Biochemical reaction thermodynamics temperature

The thermodynamic principles described in Chapter 2 of this volume can be used to indicate whether or not a reaction can take place spontaneously. They do not, however, provide information about the rate at which a reaction will proceed. Most biochemical reactions proceed so slowly at physiological temperatures that catalysis is essential for the reactions to proceed at a satisfactory rate in the cell. 

Because cryosolvents must be used in studies of biochemical reactions in water, it is important to recall that the dielectric constant of a solution increases with decreasing temperature. Fink and Geeves describe the following steps (1) preliminary tests to identify possible cryosolvent(s) (2) determination of the effect of cosolvent on the catalytic properties (3) determination of the effect of cosolvent on the structural properties (4) determination of the effect of subzero temperature on the catalytic properties (5) determination of the effect of subzero temperature on the structural properties (6) detection of intermediates by initiating catalytic reaction at subzero temperature (7) kinetic, thermodynamic, and spectral characterization of detected intermediates (8) correlation of low-temperature findings with those under normal conditions and (9) structural studies on trapped intermediates. 

When an enzyme-catalyzed biochemical reaction operating in an isothermal system is in a non-equilibrium steady state, energy is continuously dissipated in the form of heat. The quantity J AG is the rate of heat dissipation per unit time. The inequality of Equation (4.13) means that the enzyme can extract energy from the system and dissipate heat and that an enzyme cannot convert heat into chemical energy. This fact is a statement of the second law of thermodynamics, articulated by William Thompson (who was later given the honorific title Lord Kelvin), which states that with only a single temperature bath T, one may convert chemical work to heat, but not vice versa. 

R. A. Alberty, Effect of temperature on the standard transformed thermodynamic properties of biochemical reactions with emphasis on the Maxwell equations, J. Phys. Chem. 107 B, 3631-3635 (2003). (Supporting Information is available.)... 

An important concept in thermodynamics is the equilibrium state, which will be di,s-cussed in detail in the following. sections. Here we merely note that if a system is not subjected to a continual forced flow of mass, heat, or work, the system will eventually evolve to a time-invariant state in which there are no internal or external flows of heat or mass and no change in composition as a result of chemical or biochemical reactions. This state of the system is the equilibrium state. The precise nature of the equilibrium state depends on both the character of the system and the constraints imposetl on the system by its immediate sutrounding.s and its container (e.g., a constant-volume container fixes the system volume, and a thermostatic bath fixes the system temperature see Problem 1.1). 

The Gibbs free energy is a remarkable thermodynamic quantity. Because so many chemical reactions are carried out under conditions of near-constant pressure and temperature, chemists, biochemists, and engineers use the sign and magnitude of AG as exceptionally useful tools in the design of chemical and biochemical reactions. We will see examples of the usefulness of AG throughout the remainder of this chapter and this text... 

Almost all biochemical reactions are catalysed by enzymes, which are found both inside and outside body cells. Enzymes are a special kind of catalyst which are proteins and which are effective in extremely small concentrations. Their mode of action remains imperfectly understood, but they make possible many chemical reactions which would not otherwise occur at the dilutions and comparatively low temperatures at which life cells operate. Many different enzymes are produced by a single variety of plant or animal species moreover, the same enzymes are usually found in many different varieties of life forms. Enzymes are essential for the normal functioning and development of the human body, and failure to produce even one of them may result in a metabolic disorder. Enzymes are generally far more efficient than ordinary catalysts and can increase reaction rates by as much as 10 -10 times or even more. They are not used up in the reaction and do not influence any equilibrium point. Enzymes lower the activation energy of a reaction but cannot make a thermodynamically unfavourable process favourable. Enzyme-catalysed reactions can give yields of nearly 100% without any by-products. 

In the frame of the conventional thermodynamic approach, the change in AH and AS with the temperature is caused exclusively by the change in the Boltzmann distribution of reacting molecules, while the profiles of the potential wells (Fig. 2.1) remain temperature-independent. As we will see below, the latter condition could be violated in many biochemical reactions. 

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