Tyrosine definition

By means of a procedure described above, Hanson and Fittkau (HI) isolated seventeen different peptides from normal urine. One of them, not belonging to the main peptide fraction, consisted of glutamic acid, and phenylalanine with alanine as the third not definitely established component. The remaining peptides contained five to ten different amino acid residues and some unidentified ninhydrin-positive constituents. Four amino acids, i.e., glutamic acid, aspartic acid, glycine, and alanine, were found in the majority of the peptides analyzed. Twelve peptides contained lysine and eight valine. Less frequently encountered were serine, threonine, tyrosine, leucine, phenylalanine, proline, hydroxyproline, and a-aminobutyric acid (found only in two cases). The amino acid composi-... 

Cysteine and tyrosine are described as conditionally essential (see text). Selenocysteine is not always included in lists of amino acids but it obeys the definition of an amino acid, it is present in the diet, is present in some mammalian proteins, and it is incorporated directly into these proteins as selenocysteine during the normal process of translation there is a specific tRNA for this amino acid the anticodon is AGU. 

Peptide toxins. Of all the toxins produced by freshwater cyanobacteria, the peptide toxins of Microcystis aeruginosa have received the most attention. All research on these peptide toxins indicates they are small, possibly cyclic, with molecular weights estimated at 1200 to 2600 (10,11). Recent work has become more definitive in the estimation of molecular weight and amino acid composition. In 1978 Elleman et al. (12) reported that they had isolated and characterized the peptide toxin of an Australian strain Microcystis which was a pentapeptide with a minimum molecular weight of 654. It consisted of equimolar amounts of alanine, tyrosine, methionine, glutamic and 3-methyl aspartic acid and methylamine. 

The occurrence of alanine in proteins was first shown by Schutzen-berger, who did not actually identify his product with the synthetical one Weyl in l88i obtained it as a decomposition product of silk and showed that his preparation was similar in properties to Strecker s synthetical alanine. He thus established it as a constituent of a protein molecule. The researches of Emil P ischer have shown that alanine is a constant constituent of all proteins. It is worthy of note that of the eighteen definitely determined units of a protein molecule, six of them, namely, isoleucine, phenylalanine, tyrosine, serine, histidine and tryptophane, are derivatives of a-aminopropionic acid. 

More definite evidence for the transient existence of the un-cyclized l-(jS-aminoethyl)-3,4-benzoquinones has been obtained recently by Kodja and Bouchilloux,77 78 who noted that a transient yellow color (Amax ca. 385 mp) was occasionally observed during the enzymic oxidations of catecholamines (particularly in unbuffered systems at low temperatures). This phenomenon was probably due to the formation of the transient o-quinones. (The absorption maximum of o-benzoquinone, the effective chromophore of the open-chain quinones, is known to occur at ca. 390 mp.79) An absorption maximum at 390 mp is characteristic of the formation of the dopa-quinone chromophore during oxidation of small C -terminal tyrosine peptides in the presence of tyrosinase.37 48 Similar spectroscopic features were observed when the oxidations were carried out with lead dioxide in sulfuric acid solutions (pH> 1). If the initial oxidation was carried out for a short period of time, it was possible to regenerate the original catecholamines by reduction (e.g. with sodium bisulfite, potassium iodide, and zinc powder) and to show that the 385 mp peak disappeared.77,78 Kodja and Bouchilloux were also able to identify 2,4-dinitrophenylhydrazones of several of the intermediate non-cyclized quinones by paper chromatography and spectroscopy (Amax n weakly acid solution ca. 350 mp with a shoulder at ca. 410 mp).77,78... 

Most of the lysosomal proteases called cathepsins are small 20- to 40-kDa glycoproteins found in all animal tissues.313 Most are cysteine proteases which function best and are most stable in the low pH reducing environment of lysosomes. They resemble papain in size, amino acid sequence, and active site structures. Papain is nonspecific but most cathepsins have definite substrate preferences. Cathepsin B is the most abundant. There are smaller amounts of related cathepsins H (an aminopeptidase)314 and L315 and still less of cathepsins C, K, and others. Cathepsin B is both an endopep-tidase and an exopeptidase.316 It acts on peptides with arginine at either Pj or P2 but also accepts bulky hydro-phobic residues in Pj and prefers tyrosine at P3.317 Cathepsin S is less stable at higher pH than other cathepsins and has a more limited tissue distribution, being especially active in the immune system.318 319... 

Oxidation of two out of 13 tryptophan residues in a cellulase from Penicillium notatum resulted in a complete loss of enzymic activity (59). There was an interaction between cellobiose and tryptophan residues in the enzyme. Participation of histidine residues is also suspected in the catalytic mechanism since diazonium-l-H-tetrazole inactivated the enzyme. A xylanase from Trametes hirsuta was inactivated by N-bromosuc-cinimide and partially inactivated by N-acetylimidazole (60), indicating the possible involvement of tryptophan and tyrosine residues in the active site. As with many chemical modification experiments, it is not possible to state definitively that certain residues are involved in the active site since inactivation might be caused by conformational changes in the enzyme molecule produced by the change in properties of residues distant from the active site. However, from a summary of the available evidence it appears that, for many / -(l- 4) glycoside hydrolases, acidic and aromatic amino acid residues are involved in the catalytic site, probably at the active and binding sites, respectively. 

Securinine.—Further details of one group s study of the biosynthesis of securinine (13) have been published.21 The origins of this alkaloid are well defined,22 and information which adds to this definition is that tyrosine is incorporated without loss of tritium from the carbon atoms flanking the phenolic hydroxy-group.21... 

It is possible to alter amino-acid residues to see if this perturbs the EPR signal, e.g. by iodination of tyrosine residues [142]. Recently site-directed mutagenesis has been used extensively for this purpose. Whilst this can yield definitive negative results it is unlikely to lead to clear positive identification of a... 

The fantastic stmctural diversity of the natural brominated tyrosines has led to equally ingenious biosynthesis proposals, but only a few definitive labeling studies have been described. The early study by Tymiak and Rinehart on the biosynthesis of dibromotyrosine metabolites by the sponge Aplysina fistularis supports the incorporation of both phenylalanine (127) and tyrosine (128) into dienone 133 and dibromohomogentisamide (134) (Scheme 19.6) [120]. Metabolites 131,132, 135, and 136 were also identified along with 133 and 134 in this study, which utilized... 

The position of several amino acid residues at certain definite sites of the chain suggests that these residues play a specific role in the formation of the secondary and tertiary structure (Watson and Kendrew, 1961). Thus, e.g., phenylalanines in positions 43 and 46 (a-chain) and histidine in position 87 (a-chain) obviously participate in the stabilization of the heme. The hydrogen bond linking tyrosine in position 42 and aspartic acid in position 94 stabilizes two sections of the o-helix. At those sites where the chains... 

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