Vibrio cholerae (also Kommabacillus) is a gram negative curved-rod shaped bacterium with a polar flagellum that causes cholera in humans. V. cholerae and other species of the genus Vibrio belong to the gamma subdivision of the Proteobacteria. There are two major strains of V. cholerae, classic and El Tor, and numerous other serogroups.
V. cholerae was first isolated as the cause of cholera by Italian anatomist Filippo Pacini in 1854, but his discovery was not widely known until Robert Koch, working independently thirty years later, publicized the knowledge and the means of fighting the disease.
V. cholerae colonizes the gastrointestinal tract, where it adheres to villi. It produces "Zona Occludans Toxin". This toxin specifically attacks the zona occludans or "tight" junctions joining epithelial cells. V. cholerae strains possessing this higher virulence were found to possess an increase in transcription of genes coding for biosynthesis of amino acids, iron uptake systems, ribosomal proteins, and formation of a periplasmic nitrate reductase complex that may allow for respiration under low oxygen tension, all of which enhance proliferation in the rice water stool that is characteristic of cholera infection.
The 4.0 Mbp genome of N16961, an O1 serogroup, El Tor biotype, 7th pandemic strain of V. cholerae, is comprised of two circular chromosomes of unequal size that are predicted to encode a total of 3,885 genes. The genomic sequence of this representative strain has furthered our understanding of the genetic and phenotypic diversity found within the species V. cholerae.
In plants and microorganisms, all the key aromatic compounds involved in primary metabolism, including the three aromatic amino acids found in proteins, are produced via shikimate pathway. The shikimate pathway is essential in algae, higher plants, bacteria, and fungi, but absent from mammals that depend on these compounds for your diet. Vibrio cholerae appears to be capable of synthesizing an extensive variety of enzyme cofactors and prosthetic groups, including biotin, folic acid, pantothenate and coenzyme A, ubiquinone, glutathione, thioredoxin, glutaredoxin, riboflavin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), pyrimidine nucleotides, porphyrin, thiamin, pyridoxal 50-phosphate, and lipoate.
The shikimate pathway consists of seven enzymes that catalyze the sequential conversion of erythrose-4-phosphate and hosphoenolpyruvate to chorismate. All pathway intermediates can also be considered branch point compounds that may serve as substrates for other metabolic pathways.
The molecular organization and structure of the shikimate pathway enzymes varies considerably between microrganism groups. Bacteria have seven individual polypeptides, each possessing a single enzyme activity, which are encoded by separated genes. Plants have a molecular arrangement similar to bacteria, i.e., separated enzymes encoded by separated genes, with the exception of dehydroquinase and shikimate dehydrogenase, which have been shown to be present as separated domains on a bifunctional polypeptide. Plant enzymes, although nuclear encoded, are largely active in the chloroplast and accordingly possess an N-terminal transit sequence. In contrast, all fungi examined to date have monofunctional 3-deoxy-D-arabino-heptulosonate-7-phosphate synthases and chorismate synthases and a pentafunctional polypeptide termed AROM.
Chorismate is converted by five distinct enzymes to prephenate, anthranilate, aminodeoxychorismate, isochorismate, and p-hydroxybenzoate. These metabolites comprise the first committed intermediates in the biosynthesis of Phe, Tyr, Trp, folate, menaquinone, and the siderophore enterobactin, and ubiquinone, respectively. The synthesis of these precursors is in most cases highly regulated. In monogastric animals, Phe and Trp are essential amino acids that have to come with the diet and Tyr is directly derived from Phe. Since bacteria use in excess 90% of their metabolic energy for protein biosynthesis, for most prokaryotes, the three aromatic amino acids represent nearly the entire output of aromatic biosynthesis, and regulatory mechanisms for shikimate pathway activity are triggered by the intracellular concentrations of Phe, Tyr, and Trp. Among the many aromatic secondary metabolites are flavonoids, many phytoalexins, indole acetate, alkaloids such as morphine, UV-light protectants, and, most important, lignin. In microrganisms, the shikimate pathway is regulated both by feedback inhibition and by repression of the first enzyme 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHPS).
Shikimate pathway enzymes are attractive targets for drug development. Knowledge of the three-dimensional structures of the enzymes will undoubtedly aid in the design of useful inhibitors that may be used as a bactericide against V. cholerae.
The third step of the shikimate pathway, dehydration of DHQ to give 3-dehydroshikimate (DHS), is catalyzed by DHQD.
This enzyme belongs to the super family of NAD(P)H dependent oxidoreductases, which function in anabolic and catabolic enzyme pathways as well as in xenobiotic detoxification. This super family is usually subdivided into several families, including short chain dehydrogenases, medium chain dehydrogenases, aldo-keto reductases, iron-activated alcohol dehydrogenases, and long chain dehydrogenases .
The overall general topology of the DHQD consists of a five-stranded b-sheet core flanked by four a-helices.
The enzymes catalyze a trans (anti)-dehydration which results in the loss of the 2-pro-S hydrogen, probably via an enolate intermediate. The systematic name of this enzyme class is 3-dehydroquinate hydro-lyase (3-dehydroshikimate-forming). Other names in common use include 3-dehydroquinate hydrolase, DHQase, dehydroquinate dehydratase, 3-dehydroquinase, 5-dehydroquinase, dehydroquinase, 5-dehydroquinate dehydratase, 5-dehydroquinate hydro-lyase, and 3-dehydroquinate hydro-lyase. This enzyme participates in phenylalanine, tyrosine and tryptophan biosynthesis.
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