102 GENERAL BIOCHEMISTRY 



NH,+ COO- H 



I I 1 



CH3— C— COO- or H3N+— C— H CH3— C— COO- 



I 1 1 



H CHs NH3+ 



L-alanine D-alanine 



This representation places the ammonium ion clockwise from the 

 carboxyl ion in the d forms and counterclockwise in the l forms 

 regardless of the actual optical rotations. Initially this choice was 

 made arbitrarily, but it is now known from X-ray studies that the 

 convention represents the actual projection of the groups in two 

 dimensions. 



Several amino acids possess two asymmetric carbon atoms as in 

 hydroxyproline, and more than two isomers are possible. Although 

 the actual configuration of the extra asymmetric atom is not known 

 in most of these cases, there seems to be a pronounced preference for 

 a particular configuration in any given natural source. There are other 

 amino acids with two asymmetric structures but with a point of overall 

 molecular symmetry, as in cystine. Optically inactive meso isomers 

 are then possible, but only one such form is known to occur naturally. 



Returning to the configuration of the a-carbon atom, we may say 

 that very extensive studies have failed to discover d forms of any of 

 the amino acids in proteins. This apparently complete preference for 

 L isomers in proteins extends to all the great number of individual 

 proteins and species studied. The origin, preservation, and value of 

 this high degree of specificity raise a number of interesting problems, 

 none of them settled and some not even subject to experimentation at 

 present. If, for example, only L-amino acids are available in the diet 

 of an animal, it ought to be easier to synthesize protein from these 

 directly without inverting the configuration. Moreover, the metabolic 

 machinery of the cell should be simpler if a single isomer of each 

 amino acid is handled. In addition, it is felt that specific configura- 

 tions may be critical in the enzymatic and other biological functions 

 of proteins. These points and other related reasoning suggest the 

 value of optical activity, but real experimental proof is not available. 



D-Amino acids do occur in certain peptides and apparently also in 

 free form in very small amounts. Hence animals might consume 

 limited quantifies of these isomers. Although d isomers are apparently 

 not absorbed and utilized as well as the much more common l forms, 

 many species can metabolize them to some extent. The process is 

 thought to involve conversion either to optically inactive compounds 

 like keto acids or to L-amino acids at an early stage rather than involv- 

 ing metabolism along a complete pathway parallel to that for l 



