Metabolic reactions, inhibition organic acids

Lycorine (1), the most frequent and characteristic of the Amaryllidaceae alkaloids, has been reported to be a powerful inhibitor of ascorbic acid (l-Asc) biosynthesis (436,437), and thus has proved to be a useful tool in studying Asc-dependent metabolic reactions in L-Asc-synthesizing organisms (438,439). Lycorine is actually a powerful inhibitor of the activity of L-galactono-y-lactone dehydrogenase, the terminal enzyme of l-Asc biosynthesis (440-443), which appears to be localized in the mitochondrial membrane (444,445). Galanthine (7) also has a high capacity to inhibit ascorbic acid biosynthesis (437). 

Dihydropteroic acid (85) is an intermediate to the formation of the folic acid necessary for intermediary metabolism in both bacteria and man. In bacteria this intermediate is produced by enzymatic condensation of the pteridine, 86, with para-amino-benzoic acid (87). It has been shown convincingly that sulfanilamide and its various derivatives act as a false substrate in place of the enzymatic reaction that is, the sulfonamide blocks the reaction by occupying the site intended for the benzoic acid. The lack of folic acid then results in the death of the microorganism. Mammals, on the other hand, cannot synthesize folic acid instead, this compound must be ingested preformed in the form of a vitamin. Inhibition of the reaction to form folic acid Ls thus without effect on these higher organisms. 

Nevertheless, for the production of the flavour-active aromatic alcohol derivatives, such as the corresponding aldehydes and acids, metabolic engineering approaches have to compete with conventional oxidative biocatalysis starting from the natural alcohol as a substrate. For instance, the whole-cell oxidation system based on Pichia pastor is AOX already described in Sect. 23.4.1.2 can also be used to convert benzyl alcohol to benzaldehyde in aqueous media although product inhibition restricted the final product concentration to about 5 g L h indicating the need for aqueous-organic two-phase reaction media [51]. Phenylacetalde-... 

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