Nonpolar neutral amino acids

Methionine (Met or M) ((5)-2-amino-4-(methylsulfanyl)-butanoic acid) is a nonpolar, neutral, amino acid with the formula HOOCCH(NH2)CH2CH2SCH3. Together with Cys, Met is one of the two sulfur-containing proteinogenic amino acids and a great antioxidant. Its derivative 5-adenosyl methionine (SAM) serves as a methyl donor. ... 

Alanine is a nonpolar neutral amino acid with one amino group and one carboxyl group, that is, a-amino propionic acid. 

Carboxypeptidase A catalyzes the hydrolysis of carboxyl-terminal acidic or neutral amino acids however, the rate of hydrolysis depends on the structure of the side chain R. Amino acids with nonpolar aryl or alkyl side chains are cleaved more rapidly. Carboxypeptidase B is specific for the hydrolysis of basic COOH-terminal amino acids (lysine and arginine). Neither peptidase functions if proline occupies the COOH-terminal position or is the next to last amino acid. 

TABLE 16.5 Ionized Forms of Nonpolar and Polar Neutral Amino Acids... 

Substitution of polar for nonpolar amino acids in the interior of the subunit could destabilize the hydrophobic interactions within the interior (Bunn et al., 1977). The polar amino acid substitutions could allow water into the hydrophobic interior, leading to alteration of the tertiary structure. Neutral amino acid substitutions for one of the interior amino acids may alter the stereochemical congruity of the interior amino acids. Several hemoglobins have amino acid substitutions in the subunit interior Sogn, Riverdale-Bronx, Russ, Ann Arbor, Wien, Bristol, Shepherds Bush, Boras, and Olmsted. Of the unstable hemoglobin variants, 17 involve substitution of one noncharged amino acid for another (Bunn et al., 1977). 

If a sample contains groups that can take up or lose a proton, (N//, COO//), then one must expect the pH and the concentration to affect the chemical shift when the experiment is carried out in an acidic or alkaline medium to facilitate dissolution. The pH may affect the chemical shift of more distant, nonpolar groups, as shown by the amino acid alanine (38) in neutral (betaine form 38a) or alkaline solution (anion 38b). The dependence of shift on pH follows the path of titration curves it is possible to read off the pK value of the equilibrium from the point of inflection... 

What about tertiary structure Why does any protein adopt the shape it does The forces that determine the tertiary structure of a protein are the same forces that act on ail molecules, regardless of size, to provide maximum stability. Particularly important are the hydrophilic (water-loving Section 2.13) interactions of the polar side chains on acidic or basic amino acids. Those acidic or basic amino acids with charged side chains tend to congregate on the exterior of the protein, where they can be solvated by water. Those amino acids with neutral, nonpolar side chains tend to congregate on the hydrocarbon-like interior of a protein molecule, away from the aqueous medium. 

Alanine (abbreviated Ala or A) ((S)-2-aminopropanoic acid ct-aminopropionic acid) is a nonpolar, neutral, aliphatic amino acid with the formula HOOCCH(NH2)CH3 Ala plays a major role in the transport of nitrogen from skeletal muscles to the liver. 

Glycine (Gly or G) (aminoacetic acid, aminoethanoic acid) is a nonpolar, neutral, aliphatic amino acid with the formula HOOCCH(NH2)H. Gly is the simplest amino acid and plays important roles in peptide and protein chains. It does not contain a side chain and can thus fit into secondary structures where larger amino acids cannot. Gly acts as a transmitter in the CNS where it accomplishes several functions. Gly is a precursor of porphyrins. Gly, Pro, aspartate, Ser, and Asn enable reverse turns. The acylated amino group of Gly can accept a second acyl group to give rise to a diacylamide. ... 

Isoleucine (He or I) ((25,35)-2-amino-3-methylpentanoic acid) is a nonpolar, neutral, aliphatic amino acid with the formula HOOCCH(NH2)CH(CH3)CH2CH3. Having a hydrocarbon side chain. He is classified as a hydrophobic amino acid. Together with Thr, He is one of the two common amino acids that have a chiral center. Although four stereoisomers of He are possible. He present in nature exists in only one enantiomeric form, that is, (25,35)-2-amino-3-methylpentanoic acid. ... 

Phenylalanine (Phe or F) (2-amino-3-phenyl-propanoic acid) is a neutral, aromatic amino acid with the formula HOOCCH(NH2)CH2C6H5. It is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. Tyr and Phe play a significant role not only in protein structure but also as important precursors for thyroid and adrenocortical hormones as well as in the synthesis of neurotransmitters such as dopamine and noradrenaline. The genetic disorder phenylketonuria (PKU) is the inability to metabolize Phe. This is caused by a deficiency of phenylalanine hydroxylase with the result that there is an accumulation of Phe in body fluids. Individuals with this disorder are known as phenylketonurics and must abstain from consumption of Phe. A nonfood source of Phe is the artificial sweetener aspartame (L-aspartyl-L-phenylalanine methyl ester), which is metabolized by the body into several by-products including Phe. The side chain of Phe is immune from side reactions, but during catalytic hydrogenations the aromatic ring can be saturated and converted into a hexahydrophenylalanine residue. ... 

In some instances, combinations of Cig and silica columns are also used for better purification of the crude extracts (431, 445). A combination of Cg, silica, and amino solid-phase extraction columns has been successfully employed to fractionate anabolic and catabolic steroid hormone residues from meat in polar and nonpolar neutral and phenolic compounds, and to purify further each fraction effectively (452). Another combination of two solid-phase extraction columns, one using a graphitized carbon black sorbent and the other Amberlite resin in the hydroxyl form, allowed neutral anabolics to be isolated and separated from acidic anabolics and their metabolites (453). A combination of basic alumina column placed in tandem with an ion-exchange column has also been applied for the purification of the crude extracts in the determination of diethylstilbestrol and zeranol (427), and estradiol and zeranol in tissues (450). 

Identify the following amino acid, and tell from the electrostatic potential map whether its side chain is acidic, basic, neutral nonpolar, or neutral polar. 

Table 26.1 lists the pKa s of the amino acids along with their isoelectric points (pi). Both those with nonpolar side chains and those with polar side chains have pKa values near those of glycine. Therefore, these all have isoelectric points near 6 and exist predominantly as dipolar ions at neutral pH. 

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