Prosthetic group metalloprotein

HEMOPROTEINS. These proteins are actually a subclass of metalloproteins because their prosthetic group is heme, the name given to iron protoporphyrin IX (Figure 5.15). Because heme-containing proteins enjoy so many prominent biological functions, they are considered a class by themselves. 

Classes of metalloproteins. Transition ion prosthetic groups in proteins are... 

Note, however, that the -59In mV change per pH-unit is seldom found for prosthetic groups in proteins because association of protons is usually not directly on the coordination complex (which could result in loss of the metal) but rather on a nearby (or not-so-nearby) amino-acid side chain. So, the change can be anywhere between 0 and -59In mV. This information can be quite valuable for an understanding of the mechanism of action of the metalloprotein, but it does mean that we have to carry out EPR-monitored redox titrations at several different pH-values. 

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. 

B. Redesign of Nonheme Iron Proteins. In heme protein redesign described above, the heme prosthetic group largely dictates the active site structure. Redesign focuses mainly on the proximal and distal sides of the heme, causing minimal effects on the overall protein scaffolds. This is not necessarily the case for nonheme metalloproteins in which metal sites are not as dominant and small changes may have more dramatic effects on the protein folds and stability. 

Figure 22 Introducing normative prosthetic group into metalloproteins. (a) by chemical modification of heme propionate. (Reprinted with permission fi-om Ref. 25. 2002 the American Chemical Society) (b) by noncovalent addition strategy. The crystal stmcture of the Fe (3,3-Me2-salophen) incorporated into AlalTGlaMb. (Reprinted with permission from Ref 287. 2004 the American Chemical Society) (c) by a single attachment strategy. The computer model of adipoc)4e lipid binding protein-phenanthroline complex. (Reprinted with permission from Ref. 291. 1997 the American Chemical Society) (d) by a dual covalent attachment strategy. The computer model of Mb(L72C/Y 103C) with a Mn "-Salen complex covalently attached at two-points and overlayed with heme. (Reprinted with permission from Ref 288. 2004 the American Chemical Society)...

Illustrative examples for such a possibility are found with the cytochromes. The name of these proteins comes from the Greek words meaning colored substances in the cell. Cytochromes are intensely red-colored redox enzjunes containing a heme prosthetic group as their dominant chromophore. Hemes are iron complexes of protoporph5uin IX derivatives (10,26). One of the most frequently studied metalloproteins of this family is cytochrome c (27). The ribbon structure of a cytochrome c enzyme together with the protein-bound heme c cofactor 6 is shown in Fig. 4. 

Metalloproteins constitute a distinct subclass of proteins that are characterized by the presence of single or multiple metal ions bound to the protein by interactions with nitrogen, sulfur, or oxygen atoms of available amino acid residues or are complexed by prosthetic groups, such as heme, that are covalently linked to the protein. These metals function either as catalysts for chemical reactions or as stabilizers of the protein tertiary structure. Protein-bound metals may also be labile and, as such, be subject to transport, transient storage, and donation to other molecular sites of requirement within tissues and cells. 

It is known that superoxide reacts very slowly with all amino-acids since all rate constants are below 100 mol l 1 s (89). Hence its reactivity with proteins without prosthetic group is low (89). One exception seems to be collagen, in which proline residues are oxidized into hydroxyproline (90). On the other hand, superoxide reacts efficiently with free radicals such as tryptophanyl radical (91). Reaction is fast with metalloproteins. It proceeds mostly by oxidizing or reducing the metal center. Some characteristics and rate constants of reactions with metalloproteins are given in table 7. It is obvious that products are often unknown and that the mechanism is sometimes unclear. It seems that there is no reaction with transferrin (92) and horseradish and lacto-peroxidase compounds II (93). The reason is unknown. 

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