Thermodynamics apparent equilibrium constant

The second reaction studied using lipase as catalyst was the reversible re-gioselective esterification of propionic acid and 2-ethyl- 1,3-hexanediol [180]. While the previously described reaction was almost irreversible, this reaction is equilibrium limited with an apparent equilibrium constant of 0.6 0.1. In addition, the accumulated water inhibits the enzyme. Therefore, only the removal of the water from the reaction zone assures high enzymatic activity as well as drives the reaction beyond thermodynamic equilibrium. Experiments with two... 

Whenever reporting equilibrium constants, detailed information concerning the reaction conditions should always be indicated. Alberty has also presented an important review of biochemical thermodynamics in which he discusses the apparent equilibrium constant for biochemical reactions (K ) in terms of sums of reactant species. 

Activity coefficients and concentration equilibrium constants. Strictly speaking, Eq. 6-31 applies only to thermodynamic equilibrium constants -that is, to constants that employ activities rather than concentrations. The experimental determination of such constants requires measurements of the apparent equilibrium constant or concentration equilibrium constant21 Kc at a series of different concentrations and extrapolation to infinite dilution (Eq. 6-32). 

This introductory chapter describes the thermodynamics of biochemical reactions in terms of equilibrium constants and apparent equilibrium constants and avoids references to other thermodynamic properties, which are introduced later. 

The procedure for calculating standard formation properties of species at zero ionic strength from measurements of apparent equilibrium constants is discussed in the next chapter. The future of the thermodynamics of species in aqueous solutions depends largely on the use of enzyme-catalyzed reactions. The reason that more complicated ions in aqueous solutions were not included in the NBS Tables (1992) is that it is difficult to determine equilibrium constants in systems where a number of reactions occur simultaneously. Since many enzymes catalyze clean-cut reactions, they make it possible to determine apparent equilibrium constants and heats of reaction between very complicated organic reactants that could not have been studied classically. 

A good deal of work will have to be done to extract species information from the apparent equilibrium constants that have been reported for about 500 reactions. Beyond that, use can be made of analogies with known reactions for example, the various ribonucleotide phosphates (AMP, GMP, CMP, UMP, and dTMP) are believed to have the same hydrolysis constants and pKs. Beyond that, the group additivity method (Alberty, 1998c) can be used to estimate thermodynamic properties. 

Since the standard thermodynamic properties of adenosine have been determined (ref. 7), new values are given for the ATP series. These changes do not change the values of apparent equilibrium constants that are calculated between reactants in this series, but will be useful in investigating the production of adenosine.. 

These tables have been given to 0.01 kJ mol-1. In general this overemphasizes the accuracy with which these formation properties are known. However for some reactants for which species are in classical tables, this accuracy is warranted. An error of 0.01 kJ mol-1 in the standard transformed Gibbs energy of a reaction at 298 K corresponds with an error of about 1 % in the value of the apparent equilibrium constant. It is important to understand that the large number of digits in these tables is required because the thermodynamic information is in differences between entries. 

This field owes a tremendous debt to the experimentalists who have measured apparent equilibrium constants and heats of enzyme-catalyzed reactions and to those who have made previous thermodynamic tables that contain information needed in biochemical thermodynamics. 

Conditional (apparent) equilibrium constants - Equilibrium constants that are determined for experimental conditions that deviate from the standard conditions used by convention in - thermodynamics. Frequently, the conditional equilibrium conditions refer to - concentrations, and not to - activities, and in many cases they also refer to overall concentrations of certain species. Thus, the formal potential, i.e., the conditional equilibrium constant of an electrochemical equilibrium, of iron(II)/iron(III) may refer to the ratio of the overall concentrations of the two redox forms. In the case of complex equilibria, the conditional - stability constant of a metal ion Mm+ with a ligand L" refers to the overall concentration of all complex species of Mm+ other than Conditional equilibrium... 

Equation (2.12) is the convenient form for biochemists who are interested in tracking reactant concentrations and apparent equilibrium constants and free energies Equation (2.13) is the physicochemically rigorous equation from which we can build relationships for the apparent thermodynamics of Equation (2.12). 

For a given reaction, the expression used to model the flux must be constrained based on the apparent equilibrium constant and overall transformed thermodynamic driving force. Specifically it is required that the flux goes to zero when the reaction reaches equilibrium and that the forward and reverse fluxes satisfy the relationship AG = —RT n(J+/J ) introduced in Section 3.1.2. 

Generally only one equilibrium constant is used to describe the reaction between gaseous carbon dioxide and H2C0. The thermodynamic and apparent equilibrium constants for the above reactions are ... 

Calculation of Standard Thermodynamic Properties of Species of a One-species Reactant from the Apparent Equilibrium Constant at 298.15 K and the Standard Transformed Enthalpy of Reaction at 313.15 K... 

Chapters 3-5 have described the calculation of various transformed thermodynamic properties of biochemical reactants and reactions from standard thermodynamic properties of species, but they have not discussed how these species properties were determined. Of course, some species properties came directly out of the National Bureau of Standard Tables (1) and CODATA Tables (2). One way to calculate standard thermodynamic properties of species not in the tables of chemical thermodynamic properties is to express the apparent equilibrium constant K in terms of the equilibrium constant K of a reference chemical reaction, that is a reference reaction written in terms of species, and binding polynomials of reactants, as described in Chapter 2. In order to do this the piiTs of the reactants in the pH range of interest must be known, and if metal ions are bound, the dissociation constants of the metal ion complexes must also be known. For the hydrolysis of adenosine triphosphate to adenosine diphosphate, the apparent equilibrium constant is given by... 

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