Tyrosine, iron complexation

The enzyme catalyzing the formation of retinal 2 by means of central cleavage of P-carotene 1 has been known to exist in many tissues for quite some time. Only recently, however, the active protein was identified in chicken intestinal mucosa (3) following an improvement of a novel isolation and purification protocol and was cloned in Escherichia coli and BHK cells (4,5). Iron was identified as the only metal ion associated with the (overexpressed) protein in a 1 1 stoichiometry and since a chromophore is absent in the protein heme coordination and/or iron complexation by tyrosine can be excluded. The structure of the catalytic center remains to be elucidated by X-ray crystallography but from the information available it was predicted that the active site contains a mononuclear iron complex presumably consisting of histidine residues. This suggestion has been confirmed by... 

Warner and Weber (133) and Wishnia et al. (139) noted differences between the absorption spectra of chicken ovotransferrin and those of its iron complex in the alkaline range where tyrosines are assumed to show changes in ionization, as measured spectrophotometrically. Fig. 6... 

Essentially on the basis of observations 1, 2, and 3, above, Warner 131) proposed the formula reproduced in Fig. 19. This formula for the iron complex involved three tyrosines and a carboxyl from the protein side chains and a bicarbonate. The copper complex was assumed to involve two tyrosines. 

Dinitrosyl-diglutathionyl-iron complex binds with extraordinary affinity to the active site of aU these dimeric enzymes, showing values of 10 -10 M [59, 60, 341], One of the glutathiones in the irmi complex binds to the enzyme G-site, whereas the other GSH molecule is lost and replaced by a tyrosine phenolate in the coordination of the ferrous ion [60] Thus the bound complex is a... 

The ternary complex III is also in the high-spin ferric state (90, 92) and still possesses a strong tyrosinate-iron charge transfer interaction (92). Moreover, the ternary complex resembles enzyme-bound product rather than enzyme-bound reactants (96, 99). 

Binding of metal ions (2 moles Mn +, Fe +, Cu + or Zn2+ per mole of protein) at pH 6 or above is a characteristic property of conalbumin. Table 11.6 lists the absorption maxima of several complexes. The occasional red discoloration of egg products during processing originates from a conalbumin-iron complex. The complex is fully dissociated at a pH less than 4. Tyrosine and... 

The evidence from Suda s laboratory indicates that in pyrocatechase and homogentisic oxidase the iron is bound to a tyrosine residue. In tryptophan pyrrolase the iron is present as heme. In the case of reactions involving bonds adjacent to a catechol or o-amino phenol group, the hydroxyl or amino group away from the bond to be attacked may participate in the iron complex. In all cases the iron may be assumed to be bound in a complex that permits very little dissociation (as measured by exchange with free ferrous ions) at neutral or alkaline pH values. 

In hemoglobin M, histidine F8 (His F8) has been replaced by tyrosine. The iron of HbM forms a tight ionic complex with the phenolate anion of tyrosine that stabilizes the Fc3 form. In a-chain hemoglobin M variants, the R-T equilibrium favors the T state. Oxygen affinity is reduced, and the Bohr effect is absent. P Ghain hemoglobin M variants exhibit R-T switching, and the Bohr effect is therefore present. 

Synthesis and reactions of nitric oxide (NO). l-NMMA inhibits nitric oxide synthase. NO complexes with the iron in hemoproteins (eg, guanylyl cyclase), resulting in the activation of cGMP synthesis and cGMP target proteins such as protein kinase G. Under conditions of oxidative stress, NO can react with superoxide to nitrate tyrosine. 

Some acid phosphatases from animals and plants are violet in color and contain iron (Chapter 16) and an Mn3+-containing acid phosphatase has been isolated from sweet potatoes.720 These enzymes have dimetal centers, often containing one Zn2+ and one Fe3+ with bridging carboxylate and hydroxide ions between the metals. Imidazole, tyrosinate, and carboxylate side chains hold the metals as in Fig. 16-20. A water molecule bound to the Fe3+ is thought to dissociate with a low pKa of 4.8 to give an Fe3+ OH complex. The hydroxyl ion can then attack the phospho groups, one... 

Ribonucleotide reductases are discussed in Chapter 16. Some are iron-tyrosinate enzymes while others depend upon vitamin B12, and reduction is at the nucleoside triphosphate level. Mammalian ribonucleotide reductase, which may be similar to that of E. coli, is regarded as an appropriate target for anticancer drugs. The enzyme is regulated by a complex set of feedback mechanisms, which apparently ensure that DNA precursors are synthesized only in amounts needed for DNA synthesis.273 Because an excess of one deoxyribonucleotide can inhibit reduction of all... 

Downward movement of the iron atom and the complexed polypeptide chain on binding oxygen. The structure is shown before (black) and after (blue) binding oxygen. Movement of His F8 is transmitted to valine FG5, straining and breaking the hydrogen bond to the penultimate tyrosine. Only the < chain is shown here. 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

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