UDP glucose

The properties of the crystalline enzyme from calf liver have been studied by Levine et al. (1969). Despite several recrystallizations the enzyme preparations showed up to three minor components on analytical ultracentrifugation, sucrose density gradient centrifugation, or polyacrylamide gel electrophoresis. This was not due to the presence of impurities, but rather to the occurence of multimers of the enzyme, i.e., monomer, dimer, trimer, and tetramer. Evidence to this effect was obtained from the close correspondence between protein content and enzyme activity in the fractions isolated by density gradient centrifugation and gel electrophoresis. Furthermore, the separated fractions slowly redistributed to yield analytical patterns similar to that of the native enzyme, indicating that interconversion between the various molecular species was taking place. 

The molecular weight of the monomeric form of the enzyme was approximately 480,000, as calculated from sedimentation and diffusion coefficients. Dissociation in guanidine hydrochloride yielded subunits of mol. w. 69,100, suggesting the presence of 7 subunits per molecule. However, there was some uncertainty as to the true value for the partial specific volume, and the molecular weight of the subunits could actually be about 60,000. This value would correspond to 8 subunits per molecule, and electron micrographs of the crystalline enzyme rather lent support to an octomer model. 

UDP-glucose pyrophosphorylase has been purified approximately 16,000-fold from human erythrocytes to a specific activity about 65% of that reported for recrystallized liver enzyme (Tsuboi et al., 1969). The 

Studies of the kinetics of the purified erythrocyte enzyme showed a highly selective product inhibition by UDP-glucose. Distinct differences were observed in the rate patterns between the forward and reverse reactions the reverse reaction showed a constant reaction rate, whereas the rate of the forward reaction, i.e., in the direction of UDP-glucose synthesis, rapidly decreased. This was the result of inhibition by the UDP-glucose formed, and further kinetic analysis showed that there was competition between UDP-glucose and UTP for free enzyme. 

The pyrophosphorylase reaction had an absolute requirement for divalent cation, and magnesium, manganese, cobalt, and calcium were all active, the efficiency decreasing in this order. The activation by magnesium was examined for both the forward and the reverse reactions, and it was found that maximal effect was reached at concentrations which were equimolar to those of UTP and pyrophosphate, respectively, whereas the magnesium activation was independent of the concentrations of glucose 1-phosphate and UDP-glucose. 

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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 ... 

FIGURE 23.18 The UDP-glucose pyrophosphorylase reaction is a phosphoanhydride exchange, with a phosphoryl oxygen of glu-cose-l-P attacking the m-phosphorus of UTP to form UDP-glucose and pyrophosphate. 

Galactose, a constituent of the disaccharide lactose found in dairy products, is metabolized by a patiiwav that includes the isomerization of UDP-galactose to UDP-glucose. where UDP = uridylyl diphosphate. The enzyme responsible for the transformation uses NAD+ as cofactor. Propose a mechanism. 

Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-glucose by galactose 4-epimerase, where UDP = uridylyl diphosphate (a ribonucleotide diphosphate Section 28.1). The enzyme requires NAD+ for activity (Section 17.7), but it is not a stoichiometric reactant. and NADH is not a final reaction product. Propose a mechanism. 

P2Y14 Placenta, adipose tissue, stomach, intestine, discrete brain regions UDP glucose = UDP-galactose Gq/Gu... 

Figure 32-14. Conjugation of bilirubin with glucuronic acid. The glucuronate donor, UDP-glucuronic acid, is formed from UDP-glucose as depicted. The UDP-glucuronosyl-transferase is also called bilirubin-UGT.

Sasaki, N. et al.. Detection of UDP-glucose cyclo-DOPA 5-O-glucosyltransferase activity in four o clocks (Mirabilis jalapa L.), EEBS Lett, 568, 159, 2004. 

The next step involves a transfer of the glucose residue from UDP-glucose onto the glycogen primer through the aid of the enzyme glycogen synthetase ... 

If the synthesis starts from glucose molecules, then the initial step is the transfer of glucose residues from UDP-glucose onto an intermediary acceptor-dolichol phosphate (membrane-bound polyprenol phosphate). Dolichol phosphate assists in the synthesis of an... 

As evidence for reaction (1), 1 mole of D-glucosyl phosphate was formed per mole of UDP-glucose and D-galactosyl phosphate consumed when the last two compounds were incubated with an extract of Saccharomyces fragilis.13,w Experiments188 with C14-labeled D-glucosyl phosphate also favor the mechanism represented by reactions (1) and (2). 

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