AMINO ACIDS , PEPTIDES AND PROTEINS 209 



proteins. Glycine, alanine, valine, leucine and isoleucine are "neutral" amino acids and 

 differ from each other only with respect to the aliphatic side chain. Of these, glycine is 

 the only one without an asymmetric carbon atom and is therefore the only one that cannot 

 occur in an optically active form. 



Serine and threonine contain one hydroxyl group adjacent to the amino function. Be- 

 cause of this they are unstable in hot sodium hydroxide solution and react quantitatively 

 with sodium periodate (2). Cysteine, in which a sulfhydryl group replaces the hydroxyl 

 group of serine also gives the above reactions. However, cysteine can be easily distin- 

 guished from the oxygen analogue by the red color obtained with nitroprusside reagent. 

 Also cysteine is very easily oxidized aerobically to the disulfide cystine and to cysteic 

 acid. Therefore, the isolation of cystine or cysteic acid is no proof that the compounds 

 existed in that oxidation state in the intact cell. Methionine, another important sulfur-con- 

 taining amino acid, can also yield spurious oxidation products. In this case, methionine 

 sulfoxide or methionine sulfone are formed. Methionine is known to play an important bio- 

 logical role in transmethylation reactions, and there is ample evidence that the methyl 

 group attached to the sulfur of methionine can be transferred in plants to yield oxygen and 

 nitrogen methylated substances (3). The actual reactant in these methyl group transfers is 

 S-adenosylmethionine ("active" methionine) formed by reaction of methionine with ATP: 



L-methionine + ATP -» S-adenosylmethionine + pyrophosphate + orthophosphate 



Proline is a cyclic aliphatic amino acid and differs from the above compounds due 

 to the presence of a secondary amino group. It is quite soluble in 95% ethanol. 



Phenylalanine, tyrosine and tryptophan contain aromatic rings and therefore absorb 

 light in the ultraviolet region. In fact, the light absorption of the simple proteins is at- 

 tributable to the presence of these amino acids. Tyrosine, has abnormally low water solu- 

 bility, 45 mg. in 100 ml. at 25° C. This property can be taken advantage of in isolation 

 procedures. Tryptophan is readily oxidized in hot acidic solutions and is, therefore, com- 

 pletely destroyed during acid-catalyzed hydrolysis of proteins. It can be recovered from 

 proteins by using either alkaline or enzymatic hydrolysis. Evidence is accumulating that 

 tryptophan is a precursor of indole-3-acetic acid (4), a compound with well known growth 

 hormonal action in plants, (cf. Chap. 14) 



The acidic amino acids aspartic acid and glutamic acid as well as their amides 

 asparagine and glutamine are very widely distributed in higher plants and play a key role 

 in metabolic reactions. Glutamine is quite reactive and is readily decomposed in boiling 

 water to ammonium pyrrolidone carboxylate. Glutamic acid can also be cyclized but re- 

 quires somewhat more vigorous reaction conditions than glutamine. Due to unfavorable 

 steric factors asparagine and aspartic acid do not give these reactions. This is not always 

 recognized. Thus, the product obtained on treatment of asparagine in phosphate buffer of 



H«C CH 



2i I 2 



C CHCO^ NH/ 



0^\ / ^ 



N ammonium pyrrolidone carboxylate 



H 



