Eukaryotes, catalase

Enzymes with an intermittent role may be much more important than we thought in 1963. This was perhaps first clearly emphasized by Deis-seroth and Bounce in their catalase review of 1970 (25). These authors also pointed out the likelihood that the specific location of most eukaryotic catalase in the peroxisomes represents a functional response to the need to decompose hydrogen peroxide generated by the aerobic oxidases present in these same organelles, including hydroxy-acid oxidases and D-amino-acid oxidases. 

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. 

Protection from unwanted side products of oxygen reactions uses the cytoplasmic Cu/Zn superoxide dismutase and vesicular haem catalases as in all eukaryotes as... 

On the other hand, several ROS are highly cytotoxic. Consequently, eukaryotic cells have developed an elaborate arsenal of antioxidant mechanisms to neutrahze their deleterious effects (enzymes such as superoxide dismutases, catalases, glutathione peroxidases, thioredoxin inhibitors of free-radical chain reaction such as tocopherol, carotenoids, ascorbic acid chelating proteins such as lactoferrin and transferrin). It can be postulated that ROS may induce an oxidative stress leading to cell death when the level of intracellular ROS exceeds an undefined threshold. Indeed, numerous observations have shown that ROS are mediators of cell death, particularly apoptosis (Maziere et al., 2000 Girotti, 1998 Kinscherf et al., 1998 Suzuki et al., 1997 Buttke and Sanstrom, 1994 Albina et al., 1993). 

Regulation of catalase expression in eukaryotes takes place as part of a generalized response mechanism. In yeast, promoter elements of the peroxisomal catalase CTA-1 respond to glucose repression and activation by fatty acids as part of organelle synthesis. The cytosolic catalase CTT-1 responds as part of a generalized stress response to starvation, heat, high osmolarity, and H2O2, and there is even evidence of translational control mediated by heme availability 26). 

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. 

Follow-up experiments with the similar Klebsiella pneumoniae enzyme (66, 67) also showed that the pH profiles for CatPx are quite different from those for classical catalases. The latter s catalatic activities are essentially pH-independent from pH 5 to 10 (6S) CatPx of Klebsiella showed a sharp pH optimum between pH 6 and 7 (66). A similar eukaryotic fungal CatPx (69) was also characterized by a sharp pH sensitivity and saturation kinetics K 3.4 mM) and, like the bac-... 

Mukherjee M, Sievers SA, Brown MT, Johnson PJ (2006b) Identification and biochemical characterization of serine hydroxymethyl transferase in the hydrogenosome of Trichomonas vaginalis. Eukaryot Cell 5 2072-2078 Muller M (1973) Biochemical cytology of trichomonad flagellates. I. Subcellular localization of hydrolases, dehydrogenases, and catalase in Tritrichomonas foetus. J Cell Biol 57 453-474... 

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)). ... 

Cafalase from mosf eukaryotic species is tetra-meric. The protein from beef liver consists of 506-residue subrmifs. Human catalase is similar. ... 

A relation between oxidative stress and carotenogenesis was further supported by study of P. rhodozyma s defenses against 02 and H2O2. Eukaryotic cells are known to possess various detoxification systems for 02% H2O2, and peroxyl radicals including superoxide dismutase (SOD), catalase, and glutathione peroxidase. SOD... 

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