Aromatic Amino-acids

Aromatic Amiiio-acids.— Hydroxylation of 3-(3-carboxyphenyl)alanine (39) to give 3-(3-carboxy-4-hydroxyphenyl)alanine (40) has been demonstrated in Reseda lutea and R. odorata. The two amino-acids have been previously shown to be shikimic acid-dcrived. On the basis of experiments in R. lutea 

Larsen and R Serensen, Biochim. Biophys. Actm 196S, 156,190. p. O. Larsen, Biochim. Biophys. Acta, 1967,141,27. 

Another type of naturally occurring tetrahydroisoquinoline acid, represented by l,2,3,4-tctrahydro-8-hydroxy-6,7 imethoxyisoquinolinc-l-carboxylic acid (49) and its 1-methyl derivative, both constituents of peyote cactus, is produced by condensation of 3 -hydroxy-4, 5 -dimethoxyphenylethylamine with glyoxalic acid or pyruvic acid, with subsequent decarboxylation to give the alkaloids 

5-Hydroxy-L-tryptophan and 5-hydroxytryptamine occur in Griffonia simplicifolia (Leguminosae), from which an enzyme has been partly purified which catalyses the hydroxylation of tryptophan to 5-hydroxytryptophan.  

D-Amino-acids.—In recent years it has become obvious that, in higher plants, various o-amino-acids are metabolized by routes differing from those of the L-amino-acids. The deviating metabolism of o-lysine has been discussed above. The principal pathway for transformation of D-amino-acids seems to be malonylation. This was first demonstrated for D-tryptophan, undergoing acylation to N -malonyl-o-tryptophan, a compound widely occurring in 

Tryptophan fluorescence is very sensitive to the local environment. In an environment with a low polarity, tryptophan emits at a maximum of 320 nm. The peak position shifts to 355 nm in the presence of a polar environment. The loss of the protein tertiary structure (complete denaturation) induces a shift in tryptophan fluorescence to 355 nm. 

upon binding of a ligand to a protein, Trp observables (intensity, polarization, and lifetime) can be altered, and so one can follow this binding with Trp fluorescence. In proteins, tryptophan fluorescence dominates. Zero or weak tyrosine and phenylalanine fluorescence results from energy transfer to tryptophan and/or neighboring amino acids. 

Burstein et al. (1973) classified tryptophan in proteins into three categories, according to the position of their fluorescence maximum (Xrnax) and the bandwidth (AX) of their spectrum  

When a protein contains two classes of Trp residues, the recorded fluorescence emission spectrum is the result of each class contribution. 

Tyrosine is more fluorescent than tryptophan in solution, but when present in proteins, its fluorescence is weaker. This can be explained by the fact that the protein tertiary structure inhibits tryosine fluorescence. Also, energy transfer from tyrosines to tryptophan residues occurs in proteins inducing a total or important quenching of tyrosine fluorescence. This tyrosine — tryptophan energy transfer can be evidenced by nitration of tyrosine residues with tetranitromethane (TNM), a highly potent pulmonary carcinogen. Because TNM specifically nitrates tyrosine residues on proteins, the effects of TNM on the phosphorylation and dephosphorylation of tyrosine, and the subsequent effects on cell proliferation, can be investigated. 

It follows from eqn. (69) that the first-order rate coefficient should attain a limiting value, k = if [H30+] becomes large in 

001 with 2 M sulfuric acid but 1.037 with 10 M sulfuric acid [253], for the decarboxylation of 4-methoxyanthranilic acid at 60 °C, kl2/ki3 = 1002 at pH = 4 but 1.042 in 2 M hydrochloric acid [257]. Solvent isotope effects are less suitable for a discussion of the rate-determining step in 

Rate coefficients [254] and carbon isotope effects [257] in the decarboxylation of 4-methoxy-anthranilic acid at 60 °C (/Li = 0.50) 

For a quantitative treatment of the dependence of the first-order rate coefficients on the acidity of the solution, the method of the stationary state is applied to the sum of concentrations of the intermediates X and 

The first factor in eqn. (71) is identical with the right-hand side of eqn. (69). The second factor takes care of the decrease of the overall rate due to return from the intermediates HX+ and X. If the two conditions fe  

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Pish protein concentrate and soy protein concentrate have been used to prepare a low phenylalanine, high tyrosine peptide for use with phenylketonuria patients (150). The process includes pepsin hydrolysis at pH 1.5 ptonase hydrolysis at pH 6.5 to Hberate aromatic amino acids gel filtration on Sephadex G-15 to remove aromatic amino acids incubation with papain and ethyl esters of L-tyrosine and L-tryptophan, ie, plastein synthesis and ultrafiltration (qv). The plastein has a bland taste and odor and does not contain free amino acids. Yields of 69.3 and 60.9% from PPG and soy protein concentrate, respectively, have been attained. 

The mode of action is by inhibiting 5-enolpymvyl-shikimate-3-phosphate synthase. Roundup shuts down the production of the aromatic amino acids phenylalanine, tyrosine, and tryptophane (30). Whereas all these amino acids are essential to the survival of the plant, tryptophane is especially important because it is the progenitor for indole-3-acetic acid, or auxin, which plays an important role in growth and development, and controls cell extension and organogenesis. 

Herbicides also inhibit 5- (9/-pymvylshikiniate synthase, a susceptible en2yme in the pathway to the aromatic amino acids, phenylalanine, tyrosine and tryptophan, and to the phenylpropanes. Acetolactate synthase, or acetohydroxy acid synthase, a key en2yme in the synthesis of the branched-chain amino acids isoleucine and valine, is also sensitive to some herbicides. Glyphosate (26), the sulfonylureas (136), and the imida2oles (137) all inhibit specific en2ymes in amino acid synthesis pathways. 

Appllca.tlons. Various A/-derivatives of amino acids (qv) are resolvable on BSA columns. These /V-amino acid derivatives include ben2enesulfonyl-, phthalimido-, S-dimethylarnino-l-naphthalenesulfonyl- (DANSYL-), 2,4-dinitrophenyl- (DNP-), and 2,3,6-trinitrophenyl- (TNP-) derivatives (30). Amines such as Prilocain, ( )-2-(prop5lamino)-(9-propiono-toluidide, a local anesthetic (Astra Pharm. Co.), are also resolved on BSA. The aromatic amino acids DL-tryptophan, 5-hydroxy-DL-tryptophan, DL-kynurenine [343-65-7] C qH 2N 2 3 3-hydroxy-DT.-kynurenine [484-78-6] and dmgs... 

The earliest references to cinnamic acid, cinnamaldehyde, and cinnamyl alcohol are associated with thek isolation and identification as odor-producing constituents in a variety of botanical extracts. It is now generally accepted that the aromatic amino acid L-phenylalanine [63-91-2] a primary end product of the Shikimic Acid Pathway, is the precursor for the biosynthesis of these phenylpropanoids in higher plants (1,2). 

Benzyl carbamates are readily cleaved under strongly acidic conditions HBr, AcOH 50% CF3COOH (25°, 14 days, partially cleaved) - 70% HF, pyridine CF3S03H FSOaH, or CHjSO.H.- In cleaving benzyl carbamates from peptides, 0.5 M 4-(methylmercapto)phenol in CF3CO2H has been recommended to suppress Bn additions to aromatic amino acids. To achieve deprotection via an Sn2 mechanism that also reduces the problem of Bn addition, HF-Me2S-p-cresol (25 65 10, v/v) has been recommended for peptide deprotection. 

A prior distribution for sequence profiles can be derived from mixtures of Dirichlet distributions [16,51-54]. The idea is simple Each position in a multiple alignment represents one of a limited number of possible distributions that reflect the important physical forces that determine protein structure and function. In certain core positions, we expect to get a distribution restricted to Val, He, Met, and Leu. Other core positions may include these amino acids plus the large hydrophobic aromatic amino acids Phe and Trp. There will also be positions that are completely conserved, including catalytic residues (often Lys, GIu, Asp, Arg, Ser, and other polar amino acids) and Gly and Pro residues that are important in achieving certain backbone conformations in coil regions. Cys residues that form disulfide bonds or coordinate metal ions are also usually well conserved. 

Since the fluorescence intensity of the zones on silica gel layers is reduced after a few minutes the determination of aromatic amino acids is usually performed on... 

Table 3. Comparison of Electrophilic Fluorination of Aromatic Amino Acids with [ F]fluorine or [ F]acetyl Hypofluorite" [25]...

FIGURE 8.5 SEC of aromatic amino acids and dipeptides. Column Same as Fig. 8.1. Flow rate 0.6 ml/min. Mobile phase 50 m/VI formic acid. Detection Ajj, = 0.5 AUFS. 

The nonpolar amino acids (Figure 4.3a) include all those with alkyl chain R groups (alanine, valine, leucine, and isoleucine), as well as proline (with its unusual cyclic structure), methionine (one of the two sulfur-containing amino acids), and two aromatic amino acids, phenylalanine and tryptophan. Tryptophan is sometimes considered a borderline member of this group because it can interact favorably with water via the N-H moiety of the indole ring. Proline, strictly speaking, is not an amino acid but rather an a-imino acid. 

FIGURE 4.15 The ultraviolet absorption spectra of the aromatic amino acids at pH 6. 

Chymotrypsin shows a strong preference for hydrolyzing peptide bonds formed by the carboxyl groups of the aromatic amino acids, phenylalanine, tyrosine, and tryptophan. Flowever, over time chymotrypsin also hydrolyzes amide bonds involving amino acids other than Phe, Tyr, or Trp. Peptide bonds having leucine-donated carboxyls become particularly susceptible. Thus, the specificity... 

Surveys of the frequency with which various residues appear in helices and sheets show that some residues, such as alanine, glutamate, and methionine, occur much more frequently in a-helices than do others. In contrast, glycine and proline are the least likely residues to be found in an a-helix. Likewise, certain residues, including valine, isoleucine, and the aromatic amino acids, are more likely to be found in /3-sheets than other residues, and aspartate, glutamate, and proline are much less likely to be found in /3-sheets. 

L Na -iiidepeiideiit Braiiched-chaiii and aromatic amino acids Ehrlich ascites cells Chinese hamster ovary cells Hepatocytes... 

As noted previously in Section 11.10, biological dehydrations are also common and usually occur by an ElcB mechanism on a substrate in which the -OH group is two carbons away from a carbonyl group. An example occurs in the biosynthesis of the aromatic amino acid tyrosine. A base first abstracts a proton from the carbon adjacent to the carbonyl group, and the anion intermediate... 

The second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. 

This step is almost always performed to get rid of the coloured impurities in the fermentation broth. The method is based on the fact that amino adds (esperially the non-aromatic amino acids) do not adsorb onto activated charcoal. Although the treatment is very effective, some of the amino acid is lost during this step. 

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