Iron-tyrosinate proteins

Operation of the NIH shift can cause migration of a large substituent as is illustrated by the hydroxylation of 4-hydroxyphenylpyruvate (Eq. 18-49), a key step in the catabolism of tyrosine (Chapter 25). Human 4-hydroxyphenylpyruvate dioxygenase is a dimer of 43-kDa subunits.439 A similar enzyme from Pseudomonas is a 150-kDa tetrameric iron-tyrosinate protein, which must be maintained in the reduced Fe(II) state for catalytic activity.440 Although this enzyme is a... 

Binudear, Oxo-bridged Iron Centres and the Iron-tyrosinate Proteins 634... 

Table 4. Resonance Raman frequencies of phenolate ring vibrations in iron-tyrosinate proteins and model complexes...

Roe AL, Sehneider DJ, Mayer RJ, Pyrz JW, Widom J, Que L Jr (1984) X-ray absorption spectroscopy of iron-tyrosinate proteins. J Am Chem Soc 106 1676-1681 Ruiz-Lopez MF, Munoz-Paez AM (1991) A theoretical study of the XANES spectra of ratile and anatase. J Phys Condens Matter 3 8981-8990... 

In 1971, adrenodoxin, an iron-sulfur protein with a single tyrosine residue and no tryptophan was shown to fluoresce at 331 nm upon 280-nm excitation at neutral pH/20 1 On cooling from room temperature to 77 K, the emission maximum shifts to 315 nm. The redox state of the iron does not have any effect on the tyrosine emission. From these results, an exciplex between the excited singlet state of tyrosine and an unidentified group was suggested as the cause of the anomalous emission energy/2031 Later studies have shown that the excitation spectrum is a red-shifted tyrosine spectrum, that removal of the iron to form the apoprotein has no effect on the emission, and that heat, low pH, guanidine hydrochloride, urea, and LiCl all cause the emission... 

The emphasis on the study of hemoproteins and the iron-sulfur proteins often distracts attention from other iron proteins where the iron is bound directly by the protein. A number of these proteins involve dimeric iron centres in which there is a bridging oxo group. These are found in hemerythrin (Section 62.1.12.3.7), the ribonucleotide reductases, uteroferrin and purple acid phosphatase. Another feature is the existence of a number of proteins in which the iron is bound by tyrosine ligands, such as the catechol dioxygenases (Section 62.1.12.10.1), uteroferrin and purple acid phosphatase, while a tyrosine radical is involved in ribonucleotide reductase. The catecholate siderophores also involve phenolic ligands (Section 62.1.11). Other relevant examples are transferrin and ferritin (Section 62.1.11). These iron proteins also often involve carboxylate and phosphate ligands. These proteins will be discussed in this section except for those relevant to other sections, as noted above. 

The transferrins belong to the iron-tyrosinate group of proteins discussed in Section 62.1.5.5.2. Charge transfer from phenolate ligands to Fem accounts for the salmon-pink colour of transferrin. The detailed coordination environment of the iron in transferrin is not known with certainty, as... 

RNRs catalyze the reduction of ribonucleotides to deoxyribonucleotides, which represents the first committed step in DNA biosynthesis and repair.These enzymes are therefore required for all known life forms. Three classes of RNRs have been identified, all of which turn out to be metalloenzymes. The so-called class I RNRs contain a diiron site (see Cobalt Bn Enzymes Coenzymes and Iron-Sulfur Proteins for the other two types of RNRs). As diagrammed in Figure 5, these enzymes generate first a tyrosyl radical proximal to the diiron site in the protein subunit labeled R2, and then a thiyl radical in an adjacent subunit (Rl) that ultimately abstracts a hydrogen atom from the ribonucleotide substrate. This controlled tyrosine/thiol radical transfer must occur over an estimated distance of 35 A, and a highly choreographed proton-coupled electron transfer (PCET) mechanism across intervening aromatic residues has been proposed. Perhaps, even more remarkably,... 

Purple acid phosphatases. Diiron-tyrosinate proteins with acid phosphatase activity occur in mammals, plants, and bacteria. Most are basic glycoproteins with an intense 510- to 550-nm light absorption band. Well-studied members come from beef spleen, from the uterine fluid of pregnant sows (uteroferrin), and from human macrophages and osteoclasts. " " One of the two iron atoms is usually in the Fe(III) oxidation state, but the second can be reduced to Fe(II) by mild reductants such as ascorbate. This half-reduced form is enzymatically active and has a pink color and a characteristic EPR signal. Treatment with oxidants such as H2O2 or hexacyannoferrate (III)... 

Fig. 20. Structure of photosynthetic electron carriers and electron-transfer proteins (A) ATP, NAD and NADP (B) quinones (C) redox-active amino acids tyrosine and histidine (D) cytochromes and f (E) iron-sulfur proteins and (F) the copper protein plastocyanin.

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