Cysteinic radical

Single electron transfers within the protein lead to the formation of a cysteine radical on Cys-439 in the enzyme active site, which abstracts, then, the C-3 hydrogen to initiate a radical mechanism, as shown in Fig. 17. Protonation of the C-2 ... 

Ribonucleotide reductases that do not contain tyrosyl radicals have been characterized in other organisms. Instead, these enzymes contain other stable radicals that are generated by other processes. For example, in one class of reductases, the coenzyme adenosylcobalamin is the radical source. Despite differences in the stable radical employed, the active sites of these enzymes are similar to that of the E. coli ribonucleotide reductase, and they appear to act by the same mechanism, based on the exceptional reactivity of cysteine radicals. Thus, these enzymes have a common ancestor but evolved a range of mechanisms for generating stable radical species that function well under different growth... 

The primary structure of enzymes is determined by peptide covalent bonds joining each amino acid to the neighboring one, as well as by disulfide bonds joining sulfur atoms in two cysteine radicals. 

The study of highly purified preparations of the enzyme has revealed that there are two FAD molecules, two molybdenum atoms, and eight iron atoms per protein molecule. Molybdenum is combined with the histidine and cysteine radicals of the protein molecule. In addition, a bond with FAD, of unknown... 

CsHgNOsS] Radical Y 7-irradiation of single crystal of N-acetyl-L-cysteine Radical Y 93Matl... 

A particularly difficult case has been met for cysteine radicals. Such radicals are important models for thiyl radicals in proteins. Such radicals are known to display very large g-shifts that furthermore are strongly dependent on the nature of the environment. Recently a detailed experimental and theoretical study was undertaken in order to clarify the observed effects [128]. In die first step, die conformations of the cysteine radical were studied with respect to variations in the dihedral angle S-Cp-Ca-C. 

The calculation of the spectroscopic parameters at the three minima showed that the g-tensor is surprisingly sensitive to the conformation of the cysteine radical and that, indeed, very large g-shifts are calculated. However, the calculated g-shifts... 

The reason for the large errors in the ealeulations and the large g-shifts observed in crystals or frozen solutions containing the cysteine radical are readily understood upon examination of the electronic structure of this radical. The diagram in Figure 4 shows that the sulfur atom has two available lone pairs that are oriented perpendicular to the C-S bond direction. 

Figure 4. The sulfiir lone pair orbitals of the cysteine radical (in the first minimum) and the relation of their energy separation to the g-shift (to second order).

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