Amino acid dehydrogenases

Abstract

Amino acid dehydrogenases (EC 1.4.1) are enzymes that catalyze the reversible oxidative deamination of amino acids to form the corresponding keto-acids, using NAD+ or NADP+ as cofactors. This chapter will focus on the homologous glutamate, leucine, valine and phenylalanine dehydrogenases that catalyze the oxidative deamination of the α-amino of the substrate, represented by the general equation: HOOC-R-C-H NH 2+NAD(P)++H2O→HOOC R-C=O+NH +4 +NAD(P)H Glutamate dehydrogenase (GluDH) is found in all three domains of life, being important for ammonia assimilation and for bridging carbon and nitrogen metabolism (Hudson and Daniel, 1993). Valine dehydrogenase (ValDH) was first reported in pea shoots and was subsequently found in members of the genus Streptomyces. ValDH is involved in the biosynthesis of n-butyrate which is a precursor of macrolide and polyether antibiotics in these bacteria (Tang et al., 1994; Leiser et al., 1996). ValDH has also been isolated from an Antarctic psychrophile Cytophaga sp. KUC-1 (Oikawa et al., 2001). Both Leucine (LeuDH) and phenylalanine dehydrogenases (PheDH) are usually restricted to Gram positive aerobic bacteria. Their physiological roles are not clear, although it has been suggested that they are involved in catabolism of their respective preferred amino acid substrates to provide cellular energy and nitrogen (Asano et al., 1987a; Massey et al., 1976). GluDH from eubacteria and fungi are usually NAD+ or NADP+ specific. The vertebrate and some archaeabacteria GluDH can use either coenzyme with equal efficacy. LeuDH, ValDH and PheDH from Bacteria are all NAD+ specific. Due to the stereospecific reaction catalyzed by the amino acid dehydrogenases, they are useful for the synthesis of amino acids for dietary or pharmaceutical purposes. The enzymes can also be used for quantitative determination of amino acids and ammonia in clinical applications. Members of this family of enzymes share sequence and structural similarity but may differ in terms of substrate specificity, stability, salt tolerance etc. GluDH, for example, can be found in organisms occupying different ecological niches and the enzymes are therefore "adapted" to different environmental conditions. Comparative structure-function studies over the years, beginning with the elucidation of the structure of GluDH from Clostridium symbiosum in 1985 (Rice et al., 1985), have provided a comprehensive understanding of the molecular basis of substrate recognition and thermostability in this family of enzymes. These are important parameters to consider in the biotechnological application of enzymes and deciphering the molecular determinants of such properties offer the potential to engineer desired characteristics in these biocatalysts. © 2007 Springer.