Tyrosine, characteristics

The oxidative polymerization of 5,6-dihydroxyindole (1) and related tyrosine-derived metabolites is a central, most elusive process in the biosynthesis of eumelanins, which are the characteristic pigments responsible for the dark color of human skin, hair, and eyes. Despite the intense experimental research for more than a century,36 the eumelanin structure remains uncharacterized because of the lack of defined physicochemical properties and the low solubility, which often prevents successful investigations by modem spectroscopic techniques. The starting step of the oxidative process is a one-electron oxidation of 5,6-dihydroxyindole generating the semiquinone 1-SQ (Scheme 2.7). 

Correia, A. M. N. G., Leitao, M. C. A., Clifford, M. N., Caffeoyl-tyrosine and Angola II as characteristic markers for Angolan Robusta coffees, Food Chem., 53(3), 309, 1995. (CA123 8347x)... 

McGeer E., McGeer P. (1973). Some characteristics of brain tyrosine hydroxylase. In Mandel A., editor. New Concepts in Neurotransmitter Regulation. New York Plenum Press pp. 53-68. 

The oncogene v-erbB, unlike c-erbB, codes for a shortened form of the EGF receptor protein. Usually the binding of EGF is required to turn on the tyrosine kinase activity of the EGF receptor protein, the one coded by c-erbB. However, the tyrosine kinase activity of the receptor derived from v-erbB is switched on permanently, even in the absence of EGF. The uncontrolled grqwth characteristic of cancer cells results. 

The result of iodination at tyrosine groups can alter the spectral characteristics of the protein in solution (Hughes, 1950). The typical protein absorbency at 280 nm can shift to a maximum at about 305-315 nm due to the addition of iodine atoms to the phenolate ring of tyrosine. The degree of absorbance shift is dependent on how many iodine atoms are incorporated into the protein and whether they result in mainly mono- or di-iodotyrosine formation. In addition, as the level of iodination increases, the solubility of a protein in aqueous solution can dramatically decrease until complete insolubility results in proteins with high numbers of tyrosines. 

Despite the similar functions of each isozyme, only two regions of amino acid homology exist (X and Y), one of 150 and a second of 120 amino acid residues, which are 54% and 42% identical among the isozymes but are differentially localized within each enzyme (Fig. 20-3). The X and Y domains form the catalytic core of the enzyme. A characteristic of the (3 and 8 isoforms is that relatively few amino acids (40-110) separate the X and Y entities, whereas a much larger separation is observed for the PLCy isoform (approx. 400). In addition, in PLCy, the region between X and Y contains amino acid sequences that are found in nonreceptor tyrosine kinases (SH2 and SH3 domains). All four isoforms possess pleckstrin homology (PH) domains. The latter are considered to enable the enzyme to become tethered to the plasmalemma via an interaction with PI(4,5)P2. In addition, all PLC isoforms possess an E-F hand domain, which is located between PH and X domains, and a C2 domain, which is located close to the Y domain. 

The most direct demonstration of triplet-triplet energy transfer between the aromatic amino acids is the ODMR study by Rousslang and Kwiram on the tryptophanyl-tyrosinate dipeptide.(57) Since the first excited singlet state of tyrosinate is at lower energy than that of tryptophan, it is possible to excite tyrosinate preferentially. The phosphorescence of this dipeptide, however, is characteristic of tryptophan, which is consistent with the observation that the triplet state of tyrosinate is at higher energy than that of tryptophan, making tryptophan the expected triplet acceptor. 

Tyrosine fluorescence emission in proteins and polypeptides usually has a maximum between 303 and 305 nm, the same as that for tyrosine in solution. Compared to the Stokes shift for tryptophan fluorescence, that for tyrosine appears to be relatively insensitive to the local environment, although neighboring residues do have a strong effect on the emission intensity. While it is possible for a tyrosine residue in a protein to have a higher quantum yield than that of model compounds in water, for example, if the phenol side chain is shielded from solvent and the local environment contains no proton acceptors, many intra- and intermolecular interactions result in a reduction of the quantum yield. As discussed below, this is evident from metal- and ionbinding data, from pH titration data, and from comparisons of the spectral characteristics of tyrosine in native and denatured proteins. 

The fluorescence of purified histones has been studied by several different groups, 90 95) with the most detailed studies being on calf thymus histone HI. Histone HI, which binds to the outside of core particles, contains one tyrosine and no tryptophan. This protein exhibits a substantial increase in fluorescence intensity in going from a denatured to a folded state.<90) Collisional quenching studies indicate that the tyrosine of the folded HI is in a buried environ-ment.(91) Libertini and Small(94) have identified three emissions from this residue when in the unfolded state with peaks near 300, 340, and 400 nm. The 340-nm peak was ascribed to tyrosinate (vide infra), and several possibilities were considered for the 400-nm component, including room temperature phosphorescence, emission of a charge transfer complex, or dityrosine. Dityrosine has the appropriate spectral characteristics, but would require... 

O. Shimizu, J. Watanabe, and K. Imakubo, Effect of phosphate ion on fluorescence characteristics of tyrosine and its conjugate base, Photochem. Photobiol. 29, 915-919 (1979). 

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