Enthalpy of enzyme catalyzed reactions

Most of the others were obtained from measurement of apparent equilibrium constants and transformed enthalpies of enzyme-catalyzed reactions. pAs of species are also needed. These calculations, which are described in detail in Chapter 6, have been greatly facilitated by the publications by Goldberg and Tewari of evaluated data from the literature. 

The first is experimental data on apparent equilibrium constants and transformed enthalpies of enzyme-catalyzed reactions. R. N. Goldberg and Y. B. Tewara have evaluated these data in the literature and have published six review articles in J. Phys. Chem. Ref. Data. In addition R. N. Goldberg, Y. B. Tewara, and T. N. Bhat have put up a web site to assist in use these data. 

Temperature is a useful variable that reflects analytical information by associating with biological functional elements. An enzyme thermistor utilizing a commercialized thermistor is a kind of resistor that detects resistance changes as a function of the ambient temperature. In this system, the molar enthalpies of enzyme-catalyzed reactions are sensed by the thermal detector. 

R. A. Alberty, Use of standard Gibbs energies and standard enthalpies of adenosine (aq) and adenine(aq) in the thermodynamics of enzyme-catalyzed reactions, J. Chem. Thermo. 36, 593-601 (2004). 

In addition to its biochemical importance, CM has drawn attention from chemists looking to study enzyme activity for three primary reasons. First, the substrate 1 binds to the enzyme CM withont forming any covalent linkages, so that major electronic reorganizations do not need to be considered. Second, unlike many enzyme-catalyzed reactions, the reaction mechanism for the conversion of 1 into 2 is the same in both the enzyme environment and in solution in the absence of the enzyme. Last, the kinetics of the enzyme activity are well known, with CM increasing the rate of the reaction by 10 over the rate in aqueous solution." This corresponds to a reduction in the activation enthalpy of 20.7 0.4 kcal moL in solution to 15.9 kcal mol in the enzymatic environment. The activation entropy is -12.9 0.4 eu in solution but is reduced to essentially nil in the enzyme. In other words, AG = 24.5 kcal moL in solution but only 15.4 kcal mol in CM. ... 

When enzyme-catalyzed reactions are studied at a series of temperatures or there are calorimetric data, it is possible to calculate in addition Aj H ° and Ar 5 provided that the temperature dependencies of the p fs have been determined. In this chapter we have emphasized calculations at 298.15 K, including Ar H ° and Ar 5 but we have not fully utilized the enthalpy information. In Chapter 4, we will use the enthalpy information to calculate transformed thermodynamic properties at other temperatures. This will make it possible to utilize more Maxwell relations that show how various transformed thermodynamic properties are necessarily interrelated. 

This reaction is spontaneous mainly because of the change in the transformed entropy. Note the entropy increases because is produced. For most of these reactions, the enthalpy change at pH 7 determines whether the reaction goes to the right or the left, but sometimes the entropy change does, especially when hydrogen ions are produced by the reaction. Standard transformed thermodynamic properties are given for more enzyme-catalyzed reactions in Chapter 13. 

This chapter has been about calculating species properties from apparent equilibrium constants and transformed enthalpies of reaction, but there is a prior question. Where is the experimental data Fortunately, Goldberg, Tewari, and coworkers have searched the literature for these data, have evaluated it, and have published a series of review articles (10-15). These review articles provide thermodynamic data on about 500 enzyme-catalyzed reactions involving about KXX) reactants. In principle all these reactants can be put into thermodynamic tables. Goldberg, Tewari, and Bhat (16) have produced a web site to assist in the acquisition of data from the review articles. 

This chapter demonstrates the usefulness of BasicBiochemDataS (1). In the future this database can be extended and many reactions can be added to this list of 229 enzyme-catalyzed reactions. At the present time Af is known for the species of 94 reactants. Future measurements of A, // ° and enthalpies of dissociation of weak acids will be used to calculate Af H° of species of more reactants so that the effects of temperature can be calculated for more reactions. When more heat capacities of species have been determined, it will be possible to make calculations of K over wider ranges of temperature. BasicBiochemDataS contains functions of pH and ionic strength for A, G ° of these 229 enzyme-catalyzed reactions so that A, G ° and A, A h can be calculated for these reactions at 298.15 K, pHs in the range 5 to 9, and ionic strengths in the range zero to 0.35 M. The index given in the Appendix lists the EC numbers for reactions that involve these 167 reactants. 

This chapter has emphasized again the advantage of having A, G ° for an enzyme-catalyzed reaction as a function of temperature, pH, and ionic strength. If magnesium ions or other ions are bound by reactants, the free concentrations of more ions can be included as independent variables. This chapter has also emphasized the value of calorimetric data. More standard transformed enthalpies of reaction need to be measured so that temperature effeets can be calculated for more reactions. The database can also be extended by use of reliable estimation methods based on species properties. This may be especially useful with larger biochemical reactants where reactive sites are nearly independent. 

The enthalpy // of a chemical reaction system is of special interest because when a reaction occurs at constant temperature and pressure, the change in enthalpy Ar H is equal to the heat q of reaction. The change in enthalpy in a chemical reaction is also of interest because it determines the change in the equilibrium constant K with temperature. Similarly, the transformed enthalpy of an enzyme-catalyzed reaction is of special interest because when the reaction occurs at constant temperature, pressure, and pH, the change in transformed enthalpy // is equal to the heat q of the enzyme-catalyzed reaction. 

When the apparent equilibrium constant K for an enzyme-catalyzed reaction depends on pH and pMg, the calorimetric enthalpy of reaction Af//(cal) is given by (2)... 

Chemical reaction systems are discussed in terms of species, and many chemical thermodynamic properties can be calculated from the species properties given in the next section, for example pKs. However, in making calculations on enzyme-catalyzed reactions it is useful to take the pH as an independent variable. When this is done the principal thermodynamic properties of a reactant are the standard transformed Gibbs energy of formation Af G the standard transformed enthalpy of formation A( H the standard transformed entropy of formation Af 5 and the average number of hydrogen atoms in the reactant 77h. These properties are related by the following equations ... 

Apparent equilibrium constants cannot be determined experimentally on reactions that go nearly completion. Calorimetric measurements of enthalpies of reaction do not have this problem. Proteins may be reactants in enzyme-catalyzed reactions. When apparent equilibrium constants can be measured on reactions involving proteins, the thermodynamic properties of the reaction site in the protein can be calculated. 

The enthalpy difference between these alternative reaction pathways (<2 kcal/mol) is insufficient to define a preferred transition state structure. Thus, no firm inference regarding the conformation of the intermediate in the enzyme catalyzed reaction may be drawn on the basis of the molecular orbital calculations alone. In principle, the two pathways should differ... 

Enzyme-catalyzed reactions exhibit the same enthalpy change as spontaneous chemical reactions, but inasmuch as they increase the reaction rate, also the rate of enthalpy change is substantially enhanced. Therefore thermometric indication is universally applicable in enzyme... 

Molar Enthalpies of Some Enzyme-Catalyzed Reactions... 

Another type of sequential coupling is provided by cycling reactions. The product of the primary enzyme reaction is regenerated to the substrate of this reaction, i.e., the analyte, in a second, enzyme-catalyzed reaction. These cycles are based on the dependence of the two enzymes on different cofactors thus, the required free enthalpy exists for both reactions. The analyte molecule may be regarded as a catalyst of the reaction between the two cofactors. This results in a rate of cofactor conversion and enthalpy production that is enormously higher than that in a single enzyme reaction. These cycling reactions therefore lead to a substantial increase of sensitivity. 

Almost every reaction that occurs is accompanied by a change in enthalpy, which can be measured as a rise or fall in temperature in adiabatic systems of known specific heat capacity. Furthermore, the reaction of interest can often be coupled with other reactions that can amplify the enthalpy change, a common example being enzyme-catalyzed reactions that involve a proton exchange being performed in tris (hydroxymethyl) aminomethane buffer. 

Table 1 lists the molar enthalpy changes of some common enzyme-catalyzed reactions. Because enth-alpimetric measurements are based on the sum of all enthalpy changes in the reaction mixture, it is common to coimmobilize oxidases with catalase, which approximately doubles the sensitivity at the same time as the oxygen consumption is reduced and... 

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