes possess a dinuclear Mn active site. 

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