Phenylalanine, acidity

Phenylalanine Acidic amino acids and amide derivatives... 

As constituents of proteins the amino-acids are important constituents of the food of animals. Certain amino-acids can be made in the body from ammonia and non-nitrogenous sources others can be made from other amino-acids, e.g. tyrosine from phenylalanine and cystine from methionine, but many are essential ingredients of the diet. The list of essential amino-acids depends partly on the species. See also peptides and proteins. 

First, it is possible to excite a chromophore corresponding to the active site, and detennine which modes interact with it. Second, by using UV excitation, the amino acids with phenyl rings (tryptophan and tyrosine, and a small contribution from phenylalanine) can be selectively excited [4], The frequency shifts in the resonance Raman spectrum associated with them provide infomiation on their enviromnent. 

As an example, the empirical formula of phenylalanine may be split into a more extended form that shows the presence of a phenyl ring, as well as an amino and a carboxylic acid group (the condensed form in Figure 2-4). 

In the example of Figure 2-5, the compound has the trivial name phenylalanine but the lUPAC name is 2-amino-3-phenylpropanoic acid, which indicates a carbon... 

Figure 2-5. Phenylalanine, also known by the lUPAC name 2-amino-3-phenylpropanoic acid.

Water-soluble globular proteins usually have an interior composed almost entirely of non polar, hydrophobic amino acids such as phenylalanine, tryptophan, valine and leucine witl polar and charged amino acids such as lysine and arginine located on the surface of thi molecule. This packing of hydrophobic residues is a consequence of the hydrophobic effeci which is the most important factor that contributes to protein stability. The molecula basis for the hydrophobic effect continues to be the subject of some debate but is general considered to be entropic in origin. Moreover, it is the entropy change of the solvent that i... 

The positions of the three amino-acids are shown clearly by the colour of their zones or spots, the proline being yellow and the glycine and phenylalanine being blue. Note the Rp value for each amino-acid ... 

Hydrolysis of the azlactone leads to the acylaminooinnamic acid the latter may be be reduced catal3rtlcally (Adams PtOj catalyst 40 lb. p.s.i.) and then hydrolysed by hydrochloric acid to the amino acid. Alternatively, the azlactone (say, of a-benzylaminocinnamic acid) may undergo reduction and cleavage with phosphorus, hydriodic acid and acetic anhydride directly to the a-amino acid (d/ p phenylalanine). 

The influence of a large number of oc-amino acids on the values of and k at have been determined. These a-amino acids included glycine, L-valine, L-leucine, L-phenylalanine, L-tyrosine, L-tryptophan, NOrmethyl-L-tryptophan (L-abrine), N-methyl-L-tyrosine, N,N-dimethyl-L-tyrosine and p -me thoxy-N-me thyl -L -phenyl al anine. 

Phenylalanine and tryptophan have side chains that incorporate aromatic rings which are large and hydrophobic The aromatic portion of tryptophan is bicyclic which makes it larger than phenylalanine Tryptophan also has a more electron rich aromatic ring and is more polarizable than phenylalanine Its role is more specialized and it is less abundant m proteins than most of the other ammo acids... 

One ammo acid often serves as the biological precursor to another L Phenylala nine is classified as an essential ammo acid whereas its p hydroxy derivative L tyro sine IS not This is because animals can convert L phenylalanine to L tyrosine by hydrox ylation of the aromatic ring An arene oxide (Section 24 7) is an intermediate... 

Some people lack the enzymes necessary to convert L phenylalanine to L tyrosine Any L phenylalanine that they obtain from their diet is diverted along a different meta bolic pathway giving phenylpyruvic acid... 

Phenylpyruvic acid can cause mental retardation m infants who are deficient m the enzymes necessary to convert l phenylalanine to l tyrosine This disorder is called phenylketonuria, or PKU disease PKU disease can be detected by a simple test rou tmely administered to newborns It cannot be cured but is controlled by restricting the dietary intake of l phenylalanine In practice this means avoiding foods such as meat that are rich m l phenylalanine... 

Digestion of the tetrapeptide of Problem 27 13 with chy motrypsin gave a dipeptide that on ammo acid analysis gave phenylalanine and valine in equimolar amounts What ammo acid sequences are possible for the tetrapeptide ... 

Transfer RNA (tRNA) Transfer RNAs are relatively small nucleic acids containing only about 70 nucleotides They get their name because they transfer ammo acids to the ribosome for incorporation into a polypeptide Although 20 ammo acids need to be transferred there are 50-60 tRNAs some of which transfer the same ammo acids Figure 28 11 shows the structure of phenylalanine tRNA (tRNA ) Like all tRNAs it IS composed of a single strand with a characteristic shape that results from the presence of paired bases m some regions and their absence m others... 

Among the 76 nucleotides of tRNA are two sets of three that are especially important The first is a group of three bases called the anticodon, which is comple mentary to the mRNA codon for the ammo acid being transferred Table 28 3 lists two mRNA codons for phenylalanine UUU and UUC (reading m the 5 3 direction) Because base pairing requires the mRNA and tRNA to be antiparallel the two anticodons are read m the 3 5 direction as AAA and AAG... 

Protect glycine as its Boc derivative and anchor this to the solid support Remove the pro tecting group and treat with Boc protected phenylalanine and DCCI Remove the Boc group with HCl then treat with HBr in tnfluoroacetic acid to cleave Phe Gly from the solid support... 

Chymotrypsin (Section 27 10) A digestive enzyme that cat alyzes the hydrolysis of proteins Chymotrypsin selectively catalyzes the cleavage of the peptide bond between the car boxyl group of phenylalanine tyrosine or tryptophan and some other ammo acid... 

L-phenylalanine L-amino acid oxidase and horseradish peroxidase (E) I-... 

Alanine (ala) Phenylalanine (phe) R 1 0 - HNCHC N General formula for an amino acid residue Aspartic Acid (asp)... 

The sequence of each different peptide or protein is important for understanding the activity of peptides and proteins and for enabling their independent synthesis, since the natural ones may be difficult to obtain in small quantities. To obtain the sequence, the numbers of each type of amino acid are determined by breaking down the protein into its individual amino acids using concentrated acid (hydrolysis). For example, hydrolysis of the tetrapeptide shown in Figure 45.3 would give one unit of glycine, two units of alanine, and one unit of phenylalanine. Of course, information as to which amino acid was linked to which others is lost. 

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