Composition enzyme-catalyzed

In a special circumstance the rate equation for parallel reactions may be misleading.If two parallel reactions are catalyzed by a common catalyst, and if a significant fraction of the catalyst is tied up in the form of intermediates, then the two reactions are not independent, and the rate equation will not give the transition state composition. King has analyzed this case in terms of enzyme-catalyzed reactions. 

The protocol described in Section 7.1.2 involves isotopic competition, but with the different isotopomers held in separate containers. Equations 7.10 to 7.13 apply equally well to a type of competition experiment known in biochemistry as the perturbation method for determining KIE s of reversible enzyme catalyzed reactions. The perturbation method differs from simultaneous non-competitive measurements in several important ways. One begins by mixing equilibrium concentrations of substrate and product but with one component (substrate or product) at a different isotopic composition than the other. Thus, the mixture is in chemical, but not isotopic equilibrium. At this stage no enzyme is present and the interconversion is... 

The assays for PGase activity presented here are based on measurements of reaction rates. Hence, all experimental parameters that may affect the rate of an enzyme-catalyzed reaction (including pH, ionic strength, buffer composition, and temperature) need to be defined. 

Basch, J. J., Farrell, H. M., Jr., Walsh, R. A., Konstance, R. P., and Kumosinski, T. F. (1989). Development of a quantitative model for enzyme-catalyzed, time-dependent changes in protein composition of Cheddar cheese during storage. ]. Dairy Sci. Til, 591-603. 

Systems of biochemical reactions like glycolysis, the citric acid cycle, and larger and smaller sequential and cyclic sets of enzyme-catalyzed reactions present challenges to make calculations and to obtain an overview. The calculations of equilibrium compositions for these systems of reactions are different from equilibrium calculations on chemical reactions because additional constraints, which arise from the enzyme mechanisms, must be taken into account. These additional constraints are taken into account when the stoichiometric number matrix is used in the equilibrium calculation via the program equcalcrx, but they must be explicitly written out when the conservation matrix is used with the program equcalcc. The stoichiometric number matrix for a system of reactions can also be used to calculate net reactions and pathways. 

The equilibrium composition for an enzyme-catalyzed reaction or a series of enzyme-catalyzed reactions can be calculated by using equcalcc or equcalccrx. The first of these programs requires a conservation matrix. The second requires a stoichiometric matrix. The second program is recommended, especially when water is involved as a reactant, because the convention that when dilute aqueous solutions are considered, the activity of water is taken to be unity, means that a second Legendre transform is necessary. 

When accurate data can be obtained over a range of both concentrations and temperatures, it is possible from the Michaelis-Menton model to obtain data on the first-order rate constant kz and the constant Km = kz + kz)/ki and their apparent activation energies Ez and Unfortunately, most of the values quoted in the literature for the activation energies of enzyme-catalyzed reactions are derived from the use of overly simple first-order equations to describe the reaction. Consequently these values are a composite of Kmj kzy and the other constants in the Michaelis-Menton equation and cannot be used for interpretive purposes. Where the constants have been separated it is found that the values of Ez are low and of order of magnitude of 5 to 15 Kcal/mole. It is of interest to note that enzyme preparations from different biological sources, which may show different specific activity for a given reaction, have very nearly the same temperature coefficient for their specific rate constants. ... 

Enzyme Source mass (kDa) Composition Conversion catalyzed PDB code ... 

Equilibrium compositions of systems of chemical reactions or systems of enzyme-catalyzed reactions can only be calculated by iterative methods, like the Newton-Raphson method, and so computer programs are required. These computer programs involve matrix operations for going back and forth between conservation matrices and stoichiometric number matrices. A more global view of biochemical equilibria can be obtained by specifying steady-state concentrations of coenzymes. These are referred to as calculations at the third level to distinguish them from the first level (chemical thermodynamic calculations in terms of species) and the second level (biochemical thermodynamic calculations at specified pH in terms of reactants). 

Acidolysis. This reaction can be acid or enzyme catalyzed and may be used to modify triacylglycerol composition. Acidolysis of an oil containing only Cis and Cjg fatty acids with fatty acids rich in lauric acid (e.g., from palm-kemel oil) results in a triacylglycerol enriched in medium-chain fatty acids. 

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