Amino-acid residues tyrosine

For small peptides, the amount of color increases with the size of the peptide. The presence of any of five amino acid residues (tyrosine, tryptophan, cysteine, histidine, and asparagine) in the peptide or protein backbone further enhance the amount of color produced, because they contribute additional reducing equivalents for further reduction of the phos-phomolybdic/phosphotungstic acid complex. With the exception of tyrosine and tryptophan, free amino acids will not produce a colored product with the Lowry reagent however, most dipeptides can be detected. In the absence of any of the five amino acids listed above, proteins containing proline residues have a lower... 

The reaction that leads to BCA color formation as a result of the reduction of Cu2+ is also strongly influenced by the presence of any of four amino acid residues (tyrosine, tryptophan, cysteine, or cystine) in the amino acid sequence of the protein. Unlike the Coomassie dye-binding (Bradford) methods, which require a minimum mass of protein to be present for the dye to bi nd, the presence of only a single amino acid residue in the sample may result in the formation of a colored BC A-Cu+ chelate. This is true for any of the four amino acids cited above. Studies done with di- and tripeptides indicate that the total amount of color produced is greater than can be accounted for by the simple addition of the color produced with each BCA-reactive amino acid, so the peptide backbone must contribute to the reduction of copper as well. 

Biochemical applications have been made more recently O, ). Here, cyclic photochemical reactions are employed to generate nuclear spin-polarization in biological macromolecules. The information obtained is of a structural nature rather than mechanistic. In the case of proteins aromatic amino acid residues (tyrosine, histidine and tryptophan) can be polarized by reversible hydrogen atom or electron transfer reactions with a photo-excited dye. These reactions require direct contact of the dye with the amino acid side-chains so that they only occur for residues lying at the surface of the protein. 

This led to the conclusion that these amino acids were essential for the resolution capability and only 6 new libraries of 18 compounds had to be synthesized with these amino acid residues to define the position 3. Surprisingly, the separation abilities of all six libraries were very similar. Therefore, tyrosine was chosen for continuing deconvolution, since it is convenient as its aromatic ring can easily be detected by UV spectrometry. The last step, defining position 5, required the synthesis and testing of 6 individual hexapeptides. 

The improvements in resolution achieved in each deconvolution step are shown in Figure 3-3. While the initial library could only afford a modest separation of DNB-glutamic acid, the library with proline in position 4 also separated DNP derivatives of alanine and aspartic acid, and further improvement in both resolution and the number of separable racemates was observed for peptides with hydrophobic amino acid residues in position 3. However, the most dramatic improvement and best selectivity were found for c(Arg-Lys-Tyr-Pro-Tyr-(3-Ala) (Scheme 3-2a) with the tyrosine residue at position 5 with a resolution factor as high as 28 observed for the separation of DNP-glutamic acid enantiomers. 

The cDNA for this photoprotein has been cloned and expressed in E. coli, and the recombinant protein obtained was named mitrocomin (Fagan et al., 1993). Mitrocomin consists of 190 amino acid residues with a tyrosine residue at the C-terminus, and has three Ca2+-binding sites. 

In addition, eNOS is subject to protein phosphorylation. It can be phosphotylated on several serine (Ser), threonine (Thr), and tyrosine (Tyr) residues however, major changes in enzyme function have been reported for the phosphorylation of amino acid residues Seri 177 and Thr495 (in the human eNOS sequence) (Fig. 3). 

The Src homology 2 domain (or SH2-domain) is a protein domain of about 100 amino acid residues first identified in the tyrosine kinase Src. SH2-domain... 

Besides all the sensory and texturizing properties, GA has interesting antioxidant properties such as an efficient capacity for deactivation of excited electronic states and moderated radical scavenging capacity. There is increasing experimental evidence that associate the antioxidant function with its protein fraction, mainly by amino acid residues such as histidine, tyrosine and lysine, which are generally considered as antioxidants molecules (Marcuse, 1960,1962 Park et al., 2005). 

Neurotoxins present in sea snake venoms are summarized. All sea snake venoms are extremely toxic, with low LD5Q values. Most sea snake neurotoxins consist of only 60-62 amino acid residues with 4 disulOde bonds, while some consist of 70 amino acids with 5 disulfide bonds. The origin of toxicity is due to the attachment of 2 neurotoxin molecules to 2 a subunits of an acetylcholine receptor that is composed of a2 6 subunits. The complete structure of several of the sea snake neurotoxins have been worked out. Through chemical modification studies the invariant tryptophan and tyrosine residues of post-synaptic neurotoxins were shown to be of a critical nature to the toxicity function of the molecule. Lysine and arginine are also believed to be important. Other marine vertebrate venoms are not well known. 

Chemical modifications of proteins (enzymes) by reacting them with iV-acylimidazoles are a way of studying active sites. By this means the amino acid residues (e.g., tyrosine, lysine, histidine) essential for catalytic activity are established on the basis of acylation with the azolides and deacylation with other appropriate reagents (e.g., hydroxylamine). 

Another interesting mechanism of the repair of amino acid residues of LDL apolipoprotein B100 by flavonoids has been recently described [75]. The authors of this work suggested that LDL-bound quercetin (but not rutin) repaired the tyrosine free radical by intramolecular electron transfer. 

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. 

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