Catalase prosthetic group

Figure 5. Structures of the HPII catalase prosthetic group chlorin (proposed, see Chiu et al (24)) (top), of octaethylchlorin (the trans isomer) (middle), and of methylchlorin (2,2,4-trimethyldeuterochlorin) (bottom). The HPII catalase chlorin has a cis diol structure (24) and is isomeric to heme d chlorin, which has a trans diol structure.

Heme (C34H3204N4Fe) represents an iron-porphyrin complex that has a protoporphyrin nucleus. Many important proteins contain heme as a prosthetic group. Hemoglobin is the quantitatively most important hemoprotein. Others are cytochromes (present in the mitochondria and the endoplasmic reticulum), catalase and peroxidase (that react with hydrogen peroxide), soluble guanylyl cyclase (that converts guanosine triphosphate, GTP, to the signaling molecule 3, 5 -cyclic GMP) and NO synthases. 

Many enzymes require additional substances in order to function effectively. Conjugated enzymes require a prosthetic group before they are catalytically active, such groups being covalently or ionically linked to the protein molecule and remaining unaltered at the end of the reaction. Catalase (EC 1.11.1.6), for instance, contains a haem group while ascorbate oxidase (EC 1.10.3.3) contains a copper atom. 

A quite different approach came from Chance and others using heme enzymes (1947). Purified horseradish peroxidase has a characteristic absorption spectrum which was visibly altered in the presence of hydrogen peroxide. When an appropriate substrate was added it was oxidized by the hydrogen peroxide and the spectrum reverted to that of the original state of the enzyme. Similar studies were performed with catalase, showing that prosthetic groups in enzymes underwent reversible changes in the course of their reactions. 

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). 

Hemeproteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group. Examples of hemeproteins include cytochromes, catalase, hemoglobin, and myoglobin. 

Enzymes. Heme serves as the prosthetic group lor catalase, peroxidase, cytochrome oxidase, and the related cytochromes. Catalase and peroxidase iron are presumably present in the ferric form while the iron of Ihe cytochromes may exist in Ihe reduced or oxidized lorni. A number of tlasoproteins. including succinic dehydrogenase, contain iron in ihe molecule. Iron appears to act as coeiuyme for aeonilase. A number of other enzymes require the presence of iron for their activities,... 

Many other species also contain catalases. In bacteria these can contain either haem or dihydroporphyrin (chlorin) prosthetic groups [52], However, the presence of a weak tyrosine-ligation to the iron appears to be present in all cases. This, combined with the lack (present in peroxidases) of an H-bond between an arginine residue and the ferryl oxygen, may explain why catalase compound I is uniquely reactive to H2O2. 

There are a number of enzymes that catalyse the dismutation of superoxide in vivo, viz. the superoxide dismutases [50,51], They are metalloproteins which contain copper, zinc, manganese or iron as the prosthetic group. The enzyme catalase exists in vivo to degrade hydrogen peroxide within cells to form water and oxygen [43]. As stated earlier, there are barely detectable amounts of these two enzymes in the synovial fluid of arthritic patients and hence both superoxide radicals and hydrogen peroxide are potential mediators of damage to the biomolecules of the synovial fluid. 

Although most catalases contain the iron-protoporphyrin IX prosthetic group, it has been known for over 30 years that some bacteria are able... 

Mn Catalase. The majority of known catalases contain a heme prosthetic group to catalyze the disproportionation of hydrogen peroxide. These enzymes are long established (e.g., the beef liver catalase was crystallized in 1937 (13)) and are believed to protect respiring cells... 

Of the two catalases found in E. coli, one has a protoheme prosthetic group whereas the other, the HPII catalase, contains an iron chlorin prosthetic group, the proposed structure of which is displayed in Figure 5 (24). Although the nature of the prosthetic group has been established,... 

Figure 9. MCD spectra of the ferrous pyridine-CO complexes of the extracted prosthetic group of HPII catalase (--), octaethylchlorin (----),...

Both notions are necessarily deductive since the configuration of the apoprotein near the prosthetic group is unknown also, because the characteristics of catalase or its derivatives do not lend themselves to singular assignments (74, 76-80) of the proximal ligand (Ls, Lei cf. Table I) and... 

PropUtrabacUrium arabinosum, -gly-cerophosphate dehydrogenase of, 260 2-Propyn-l-ol, catalase and, 404 Prostaglandin, biosynthesis, 168 Prosthetic groups, succinate dehydrogenase, 234-235... 

The prosthetic group of myoglobin and hemoglobin is a heme-b, protoheme-IX (Figure 2), which is the same as that found in several peroxidases, catalases, and cytochromes. The heme is linked noncovalently at the heme pocket of the protein... 

Myoglobin has the same prosthetic group as some peroxidases, such as horseradish peroxidase (HRP) or cytochrome c peroxidase, and reacts with H2O2 to produce a ferryl species, PFe(IV)=0, observed in the native peroxidase (see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, the catalytic activity of myoglobin toward substrate oxidation is very low, because... 

These are difficult enzymes to work with and only recently have crystal structures become available for two catalase-peroxidases Haloarcula marismortui (HMCP) and Burkholderia pseudomallei (BpKatG). A typical subunit is approximately 80 kDa in molecular mass, with a single heme b prosthetic group. The primary structure of each subunit can be divided into two distinct domains, N terminal and C terminal. The N-terminal domain contains the heme and active site, while the C-terminal domain does not contain a heme binding motif and its function remains unclear. The clear sequence similarity between the two domains suggests gene duplication and fusion. Curiously, despite many years of study, the actual in vivo peroxidatic substrate of the catalase-peroxidases has not been identified. 

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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