E Tyrosine

I 21.10 Predict whether the isoelectric points for the following a-amino acids are considerably acidic, slightly acidic, or basic (a) alanine, (b) lysine, (c) aspartic acid, (d) cystine, (e) tyrosine. (See Table 21-1 and Problem 21.6.) ... 

A more strongly acidic solution is needed to repress this ionization. Aspartic acid s isoelectric point is strongly acidic (pH = 2.7). (d) Cystine is a diaminodicarboxylic acid and behaves like a monoaminomonocarboxylic acid. The isoelectric point is slightly acid (pH = 4.6). (e) Tyrosine is a monoaminomonocarboxylic acid containing a phenolic OH, which however is too weakly acidic to ionize to any significant extent. The isoelectric point is slightly acidic (pH = S.6). 

Figure 2. The effects of progressive succinylation on the viscosity (v/v — 1 )/C) and uv absorption, i.e. tyrosine (At 287) of peanut protein dispersions...

Figure 3. Some of the commonly occurring sidechains (a) glycine (Gly, G) (A) leucine (Leu, L) (c) glutamic acid (Glu, E) (d) lysine (Lys, K) (e) tyrosine (Tyr, Y) (/) tryptophan (Tip, W). All atoms, including hydrogens (black dots), are shown the a-carbons are indicated by lines, oxygens by dots, and nitrogens by crosshatching in the corresponding circles open circles are other carbon atoms and the flexible dihedral angles are indicated by arrows with the one nearest to the a-carbon called x1, the next x2> and so on.

E = tyrosine phenol lyase, whole cells from Erwfnia herbicola... 

Van der Vliet, Eiserich JP, O Neill A, Halliwell B. Cross C E. Tyrosine modification by reactive, nitrogen species a closer look. Arch Biochem Biophys 1995 319 341-9. 

Other studies performed on the second-derivative spectrum (Ragone et al. 1984) have shown that the ratio r between two peak to trough distances (287-283 and 295-290) was related to the tyrosine / tryptophan ratio and was found dependent on the surrounding medium polarity of tyrosine. Table 1.5 shows the values of r for different proteins measured in the native (r ) and denatured (r ) states. We can notice that r increases when the proteins are denatured, i.e., tyrosines are much more exposed to the solvent in the denatured state. The increase of r with the protein deiiaturation is in principle dependent on the tyrosines locations in the proteins in the native state. 

Further, owing to deamination, p-hydroxyphenylpyruvic acid (15), a well-known inhibitor of tyrosinase, was produced from tyrosine. This product underwent further transformation to yield biphenyl 16 while indolylpyruvic acid (22) and subsequently indole (19) were formed from tryptophan. Hence, in biological systems, the loss of the starting material, i.e. tyrosine, for melanin synthesis due to the formation of 15, 16 and 18 may be counteracted by tryptophan, as an alternative substrate in the pathway of melanogenesis through 22 and 19. However, such a replacement of substrate for melanin by tryptophan is not possible in... 

The hydroxylation of the amino acid tyrosine to dopa (3,4-dihydroxyphenyl-alanine) is the rate-limiting step in catecholamine biosynthesis [231,257] (Table 1, p. 103). It is catalysed by the enzyme tyrosine hydroxylase. Norepinephrine synthesis is regulated at the tyrosine hydroxylase step by feed-back inhibition, i.e. tyrosine hydroxylase activity decreases as the concentration of endogenous norepinephrine increases [230]. 

Tyrosinase activity is not limited to low molecular weight substrates, these enzymes can also oxidize the phenolic moieties of peptides and proteins (i.e. tyrosine or dihydroxyphenylalanine residues). This amino-acid-residue-specific... 

In the last decade, many efforts have been devoted to the study of the influence of chiral molecules on the enzymatic processes at the membrane surfaces. V -Acyl-L-and D-amino acid derivatives have been employed as model substances for simulating biomembranes and interfacial processes at biomembrane surfaces [32]. It has been found that chiral monolayers of V -acylamino acid methyl esters on a pure water surface showed that hydrogen bond formation via NH, COOH, and p-hydroxyphenyl groups (i.e., tyrosine side chains) lead to a pronounced chiral discrimination [33,34]. Homochiral (d-d or L-L interactions) and heterochiral (d-l interaction) discrimination can be observed depending on the area per molecule ( min)> which depends on the conformation of the amino acid residue and on the alkyl chain length. 

---