Leucine dehydrogenase properties

We shall now briefly outline some of the features of the zinc metalloenzymes which have attracted most research effort several reviews are available, these are indicated under the particular enzyme, and for more detailed information the reader is referred to these. Attention is focussed here, albeit briefly, on carbonic anhydrases,1241,1262,1268 carboxypeptidases, leucine amino peptidase,1241,1262 alkaline phosphatases and the RNA and DNA polymerases.1241,1262,1462 Finally, we examine alcohol dehydrogenases in rather more detail to illustrate the use of the many elegant techniques now available. These enzymes have also attracted much effort from modellers of the enzymic reaction and such studies, which reveal much interesting coordination chemistry and often new catalytic properties in their own right—and often little about the enzyme system itself (except to indicate possibilities), will be mentioned in the next section of this chapter. 

These characteristics of lysine producers are combined to produce much stronger lysine producing strains. In addition to these fundamental properties, further addition of leucine requiring mutation is effective to increase the amount of lysine since in the mutant dihydrodipycolinate synthase is released from repression by leucine. The precursors of lysine synthesis include phosphoenol-pyruvate, pyruvate, and acetylCoA. In addition, many mutations are induced in the lysine producers to supply sufficient amounts of these precursors in good balance. These are deletion mutants of pyruvate kinase and show low activity of pyruvate dehydrogenase, etc. Furthermore, an alanine requirement was also reported to be effective in increasing the lysine amount. 

There are several examples of d to l inversion of amino acids in the literature. D-Phenylalanine may have therapeutic properties in endogenous depression and is converted to L-phenylalanine in humans [145]. o-Leucine is inverted to the L-enantiomer in rats. When o-enantiomer is administered, about 30% of the enantiomer is converted to the L-enantiomer with a measurable inversion from l to o-enantiomer. As indicated in Fig. 13, D-leucine is inverted to the L-enantiomer by two steps. It is first oxidized to a-ketoisocarproate (KIC) by o-amino acid oxidase. This a-keto acid is then asymmetrically reaminated by transaminase to form L-leucine. In addition, KIC may be decarboxylated by branched-chain a-keto acid dehydrogenase, resulting in an irreversible loss of leucine (Fig. 13) [146]. D-Valine undergoes a similar two-step inversion process, and this can be antagonized by other amino acids such as o-leucine. The primary factor appears to be interference with the deamination process [147]. 

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