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However, catalase deficiency in mice may increase the likelihood of developing obesity, fatty liver, [26] and type 2 diabetes. [27] Some humans have very low levels of catalase , yet show few ill effects. The increased oxidative stress that occurs with aging in mice is alleviated by over-expression of catalase. [28]
Like all catalysts, enzymes increase the reaction rate by lowering its activation energy. Some enzymes can make their conversion of substrate to product occur many millions of times faster. An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds.
Enzyme catalysis is the increase in the rate of a process by an "enzyme", a biological molecule. Most enzymes are proteins, and most such processes are chemical reactions. Within the enzyme, generally catalysis occurs at a localized site, called the active site.
Additionally, enzymes including superoxide dismutase, catalase, and peroxidase contribute to the oxidation response by reducing the presence of hydrogen peroxide, which is a prevalent precursor of the hydroxyl radical (OH•).
Several costs are associated with this metabolic pathway; the production of hydrogen peroxide in the peroxisome (associated with the conversion of glycolate to glyoxylate). Hydrogen peroxide is a dangerously strong oxidant which must be immediately split into water and oxygen by the enzyme catalase .
Many algal species have been shown to not only produce reactive oxygen species under normal conditions but to increase production of these compounds under stressful situations. In particular, ROS levels have been shown to be influenced by cell size, cell density, growth stage, light intensity, temperature, and nutrient availability.
Glutathione peroxidase 4 (GPx4) has a high preference for lipid hydroperoxides; it is expressed in nearly every mammalian cell, though at much lower levels. Glutathione peroxidase 2 is an intestinal and extracellular enzyme, while glutathione peroxidase 3 is extracellular, especially abundant in plasma. [ 4 ]
Kinetically perfect enzymes have a specificity constant, k cat /K m, on the order of 10 8 to 10 9 M −1 s −1.The rate of the enzyme-catalysed reaction is limited by diffusion and so the enzyme 'processes' the substrate well before it encounters another molecule.