Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.1.1.3 (HSD)
 3,464 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Aspartate kinase (AK) and homoserine dehydrogenase (HSDH) are enzymes in the aspartate-derived amino acid biosynthetic pathway. Recent biochemical evidence indicates that an AK-HSDH bifunctional enzyme exists in maize (Zea mays L.). In this report, we characterize three genes that encode subunits of AK-HSDH. Two cDNAs, pAKHSDH1 and pAKHSDH2, containing the full-coding sequence, and one partial cDNA, pAKHSDH3, encode amino acid sequences similar to the reported monofunctional AK and HSDH enzymes from prokaryotes and yeast (Saccharomyces cerevisiae) and to AK-HSDH bifunctional enzymes of prokaryotes, yeast, carrot (Daucus carota), and Arabidopsis thaliana. Immunological and biochemical analyses verify that the cDNAs encode AK-HSDH and indicate that both the AK and HSDH activities are feedback inhibited by threonine. RNA blots identify a 3.2-kb transcript in all maize tissues examined. pAKHSDH1 and pAKHSDH2 map to chromosomes 4L and 2S, respectively. This study shows that maize contains AK-HSDH bifunctional enzyme(s) encoded by a small gene family of at least three genes. Maize AK-HSDH has conserved sequences found in communication modules of prokaryotic two-component regulatory systems, which has led us to propose that maize AK-HSDH may be involved in a similar regulatory mechanism.
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PMID:Molecular genetics of the maize (Zea mays L.) aspartate kinase-homoserine dehydrogenase gene family. 784 52

Amplification of the operon homdr-thrB encoding a feedback-insensitive homoserine dehydrogenase and a wild-type homoserine kinase in a Corynebacterium lactofermentum lysine-producing strain resulted in both homoserine and threonine accumulation, with some residual lysine production. A plasmid enabling separate transcriptional control of each gene was constructed to determine the effect of various enzyme activity ratios on metabolite accumulation. By increasing the activity of homoserine kinase relative to homoserine dehydrogenase activity, homoserine accumulation in the medium was essentially eliminated and the final threonine titer was increased by about 120%. Furthermore, a fortuitous result of the cloning strategy was an unexplained increase in homoserine dehydrogenase activity. This resulted in a further decrease in lysine production along with a concomitant increase in threonine accumulation.
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PMID:Effect of inducible thrB expression on amino acid production in Corynebacterium lactofermentum ATCC 21799. 788 27

Two genes, hom (encoding homoserine dehydrogenase) and thrB (encoding homoserine kinase), of the threonine biosynthetic pathway are clustered in the chromosome of Brevibacterium lactofermentum in the order 5' hom-thrB 3', separated by only 10 bp. The Brevibacterium thrB gene is expressed in Escherichia coli, in Brevibacterium lactofermentum, and in Corynebacterium glutamicum and complements auxotrophs of all three organisms deficient in homoserine kinase, whereas the Brevibacterium hom gene did not complement two different E. coli auxotrophs lacking homoserine dehydrogenase. However, complementation was obtained when the homoserine dehydrogenase was expressed as a fusion protein in E. coli. Northern (RNA) analysis showed that the hom-thrB cluster is transcribed, giving two different transcripts of 2.5 and 1.1 kb. The 2.5-kb transcript corresponds to the entire cluster hom-thrB (i.e., they form a bicistronic operon), and the short transcript (1.1 kb) originates from the thrB gene. The promoter in front of hom and the hom-internal promoter in front of thrB were subcloned in promoter-probe vectors of E. coli and corynebacteria. The thrB promoter is efficiently recognized both in E. coli and corynebacteria, whereas the hom promoter is functional in corynebacteria but not in E. coli. The transcription start points of both promoters have been identified by primer extension and S1 mapping analysis. The thrB promoter was located in an 87-bp fragment that overlaps with the end of the hom gene. A functional transcriptional terminator located downstream from the cluster was subcloned in terminator-probe vectors.
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PMID:Transcriptional analysis and regulatory signals of the hom-thrB cluster of Brevibacterium lactofermentum. 796 9

When cells of Escherichia coli are grown on lactate (or other carbon sources), an addition of serine to the medium causes growth inhibition. This growth inhibition is caused by inhibition by serine of homoserine dehydrogenase I, which is involved in threonine-isoleucine biosynthesis [Hama, H., Sumita, Y., Kakutani, Y., Tsuda, M., & Tsuchiya, T. (1990) Biochem. Biophys. Res. Commun. 168, 1211-1216]. We have cloned and sequenced genes which enhance the serine-sensitivity. Two open reading frames were found and designated as sseA and sseB. Introduction of either sseA or sseB gene, or both, into E. coli cells enhanced the serine-sensitivity. The sseA gene elicited stronger enhancement than sseB. The deduced amino acid sequence of SseA showed considerable similarity with that of bovine liver rhodanese, which catalyzes sulfur transfer from thiosulfate. We observed a twofold increase in rhodanese activity in E. coli cells harboring a plasmid carrying the sseA gene. The position of sseA in the genetic map is around 52'. However, sseA is different from cysM, which codes for O-acetylserine sulfhydrylase-B, an enzyme catalyzing sulfur transfer from thiosulfate to O-acetylserine, the map position of which is also around 52'.
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PMID:Enhancement of serine-sensitivity by a gene encoding rhodanese-like protein in Escherichia coli. 798 94

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.
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PMID:Activities and regulation of the enzymes involved in the first and the third steps of the aspartate biosynthetic pathway in Enterococcus faecium. 800 14

The Gram-positive bacterium Corynebacterium glutamicum is used for the industrial production of amino acids, e.g. of L-glutamate and L-lysine. In the last ten years genetic engineering methods were developed for C. glutamicum and consequently, recombinant DNA technology was employed to study the biosynthetic pathways and to improve the amino acid productivity by manipulation of enzymatic, transport and regulatory functions of this bacterium. The present review summarizes the current knowledge on the synthesis and over-production of the aspartate derived amino acids L-lysine, L-threonine and L-isoleucine in C. glutamicum. A special feature of C. glutamicum is its ability to convert the lysine intermediate piperideine2,6-dicarboxylate to diaminopimelate by two different routes, i.e. by reactions involving succinylated intermediates or by the single reaction of diaminopimelate dehydrogenase. The flux distribution over the two pathways is regulated by the ammonium availability. The overall carbon flux from aspartate to lysine, however, is governed by feedback-control of the aspartate kinase and by the level of dihydrodipicolinate synthase. Consequently, expression of lysCFBR encoding a deregulated aspartate kinase and/or the overexpression of dapA encoding dihydrodipicolinate synthase led to overproduction of lysine. As a further specific feature C. glutamicum possesses a specific lysine export carrier which shows high activity in lysine overproducing mutants. Threonine biosynthesis is in addition to control by the aspartate kinase tightly regulated at the level of homoserine dehydrogenase which is subject to feedback-inhibition and to repression. C. glutamicum strains possessing a deregulated aspartate kinase and a deregulated homoserine dehydrogenase produce lysine and threonine. Amplification of deregulated homoserine dehydrogenase in such strains led to an almost complete redirection of the carbon flux to threonine. For a further flux from threonine to isoleucine the allosteric control of threonine dehydratase and of the acetohydroxy acid synthase are important. The expression of the genes encoding the latter enzyme is additionally regulated at the transcriptional level. By addition of 2-oxobutyrate as precursor and by bypassing the expression control of the acetohydroxy acid synthase genes high isoleucine overproduction can be obtained.
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PMID:Molecular aspects of lysine, threonine, and isoleucine biosynthesis in Corynebacterium glutamicum. 809 56

The first and the third steps in the aspartate biosynthesis pathway in Streptococcus bovis are catalyzed by two different forms of aspartokinase and a single homoserine dehydrogenase, respectively. These enzymes can be separated by ammonium sulfate fractionation and gel filtration on Sephadex G-200. The two aspartokinase isozymes differ in molecular weights and are subject to differential regulation. The aspartokinase system of S. bovis is characterized by the absence of specific negative allosteric effectors among the end products of the synthesis of amino acids of the aspartic family. Homoserine dehydrogenase, which catalyzes the third step of the aspartic family amino acid synthesis, also has such negative effectors as threonine and methionine. The aspartokinase isozymes do not form multienzyme complexes with homoserine hydrogenase in S. bovis.
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PMID:[Analysis of key enzyme activities involved in aspartate amino acid biosynthesis in Streptococcus bovis]. 811 40

The gene encoding Arabidopsis thaliana aspartate kinase (ATP:L-aspartate 4-phosphotransferase, EC 2.7.2.4) was isolated from genomic DNA libraries using the carrot ak-hsdh gene as the hybridizing probe. Two genomic libraries from different A. thaliana races were screened independently with the ak probe and the hsdh probe. Nucleotide sequences of the A. thaliana overlapping clones were determined and encompassed 2 kb upstream of the coding region and 300 bp downstream. The corresponding cDNA was isolated from a cDNA library made from poly(A)(+)-mRNA extracted from cell suspension cultures. Sequence comparison between the Arabidopsis gene product and an AK-HSDH bifunctional enzyme from carrot and from the Escherichia coli thrA and metL genes shows 80%, 37.5% and 31.4% amino acid sequence identity, respectively. The A. thaliana ak-hsdh gene is proposed to be the plant thrA homologue coding for the AK isozyme feedback inhibited by threonine. The gene is present in A. thaliana in single copy and functional as evidenced by hybridization analyses. The apoprotein-coding region is interrupted by 15 introns ranging from 78 to 134 bp. An upstream chloroplast-targeting sequence with low sequence similarity with the carrot transit peptide was identified. A signal sequence is proposed starting from a functional ATG initiation codon to the first exon of the apoprotein. Two additional introns were identified: one in the 5' non-coding leader sequence and the other in the putative chloroplast targeting sequence. 5' sequence analysis revealed the presence of several possible promoter elements as well as conserved regulatory motifs. Among these, an Opaque2 and a yeast GCN4-like recognition element might be relevant for such a gene coding for an enzyme limiting the carbon-flux entry to the biosynthesis of several essential amino acids. 3' sequence analysis showed the occurrence of two polyadenylation signals upstream of the polyadenylation site. This work is the first report of the molecular cloning of a plant ak-hsdh genomic sequence. It describes a promoter element that may bring new insights to the regulation of the biosynthesis of the aspartate family of amino acids.
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PMID:Molecular analysis of the aspartate kinase-homoserine dehydrogenase gene from Arabidopsis thaliana. 820 22

The essential amino acids lysine and threonine are synthesized in higher plants by two separate branches of a common pathway. This pathway is primarily regulated by three key enzymes, namely aspartate kinase (AK), dihydrodipicolinate synthase (DHPS) and homoserine dehydrogenase (HSD), but how these enzymes operate in concert is as yet unknown. Addressing this issue, we have expressed in transgenic tobacco plants high levels of bacterial AK and DHPS, which are much less sensitive to feedback inhibition by lysine and threonine than their plant counterparts. Such expression of the bacterial DHPS by itself resulted in a substantial overproduction of lysine, whereas plants expressing only the bacterial AK overproduced threonine. When both bacterial enzymes were expressed in the same plant, the level of free lysine exceeded by far the level obtained by the bacterial DHPS alone. This increase, however, was accompanied by a significant reduction in threonine accumulation compared to plants expressing the bacterial AK alone. Our results suggested that in tobacco plants the synthesis of both lysine and threonine is under a concerted regulation exerted by AK, DHPS, and possibly also by HSD. We propose that the balance between lysine and threonine synthesis is determined by competition between DHPS and HSD on limiting amounts of their common substrate 3-aspartic semialdehyde, whose level, in turn, is determined primarily by the activity of AK. The potential of this molecular approach to increase the nutritional quality of plants is discussed.
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PMID:Concerted regulation of lysine and threonine synthesis in tobacco plants expressing bacterial feedback-insensitive aspartate kinase and dihydrodipicolinate synthase. 825 29

As an approach in the study of the evolution of threonine biosynthetic pathways throughout various organisms, the sequences of three enzymes, namely homoserine dehydrogenase, homoserine kinase and threonine synthase, originating from six organisms, namely Escherichia coli, Bacillus subtilis, Corynebacterium glutamicum, Brevibacterium lactofermentum, Pseudomonas aeruginosa and Saccharomyces cerevisiae, were compared. As a general trend all three enzymatic activities were carried out by proteins sharing sequence relatedness (except for the homoserine kinase of P aeruginosa). Unexpectedly however, for each step one or two enzymes stood out of the main stream: i) for homoserine dehydrogenase, the yeast protein is atypically similar to the E coli enzyme; ii) for homoserine kinase, the P aeruginosa protein shares no similarity with any other species; and iii) for threonine synthase, the B subtilis protein is far distant from the enzymes of other species. Hence in contrast to other biosynthetic pathways such as the tryptophan one, the threonine pathway seems not to have evolved as a whole throughout different organisms but rather each step seems to have been subjected to multiple constraints including substrate-mediated ones and host-specific ones.
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PMID:Evolutionary comparisons of three enzymes of the threonine biosynthetic pathway among several microbial species. 839 99


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