Glutamate semialdehyde amino acid synthesis

Ornithine is an amino acid. However, it is not incorporated into proteins during the process of protein synthesis because no genetic codon exists for this amino acid. Although ornithine is normally regenerated by the urea cycle (one of the products of the arginase reaction), ornithine also can be synthesized de novo if needed. The reaction is an unusual transamination reaction catalyzed by ornithine aminotransferase under specific conditions in the intestine (Fig. 38.14). The usual direction of this reaction is the formation of glutamate semialdehyde, which is the first step of the degradation pathway for ornithine. 

Fig. 3. Proline and arginine synthesis and degradation to show interrelationships between the pathways. The structures are glutamic acid (GLU), ornithine (ORN), citrulline (CIT), arginine (ARG), urea. 2-oxo-5-amino valeric acid (OAV), A -pyrroline-2-carboxylic acid (P2C), proline (PRO), A -pyrroline-5-carboxylic acid (P5C), glutamic semialdehyde (GSA).

The individual steps involved in the conversion of glutamate into ALA are shown in Scheme 6 [1,2,13]. First the glutamate is converted by an ATP-depen-dent ligase into a RNA ester 18, which appears to be the same as the gluta-myl- RNA used for protein synthesis in the plant chloroplast. This gluta-rnyl- RNA is then reduced by an NADPH-dependent reductase to glutamate 1-semialdehyde 19. As expected of an a-amino aldehyde, 19 is not particularly stable under neutral or basic conditions but can be isolated under acidic conditions, under which it cyclises to the corresponding lactol [2]. 

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