Aromatic amino acids, composition

For historical reasons many pharmaceutical enzymes are assayed with physiological or biopolymeric substrates (proteins, polysaccharides, bacteria, oil emulsions), which causes a number of theoretical and practical problems. The interpretation of results is difficult when natural substrates are converted into products that are substrates themselves for the next enzymatic attack. Reaction rates often depend on the position of the scissile bonds in the molecule and the chemical nature of the moieties. Hydrolysis can proceed simultaneously on various bonds at various rates. In proteolysis it is assumed that some products are liberated only after denaturation and that during the reaction course new peptide bonds become accessible for hydrolysis. In these cases the enzymatic mechanisms become exceedingly complex, kinetic parameters are apparent values, and experimental results are strongly influenced by the reaction conditions. Reproducibility problems can occur upon assaying proteinases with a limited specificity for particular casein types. Bromelain and pancreatic proteinase, FEP pharmaceutical enzyme standards, are assayed with a casein substrate. The extent of soluble peptide release is a measure of proteolytic activity. However, due to limited specificity, some proteinases release peptides with a nonrandom aromatic amino acid composition. Contamination of casein preparations with protein and of test enzyme substances with other proteinases biases the assay results. Under these conditions, relative assay methods are indicated. 

Native fluorescence of a protein is due largely to the presence of the aromatic amino acids tryptophan and tyrosine. Tryptophan has an excitation maximum at 280 nm and emits at 340 to 350 nm. The amino acid composition of the target protein is one factor that determines if the direct measurement of a protein s native fluorescence is feasible. Another consideration is the protein s conformation, which directly affects its fluorescence spectrum. As the protein changes conformation, the emission maximum shifts to another wavelength. Thus, native fluorescence may be used to monitor protein unfolding or interactions. The conformation-dependent nature of native fluorescence results in measurements specific for the protein in a buffer system or pH. Consequently, protein denatur-ation may be used to generate more reproducible fluorescence measurements. 

It should also be kept in mind that the Beer-Lambert law often is not vahd at higher concentrations, since there occur interactions between chromophores and other molecules . This effect is observed especially at reading of proteins in the UV. The solvent may influence the absorbance too, because, for example, some of the aromatic amino acid residues are buried within hydrophobic core of the molecule and become exposed during unfolding of the protein when the composition of the solvent is changed or the protein is denaturated by dilution. 

The ultraviolet absorbance of many proteins is due to the presence of amino acids with aromatic side chains. Gelatin, a derivative of collagen (Chap. 4), has an unusual composition, with a low proportion of aromatic amino acids. 

The liver is responsible for modifying blood protein and Aa composition, which it performs by a series of enzymatic process including transamination, deamination and reamination. The essential aromatic amino acids are degraded in the liver, whereas the branched-chain amino acids are passed to the periphery, where they are metabolised exclusively by skeletal muscle. Non-essential amino acids may be metabolised hepatically or in skeletal muscle. 

Finally, the relationship between the amino acid composition of grapes and the final aromatic composition of wine has been recently described (Hernandez-Orte et al. 2002, 2006). Therefore, it is possible that in the near future grape juice will be complemented with specific mixtures of amino acids in order to improve the aromatic quality of wine. 

Van Rensburg, H., Anterola, A.M., Levine, L.H., Davin, L.B. and Lewis, N.G. (2000) Monolignol compositional determinants in loblolly pine aromatic amino acid metabolism and associated rate-limiting steps. ACS Symp. Ser., 742,118-44. 

The absorbance of the aromatic amino acids can be used for rapid and reliable determinations of protein concentration, provided that its amino acid composition is known (see Sect. 5.1.2.3). Spectrophotometric determination of nucleic add concentrations requires information not only about base composition but also about conformation, i.e., whether the nucleic acid is single- or double-stranded (see-Sect. 5.2.1). 

Differences are apparent between the amino acid compositions of the component parts, but the values for barbs show greatest dissimilarities particularly in alanine, glycine, isoleucine, and the aromatic amino acids. Goose barbs and goose down show less marked differences in composition. 

The amino acid composition was determined for the D-mannanase preparation from Aspergillus niger 84 and revealed that this preparation contained a high proportion of aromatic and acidic amino acid constituents. The D-mannanase preparation from Bacillus subtilis83 was found to contain 5 moles of tryptophan and 9 moles of tyrosine per mole of the enzyme. 

The D-xylanase system of Stereum sanguinolentum,199 the only D-xylanase for which an amino acid composition has as yet been published, was found to contain a high proportion of acidic and aromatic amino acid residues. The M.W., as determined from the amino acid composition, is 23,900, compared with 21,600 as calculated from ultracentrifugation data. Other physical parameters that have been determined199 for this D-xylanase include the sedimentation coefficient [2.8S, which is similar to that reported for a D-xylanase isolated from Trichoderma viride,203 namely, 2.IS], the partial specific volume (0.71 cm3.g 1), and the molar extinction coefficient (6.25 X 104). Activation energies have been reported for D-xylanases from Schizophyllum commune233 (EA 28.6 kj.mol-1) and from a commercial cellulase preparation229 (EA 34.0 kj.mol-1). 

Nothing is known about the amino acid composition of the protein which absorbs light at 280 m/< indicating the presence of aromatic amino acids. There were no characteristic absorption maxima in the visible part of the spectrum. 

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