Tyrosine residues reductases

Fe atoms. It had been anticipated that the c-type cytochrome center would have His/Met coordination, but His/His is observed. The former is the more usual coordination, especially at the high potential end E° > +200 mV) ofthe typical bacterial electron transfer chain to which the nitrite reductase is connected (Fig. 2) (7). The second curious feature is that the di heme iron is also six-coordinate thus, the enzyme does not offer a substrate-binding site at either heme. In addition to an expected axial histidine ligand there was an axial tyrosine (residue 25) ligand to the d heme (Fig. 4a). Each monomer is organized into two domains. 

An unusual feature of ribonucleotide reductase is that the reaction it catalyzes involves a radical mechanism. The mammalian type of reductase initiates this reaction by the tyrosyl radical-nonheme iron. Hydroxyurea and related inhibit the majTrrrraiVarr retftrcCase 6y abolishing the radical state of the tyrosine residue. Inhibition of DNA synthesis by such compounds is secondary to this effect. 

Unfortunately, despite the great stability of the tyrosine radical of ribonucleotide reductase [69], the crystals available for diffraction contain the enzyme in its non-radical form [107]. However, the tyrosine residue (Tyr-122) known to convert to the radical is buried in the protein [99] and the position and structure of the tyrosine residue in the crystal is consistent with spectroscopic data for the radical form as with cytochrome c peroxidase, it appears unlikely that major conformational changes occur subsequent to radical formation. 

Amino acid analyses of the two forms are shown in Table VII. They differ from one another by 100 residues, or about 11,000 in molecular weight. The amino acid content of the hydrophobic domain has been calculated by difference, and the composition is dominated by apolar amino acids 74). Treatment of the detergent-extracted enzyme with chymo-trypsin results in a soluble form of the protein its amino acid content is very similar to that of the lysosomal form 74). The spectra of the lysosomal-extracted and the detergent-extracted reductase are Identical in the visible and near-ultraviolet, the extinction coefficient at 461 nm being 10.6 mM cm h Differences between these spectra in the 260-280-nm region are accounted for by the additional tryptophan and tyrosine residues the approximate extinction coefficients at the ultra-... 

Turnover number, succinate dehydrogenase, 236-237 Tyrosine residues cytochrome hi reductase, 163 cytochrome c peroxidase, 355 glyceraldehyde-3-phosphate dehydrogenase, 21... 

Ekberg, M., Sahlin, M., Eriksson, M., and Sj berg, B.-M., I996,Two conserved tyrosine residues in protein R1 participate in an intermolecular elech on transfer in ribonucleotide reductase. J. Biol. Chem. 271 20655n20659. 

Larsson, A., and Sj"herg, B.-M., 1986, Identification of the stable free radical tyrosine residue in ribonucleotide reductase. EMBO J. 5 2037n2040. 

Figure 25.11. Ribonucleotide Reductase R2 Subunit. This subunit contains a stable free radical on a tyrosine residue. This radical is generated by the reaction of oxygen at a nearby site containing two iron atoms. Two R2 subunits come together to form a dimer.

CCP provided the first example of a redox-active tryptophan residue in enzyme catalysis (S5), and a second example has just been proposed in ribonucleotide reductase turnover (86). The long half-life of the Trpl91 radical in CCP compound I (ti/2 —3 h) (87), despite the presence of neighboring tyrosine residues (84) which rapidly reduce Trp in aqueous solution (8S), suggests a special radical-stabilizing environ-... 

Figure 5.21. Upper Aromatic region of 500 MHz H-NMR spectrum of the complex between 2,4-diaminopyrimidine and selectively deuterated dihydrofolate reductase, in which the only aromatic protons remaining were the 2 6 -protons of the five tyrosine residues. Lower Difference spectrum showing the NOE effects observed on irradiating resonance in this sample.

Class I Ribonucleotide Reductases - The most widely distributed form of ribonucleotide reductase. It acts upon ribonucleoside diphosphates. The enzyme generates a free radical on a tyrosine residue, with the aid of a diferric oxygen bridge. 

Subunit B1 of . coli ribonucleotide reductase is an SH protein carrying several binding sites of different affinity for substrate and effector nucleotides these interactions will be discussed in a later paragraph. The most unusual features of the smaller subunit B 2 are its iron content and a tyrosine residue present as stable free radical. Both these components are essential for enzyme catalysis and structurally coupled. They confer characteristic light absorption to the protein, with a broad maximum around 370 nm (a =... 

The origin of the proton abstracted by the formed carbanion was also determined by site-specific mutagenesis. Several studies showed that exchanges of a strongly conserved tyrosine residue to phenylalanine or alanine leads to an overall reduced activity [8-10]. In contrast to OYEl [8], the studies with the OYEs designated as morphinone reductase (MR) and pentaerythritol tetranitrate (PETN) suggest that solvent water is the source of the proton required for reduction [9,10]. 

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