Sucrose, estimation reactions

When a mixture of UDPG and D-fructose is subjected to the reaction of enz3nne preparations from these plant sources in the presence of inorganic phosphate buffer at pH 7.0, a nonreducing disaccharide, identified as sucrose, is formed. The equilibrium constant, Ky for this reaction at 37°C was found to be between 2 and 8, and the AF was estimated to be approximately —1000 cal. as compared to +1770 cal. for the sucrose phosphorylase reaction. In contrast to the sucrose phosphorylase reaction, the equilibrium of the reaction starting with UDPG and D-fructose is in favor of sucrose synthesis. 

Glucose and fructose (Atlas S3) are simple carbohydrates with the molecular formula C6H12O6. Sucrose (Atlas S5), or table sugar, is a complex carbohydrate with molecular formula CijHjjOn that consists of a glucose unit covalently linked to a fructose unit (a water molecule is released as a result of the reaction between glucose and fructose to form sucrose). Estimate the standard enthalpy of combustion of sucrose from the standard enthalpies of formation of the reactants and products. 

Estimation of Catalytic Activity. The catalytic activities of the copolymers on the hydrolysis of polysaccharides were estimated, with the measurement of increase in reducing sugar in the reaction mixture with reaction time according to Somogyi method (22). The hydrolysis rates of sucrose were determined from the measurement of optical rotatory power... 

Evidence consistent with this conclusion has been recently obtained by Pecina et al. (2003) in the genetically-engineered DAT knockdown mice, who carry a subtotal reduction in the expression of DAT which results in an increased steady-state level of extracellular DA (Pecina et al., 2003). Compared to the wild-type mice, the knockdown mice show faster running for food in a straight runway and an increased food intake, which results in an increased body weight. This increased motivation for food was not the result of increased rewarding properties of food as estimated from the hedonic reactions to intraoral infusion of sucrose. These studies therefore are consistent with the idea that sweet reward is independent from DA and that DA plays a role in the incentive, rather than the rewarding properties of food. 

From the data of part F, estimate the rate constant for the acid-catalyzed reaction. Also calculate a turnover number for the acid-catalyzed reaction, i.e., the number of molecules of sucrose hydrolyzed per second per hydrogen ion present. Note that Eq. (10) predicts that this turnover number depends on (S). Do your data confirm this ... 

Figure 11.2 Kinetic plots (initial rate versus sucrose concentration) of fructosyltransferase from A aculeatus. Transfer activity ( ) and hydrolytic activity (o). Reactions were carried out in 0.2 M sodium acetate buffer (pH 5.5) at 60°C. Kinetic parameters were calculated estimating a molecular mass of 135 kDa for the active enzyme. Adapted from Ref [33].

As long ago as 1913, Leonor Michaelis, director of the biochemical laboratory of the Berlin Municipal Hospital, and Maud Leonora Menten, a young Canadian pathologist in Europe for further training, had published their analysis of the reaction between sucrose and invertase, the enzyme that catalyzes hydrolysis of sucrose. Michaelis and Menten dissolved sucrose in acetate buffer and, holding the solution at 25 C, measured its optical activity. At zero time they added the enzyme and observed the change in optical activity until the reaction was complete. Michaelis and Menten extrapolated the data to zero time in order to estimate the initial velocity of the reaction, and they repeated the experiments with three different concentrations of substrate and three of enzyme. 

Stitt M. 1989. Control of sucrose synthesis estimation of free energy charges, investigation of the contribution of equilibrium and non-equilibrium reactions and estimation of elasticities and flux control coefficients. In Barber, J. (ed.) Techniques and new Developments in Photosynthetic Research. Plenum Press, London, pp. 365-392. 

The diffusivity of sucrose inside the pellet was also separately measured, in the Na resin form DeA = 2.69 x 10 cmVs. More exact values for the resin form were also computed from the reaction data but as an example of the estimation of an effectiveness factor, the Na value will be used here. 

Example 16.1 Estimate the equilibrium constant for the conversion of sugar (sucrose) to ethanol at 25°C= 298.15 K. The overall reaction is... 

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