Explanation:
Catalases are one of the most studied groups of enzymes. The term catalase was first identified by Loew as hydrogen peroxide (H2O2) degrading enzyme in 1901, and the protein has been the focus of study for biochemists and molecular biologists ever since. The overall reaction for catalase can simply be described as the degradation of two molecules of hydrogen peroxide to water and oxygen (reaction 1). This catalytic reaction occurs in two distinct stages, but what each of the stages includes is mainly based on the kind of catalase . The first stage involves oxidation of the heme using first hydrogen peroxide molecule to form an oxyferryl species in which one oxidation equivalent is taken off from the iron and one from the porphyrin ring to make a porphyrin cation radical (reaction 2). In the second stage, this radical intermediate, known as compound I, is reduced by a second hydrogen peroxide to regenerate the resting state enzyme, water and oxygen (reaction 3) [2, 3]. Catalases can also function as peroxidases, in which suitable organic compound is used as an electron donor. During peroxidase reaction, compound I is converted to compound II (reaction 4), which can be oxidized by another hydrogen peroxide to produce the inactive compound III (reaction 5). For NADPH-binding catalases, it has been suggested that enzyme inhibition through the appearance of compound III can be prevented by the NADPH blocking or releasing compound II generation [4, 5, 6].
2H2O2→2H2O+O2E1
Enz(Por–FeIII)+H2O2→CpdI(Por+∙–FeIV=O)+H2OE2
CpdI(Por+∙–FeIV=O)+H2O2→Enz(Por–FeIII)+H2O+O2E3
CpdI(Por+∙–FeIV=O)+AH2→CpdII(Por–FeIV–OH)+AH∙E4
CpdII(Por–FeIV–OH)+H2O2→CpdIII(Por–FeIII–O2−∙)+H2OE5
Catalases have been classified into three groups: monofunctional heme-containing catalases, heme-containing catalase-peroxidases, and manganese-containing catalases [7]. Among them, monofunctional catalases constitute the largest and most extensively studied group of catalases [1, 2]. They all possess two-step mechanism for dismutation of hydrogen peroxide. Members of this largest class of catalases can be biochemically subdivided based on having large (75–84 kDa) subunits with heme d associated or small (55–69 kDa) subunits with heme b associated. All small subunit enzymes so far characterized, unlike larger enzymes, have been found with NADP(H) bound [1, 8]. In turn, larger subunit enzymes have been shown to exhibit significantly enhanced stability against high temperatures and proteolysis [1, 9]. The catalase-peroxidases, less widespread class, exhibit significant peroxidatic activity in addition to catalytic activity [2]. They are found in bacteria, archaebacteria, and fungi. Catalase-peroxidases have a molecular mass in the range of 120–340 kDa [10, 11]. Manganese-containing catalases are not as widespread as the heme-containing catalases, and there are only three of them so far characterized, one from lactic acid bacteria (Lactobacillus plantarum) and two from thermophilic bacteria (Thermus thermophilus and Thermoleophilum album) [1, 2]. These enzymes are also called pseudo-catalases as their active site contains a manganese-rich reaction instead of heme group [12, 13]. Crystal structures of two manganese catalases, one from T. thermophilus and the other from L. plantarum, show the presence of dimanganese group in the catalytic center [1].
Although monofunctional catalases are described as such due to the prolonged-agreed belief that their only role is hydrogen peroxide removal, this rather limited catalytic role has recently been questioned. Vetrano et al. expressed a novel oxidase activity in the absence of hydrogen peroxide [14]. Later, a catalase from S. thermophilum was shown to have an unselective phenolic oxidase activity in the absence of hydrogen peroxide [15, 16, 17]. It is thought that such bifunctional enzymes might be more common due to the evidence on the presence of oxidase/peroxidase activity in catalase enzymes from different organisms such as Bacillus pumilus [18], Thermobifida fusca [19], and Amaranthus cruentus [20]. Such studies are likely to give evidence that translates from various sources to a great deal of catalases. Bifunctional enzymes can be advantageous in many industrial applications including the removal of toxic chemicals and/or chemoprotective agent activity especially when the oxidase activity is enhanced by directed evolution or engineering.
Hydrogen peroxide is a chemical compound with the formula H
2O
2. In its pure form, it is a very pale blue[5] liquid, slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3–6% by weight) in water for consumer use, and in higher concentrations for industrial use. Concentrated hydrogen peroxide, or "high-test peroxide", decomposes explosively when heated and has been used as a propellant in rocketry.
