Enthalpy biochemical reaction

Clearly, Aid is equal to the heat transferred in a constant pressure process. Often, because biochemical reactions normally occur in liquids or solids rather than in gases, volume changes are small and enthalpy and internal energy are often essentially equal. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

Processes at constant pressure. Chemical and biochemical reactions are much more likely to be conducted at constant pressure (usually 1 atm) than they are at constant volume. For this reason, chemists tend to use the enthalpy H more often than the internal energy E. 

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

Enthalpy is a function of state that is closely related to energy but is usually more pertinent for describing the thermodynamics of chemical or biochemical reactions. The change in enthalpy (AH) is related to the change in energy... 

The change in enthalpy AH is given by the expression AH = AE + A(PV). For most biochemical reactions, AE and AH are nearly equal. The organic molecules found in cells generally have much higher enthalpies than the simpler molecules from which they are built. 

Why can we equate internal energy and enthalpy for most biochemical reactions ... 

Enthalpies of Reaction from the Standard Enthalpies of Formation of Species II 10.3 Calculation of Standard Transformed Entropies of Biochemical Reactions I, 10.4 Effect of Temperature... 

The transformed enthalpy of a biochemical reaction is a function of temperature, pH, and ionic strength. Knowledge of AfG°, AfH°, and CPm for all the species in a biochemical reaction makes it possible to calculate ArG °, ArH °, ArC P°, and K for the biochemical reaction at the desired T, pH, and ionic strength. Note that when ions are involved there is an electrostatic contribution that varies with temperature (see Section 3.7). 

When a biochemical reaction that is affected by pH and pMg is carried out in a calorimeter in a buffer, the hydrogen ions and magnesium ions that are produced or consumed react with the buffer to produce a heat effect that is characteristic of the buffer, rather than the reaction being studied. Therefore this contribution should be calculated and should be used to correct the calorimetric heat effect to obtain the standard transformed enthalpy of the biochemical reaction ArH °. The analysis by Alberty and Goldberg (1993) shows that the enthalpy change in the calorimetric experiment ArH(cal) is given by... 

If the enthalpies of formation of all the species involved are known, the standard transformed enthalpy of a biochemical reaction Ar// ° at a specific T, P, pH, etc.,... 

The determination of standard transformed enthalpies of biochemical reactions at specified pH, either from temperature coefficients of apparent equilibrium constants or by calorimetric measurements, makes it possible to calculate the corresponding standard transformed entropy of reaction using... 

Calorimetric measurements yield enthalpy changes directly, and they also yield information on heat capacities, as indicated by equation 10.4-1. Heat capacity calorimeters can be used to determine Cj , directly. It is almost impossible to determine ArCp° from measurements of apparent equilibrium constants of biochemical reactions because the second derivative of In K is required. Data on heat capacities of species in dilute aqueous solutions is quite limited, although the NBS Tables give this information for most of their entries. Goldberg and Tewari (1989) have summarized some of the literature on molar heat capacities of species of biochemical interest in their survey on carbohydrates and their monophosphates. Table 10.1 give some standard molar heat capacities at 298.15 K and their uncertainties. The changes in heat capacities in some chemical reactions are given in Table 10.2. 

R. A. Alberty and R. N. Goldberg, Calorimetric dDetermination of the standard transformed enthalpy of a biochemical reaction at specilicd pH and pMg, Biophys. Client. 47, 213-223 (1993). 

The Vj are the stoichiometric numbers of reactants in the biochemical equation (positive for reactants on the right side of the equation and negative for reactants on the left side). The prime is needed on the stoichiometric numbers to distinguish them from the stoichiomeric numbers in the underlying chemical reactions. The standard transformed enthalpy of reaction Ar H(heat of reaction) is related to the standard transformed enthalpies of formation Af //,of the reactants by... 

The values for enthalpy and Gibbs energy changes quoted in textbooks for biochemical reactions are standard values, AH° and AG0, when the reactions take place in solution under standard conditions. All components are in their standard states at standard concentration of 1 mol.dnr3 and the H ion activity is taken to be 10 7. Actual AG and AH values in cells and tissues may be markedly different in the case of AG, even to the extent of a different sign depending on the concentrations of the reactants. It must be emphasized that AG for a particular reaction provides no information about the rate of that reaction. 

This heat change is called the enthalpy change for a reaction and is symbolized by AH. Most biochemical reactions occur in solution in these cases volume changes are negligible, and thus A// = At/. 

Given information on the standard enthalpies of formation for the species in a reaction, this expression may be used to estimate the standard free energies of formation at one temperature T2 as functions of the values of A fG° at another temperature 7).5 The temperature-corrected estimates of A fG° yield temperature-corrected estimates of ArG° through Equation (2.23). This ability to correct for temperature is important because the majority of biochemical reaction data are tabulated at 25°C, while human beings and many other warm-blooded creatures operate at temperatures near 37°C. 

This program is applied to the following reactions for which standard enthalpies of formation are known for all species. It is convenient to keep track of biochemical reactions by using their EC numbers (5). 

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. 

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