1887

Abstract

The -leucine dehydrogenase (LeuDH, EC 1.4.1.9) from the extremely thermophilic ‘’ (DSM 405) has been purified to homogeneity and crystallized. Its physical and biochemical properties, such as subunit structure, , amino acid composition, isoelectric point, heat stability, pH-optima, values for coenzymes and substrates, and pattern of inhibition have been determined. The native enzyme is an octamer ( 320000) composed of identical subunits ( 39000) which contain one intrachain disulphide bridge. Only aliphatic amino acids were accepted as substrates and a preference was exhibited for branched-chain residues. Inhibition occurred only upon incubation with thiol reagents such as HgCl and -mercuribenzoate, and pyridoxal 5′-phosphate. The LeuDH was very thermostable, with 50% residual activity remaining after 30 min incubation at 80 °C. Its properties, as well as amino acid composition and N-terminal sequence, were compared with those of the phylogenetically related LeuDH from the mesophile . Both enzymes displayed very similar properties: the quaternary structures were identical, amino acid composition alike, the N-terminal sequence showed 62% homology for the first 19 residues, and values for the different substrates differed by a factor of two or less, indicating that the active centres had a similar structure. Thus, the only major difference observed was the much higher stability of the thermophilic enzyme to denaturation by heat and organic solvents.

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/content/journal/micro/10.1099/00221287-135-5-1305
1989-05-01
2019-10-19
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