532 A. E. BRAUNSHTEIN 



aldehydes and glycine, and ihe back reactions of glycine with aldehydes to form, 

 e.g., serine, threonine, phenylserine (reaction i). 



(1) R-CHOH-CH-NHj-COOH -t-O^CH-COOH f M^ ^^=^ R-CHi^-COOH ^F^ 

 f> -hydroxy amino acid glyoxylic acid ^ ^^f 



=;^=H-cQ) -(-H-K:H COOH ^f=^ NH^CHjCOOH + = CH-COOH + M * * 

 aldffhyde ^ — »-m^ glycine glyoxylic acid 



IK > 



Co 



The glycine-glyoxylate azomethine rapidly condenses with an excess of 

 glyoxylate (RCHO; R = HOOC) to form hydroxyaspartic acid. 



It may be expected that glyoxylic acid, like pyridoxal, will catalyse, under 

 suitable conditions, substitution reactions at the /?-C atom of serine, resulting 

 in the formation of cysteine, cystathionine, tryptophan, tyrosine and other ß- 

 substituted a-aminopropionic acids. Similarly to serine its /j-carboxy derivative, 

 hydroxyaspartate, can eventually be degraded by glyoxylate to H^O, ammonia 

 and oxaloacetate. The intermediate azomethines would be expected to be highly 

 reactive in condensation reactions analogous to steps in the enzymic biosynthesis 

 of amino and keto acids with longer and branched side chains. 



These considerations suggest one of the probable ways for abiogenic con- 

 version of simple amino acids to amino acids with complicated side chains in 

 the prebiological era. 



Much earlier than vitamin Bg or reactions of the citric acid cycle had made 

 their appearance, such amino acids could have been synthesized by non-enzymic 

 condensation reactions involving azomethine complexes of glycine or serine 

 with glyoxylic acid and a chelating metal atom, e.g. aluminium. If this atom was 

 fixed in the surface of clay or a similar solid, the resulting chemosorption could 

 increase the efficiency and specificity of catalysis. 



Of great importance is the recent discovery [30] that a pyridoxal enzyme is 

 involved in the first step of porphyrin biogenesis, namely, in the synthesis of I 

 S-aminolaevulinic acid by way of a Dakin-West type condensation between] 

 glycine and 'active succinate', presumably succinyl-coenzyme A (reaction 2). 

 The following steps of porphyrin biosynthesis — the Knorr type condensation of I 

 2 molecules of S-aminolaevulinic acid to porphobihnogen (reaction 3) and the 

 conversion of 4 molecules of porphobilinogen to uroporphyrin III (reaction 4) 

 are reactions known to proceed spontaneously and rapidly even in dilute aqueous 

 solutions under quite mild conditions. Uroporphyrinogen III or a similar re- 1 

 duced intermediate is considered as the common precursor of the métallo- 1 

 porphyrin biocatalysts — haems and chlorophyll. 



It is plausible that under suitable conditions a soluble or chemosorbed metal j 

 chelate of glycine-glyoxylate azomethine could condense with a reactive succinyl ' 

 compound of abiogenic origin, e.g. the mononitrile, the thio acid or an ester of j 

 succinic acid, to yield S-aminolaevulinic acid (reaction 2a), similarly to thej 



