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Activities and regulation of the enzymes involved in the first and the third steps of the aspartate biosynthetic pathway in Enterococcus faecium
Authors:E O Kalcheva  V O Shanskaya  S S Maliuta
Institution:(1) Institute of Molecular Biology and Genetics, Academy of Sciences of Ukraine, 2521 43 Kiev, Ukraine;(2) Demeevskaya Street 55/32, 252109 Kiev, Ukraine
Abstract:The enzymes aspartokinase and homoserine dehydrogenase catalyze the reaction at key branching points in the aspartate pathway of amino acid biosynthesis. Enterococcus faecium has been found to contain two distinct aspartokinases and a single homoserine dehydrogenase. Aspartokinase isozymes eluted on gel filtration chromatography at molecular weights greater than 250,000 and about 125,000. The molecular weight of homoserine dehydrogenase was determined to be 220,000. One aspartokinase isozyme was slightly inhibited by meso-diaminopimelic acid. Another aspartokinase was repressed and inhibited by lysine. Although the level of diaminopimelate-sensitive (DAPs) enzyme was not much affected by growth conditions, the activity of lysine-sensitive (Lyss) aspartokinase disappeared rapidly during the stationary phase and was depressed in rich media. The synthesis of homoserine dehydrogenase was controlled by threonine and methionine. Threonine also inhibited the specific activity of this enzyme. The regulatory properties of aspartokinase isozymes and homoserine dehydrogenase from E. faecium are discussed and compared with those from Bacillus subtilis.
Keywords:Enterococcus faecium  Aspartokinase isozymes  Homoserine dehydrogenase  Regulation of activity
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