UDP glucose, Formation

Notice that this enzyme is named for the reverse reaction in the cell, the reaction proceeds in the direction of UDP-glucose formation, because pyrophosphate is rapidly hydrolyzed by inorganic pyrophosphatase (Fig. 15-7). 

The net flux in vivo through SuSy can be altered by a relatively small shift in the mass action ratio away from its equilibrium position (Keq). To obtain net flux in favor of UDP-glucose formation, the reaction must be displaced from equilibrium... 

Sugar nucleotides are formed from sugar-l-phosphates and nucleoside triphosphates by specific pyrophosphorylase enzymes (Figure 23.18). For example, UDP-glucose pyrophosphorylase catalyzes the formation of UDP-glucose from glucose-l-phosphate and uridine 5 -triphosphate ... 

The R locus determines the presence (R) or absence (r) of anthocyanins in the seed coat. R is required (with i and T) to produce black seed [10]. However the identity of the gene product encoded by this locus has not been reported. Todd and Vodkin [25] have demonstrated that brown seed coats (r) contain proanthocyanidin (PAs) and black seed coats (R) contain anthocyanins in addition to PAs and suggested that R acts subsequent to the formation of leucoanthocyanidin but previous to the formation of anthocyanins. UDP-glucose flavonoid 3-0-glucosyltransferase (UF3GT) should be considered a candidate gene of the R locus but its identiflcation has not yet been reported. 

The formation of glycosidic bonds between sugars is endergonic. Initially, therefore, the activated form—UDP-glucose—is synthesized by reaction of glucose 1-phosphate with UTP (see p. 110). 

From extensive analysis of recombinant proteins, and the crystal structure of A. thaliana protein, detailed reaction mechanisms have been proposed. The ANS reaction likely proceeds via stereospecific hydroxylation of the leucoanthocyanidin (flavan-3,4-cA-diol) at the C-3 to give a flavan-3,3,4-triol, which spontaneously 2,3-dehydrates and isomerizes to 2-flaven-3,4-diol, which then spontaneously isomerizes to a thermodynamically more stable anthocyanidin pseudobase, 3-flaven-2,3-diol (Figure 3.2). The formation of 3-flaven-2,3-diol via the 2-flaven-3,4-diol was previously hypothesized by Heller and Forkmann. The reaction sequence, and the subsequent formation of the anthocyanidin 3-D-glycoside, does not require activity of a separate dehydratase, which was once postulated. Recombinant ANS and uridine diphosphate (UDP)-glucose flavonoid 3-D-glucosyltransferase (F3GT, sometimes... 

The standard reaction mixture contained in a Anal volume of 0.25 mL 16 mM MgCl2, 6.4 mM UDP-glucose, 3.2 mM p-nitrophenol, enzyme extract (100-500 /tg of protein) and 0.1 M Tris-HCl buffer (pH 8.0). After a 20-minute incubation at 43°C, the reaction was terminated by the addition of 20 /aL of 40% perchloric acid. The supernate obtained by centrifugation was Altered and then subjected to HPLC analysis. Formation of product was linear with time up to 40 minutes and with protein added in the range of 0 to 5.5 mg protein/mL. Alternative substrates for the enzyme include 1-naphthol and 2-naphthol. 

Johansson, H., Sterky, F., Amini, B., Lundberg, J., and Kleczkowski, L.A., 2002, Molecular cloning and characterization of a cDNA encoding poplar UDP-glucose dehydrogenase, a key gene ofhemicellulose-/pectin formation. Biochim. Biophys. Acta 1576 53-58. 

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