Homoisocitric acid

In fungi, lysine is synthesized via an a-aminoadipate pathway [33], The conversion of homoisocitric acid to a-ketoadipic acid occurs due to enzyme homoisocitrate dehydrogenase (EC 1.1.1.87, NAD-HDH) ... 

The common feature linking isocitric acid with homoisocitric acid is a malic acid backbone. The only difference between the two compounds is the length of the acid chain attached to the C-3 position. 

L-a-Aminoadipic acid is presumably the precursor of the 3-(L-a-aminoadipoyl)-side-chain of isopenicillin N formed by P. chrysogenum. Free a-aminoadipic acid isolated from the mycelium of this organism was found to be predominantly the L-isomer (Arnstein and Morris, 1960c). But L-a-aminoadipic acid- C added to the extracellular fluid of the Cephalosporium sp. is also a better precursor than the D-isomer of the 3-(D-a-aminoadipoyl) side-chain of penicillin N and cephalosporin C (Abraham et al., 1964). The does not appear to be due merely to the fact that L-a-aminoadipic acid enters the mycelium much more rapidly than D-a-amino-adipic acid. Added L-a-aminoadipic acid may be diluted in the amino acid pool of the mycelium by endogenous L-a-aminoadipic. The latter is synthesized by the Cephalosporium sp. from acetyl coenzyme A and a-oxoglutarate by the following route through homoisocitric acid (Trown et al., 1962 Trown et al., 1963), which is that by which it is also synthesized in yeast (Strassman and Ceci, 1964 Strassman, Ceci and Silverman, 1964) ... 

Strassman, M., L. N. Ceci, and B. E. Silverman Enzymatic conversion of homoisocitric acid into a-ketoadipic acid. Biochem. Biophys. Research Commun. 14, 268 (1964). 

This scheme is that acetate may condense with a-ketoglutarate to yield a homolog of citric acid, which, by undergoing a series of reactions analogous to the citric acid cycle, should yield homoisocitrate, oxalo-glutarate, a-ketoadipate, and ultimately, a-aminoadipic acid. 

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