Proteins residue radicals, enzymes

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

In view of the formation of a highly reactive Compound I ferryl species, and the fact that the porphyrin radical cation of this intermediate is reduced in enzymes such as CcP by a protein residue, it is not surprising that permanent covalent modifications are autocatalytically introduced into some protein frameworks. Two examples of autocatalytic protein modification, those of LiP and the catalase-peroxidases, are summarized here to illustrate the maturation of peroxidase protein structures that can have important functional consequences. 

Montellano et al., 1984). The cytochrome f -450-catalyzed oxidation of 2,3-bis(carbethoxy)-2,3-diazabicyclo[2.1.0]hex-5-ene, a known cyclobutadiene precursor, results in enzyme inactivation and eventual isolation of N-(2-cy-clobutenyOprotoporphyrin IX (Steams and Ortiz de Montellano, 1985). The formation of this adduct is readily rationalized by oxidative release of cyclobutadiene (or its radical cation) which binds to a nitrogen of the heme and then abstracts a hydrogen from a protein residue (Fig. 31). 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

This enzyme catalyzes the conversion of pyruvate to formate and acetyl CoA and is a key enzyme in the anaerobic degradation of carbohydrates in some Enterobacteriaceae. Using an enzyme selectively C-labeled with glycine, it was shown by EPR that the reaction involves production of a free radical at C-2 of glycine (Wagner et al. 1992). This was confirmed by destruction of the radical with O2, and determination of part of the structure of the small protein that contained an oxalyl residue originating from gly-734. 

Recently, Gunther et al. [41] proposed that nitric oxide may directly react with enzymes without intermediate formation of peroxynitrite. It is known that the oxidation of arachido-nic acid by prostaglandin H oxidase is mediated by the formation of enzyme tyrosyl radical (see Chapter 26). Correspondingly, it has been suggested that NO is able to react with this radical to form the tyrosine iminoxyl radical and then nitrotyrosine. Therefore, the NO-dependent nitration of protein tyrosine residue may occur without the formation of peroxynitrite or other nitrogen oxides. 

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