Enzymes of Deamination, Decarboxylation, Transmethylation and Intermediary Metabolism

Abstract

Plants are autotrophic with regard to the amino acids and manufacture them from inorganic sources of nitrogen. Obviously then, they must possess all the necessary enzymes bringing about the synthesis and interconversion of the various amino acids; yet very little is so far known about the enzymes of higher plants involved in the metabolism of these compounds. Numerous tracer studies have been made using labeled carbon compounds (cf. Webster, 1959) and in each case the results have pointed to the presence of reactions and pathways similar to those of microorganisms and, in some instances, of animal tissues. Tracer techniques, however, do not yield information regarding the nature of the enzymes bringing about the synthesis of a particular intermediate. From the viewpoint of comparative biochemistry it would, for instance, be interesting to know the nature of the enzyme synthesizing carbamyl phosphate in higher plants and fungi. It is well known that this compound is synthesized in animals by means of an irreversible reaction mediated by carbamyl synthetase; in bacteria, however, this compound is produced by means of a reversible reaction catalyzed by carbamate kinase. Similarly, in the synthesis of ornithine, a dichotomy of pathways is observed with different organisms. In gram positive bacteria and animal tissues, ornithine probably arises from glutamate via glutamic γ-semialdehyde, but in gram negative bacteria, like E. coli, ornithine is produced through a pathway involving N-acetylated derivatives of glutamic acid (Vogel and Bonner, 1959).