AMINO ACIDS, PEPTIDES AND PROTEINS 227 



BIOSYNTHESIS OF AMINO ACIDS 



The close interrelationships between carbohydrate and amino acid metabolism has 

 been known for a long time. Thus, pyruvic acid, oxalacetic acid and alpha-ketoglutaric 

 acid, compounds arising during the oxidation of glucose, can be converted by transamina- 

 tion or reductive amination reactions to L-alanine, L-aspartic acid and L-glutamic acid, 

 respectively. Glutamic acid and aspartic acid, are in turn, precursors of a large number 

 of other amino acids. For example the interconversion of glutamic acid and glutamine 

 has been demonstrated in higher plants (74). Evidence exists that in microorganisms 

 (75) and possibly also in higher plants (76) glutamic acid is reduced to the semi-aldehyde 

 an intermediate from which proline can form by ring closure and reduction. In the forma- 

 tion of ornithine glutamic acid is first acetylated, then reduced and transaminated. 



Ornithine in turn is a precursor of critrulline and arginine in microorganisms, and 

 possibly also in higher plants (77). Enzymes that can catalyze the decarboxylation of 

 glutamic acid to gamma-aminobutyric acid have been demonstrated to be widely distrib- 

 uted in higher plants (78). Aspartic acid is readily interconvertible with asparagine and 

 can apparently give rise to beta-alanine, homoserine and threonine. Serine and glycine 

 are interconvertable by a reaction involving a tetrahydrofolic acid derivative. This re- 

 action has been shown to occur in higher plants (79). 



The biosynthesis of tyrosine and phenylalanine from intermediates of the shikimic 

 acid (VII) pathway has been established for three different families of higher plants, 

 Labiatae, Gramineae and Polygonaceae (80). 



Shikimic acid can be consid- 

 ered a carbohydrate derivative 

 since it is formed from phosphoenol 

 pyruvate and D-erythrose-4-phosphate 

 (Chap. 2). In microorganisms, tryp- 



|-j Q Q — /» N tophan may also be formed from 



2 >\ Xi , shikimic acid-5-phosphate through 



anthranilic acid (81). 



OH 



YK 



The biosynthetic pathways of 

 leucine, isoleucine and valine are 

 quite similar. A scheme which ap- 

 pears to account for the results ob- 

 tained with microorganisms is pre- 

 sented below for the synthesis of valine. Pyruvic acid and acetaldehyde condense to yield 

 alpha-acetolactic acid (VIII); this may be reduced to the glycol IX. On rearrangement by 

 a reaction analogous to a pinacol-type rearrangement, alpha-keto isovaleric acid (X) is 

 formed. 



CO^H ^ COoH COoH CO«H 



I 



'2 ^^2^ ^^2"" I 2' 



II 



H C-C-OH — > H_C-C-OH C = 



C = + CH3CH^ H3C-U-un — * n^v ^ 



I C = H-C-QH ^3^"^"" 



CH_ I I ^1 



3 CH3 CH3 CH3 



By a transamination reaction X may be converted to valine. 



A certain amount of evidence exists that gamma-methyleneglutamic acid and related 

 compounds originate in the peanut through condensation of two molecules of pyruvic acid. 

 The amino acid analogues of the keto acids XI, XII, and XIII occur in some plants. 



