Equilibrium constant, biochemical reactions

Equilibrium constant of formation, 5-43 to 65 Equilibrium constant, biochemical reactions, 7-16 to 18... 

Enzymes increase the rate but do not influence the equilibrium of biochemical reactions Enzymes are highly efficient in their catalytic power displaying rate enhancement of 106 to 1012 times those of uncatalyzed reactions without changing the equilibrium constants of the reactions. 

The common complexes of the M(II) and M(III) transition-metal ions of the first transition series are labile, except for Cr H) and low-spin Co(III), Fe(II) and Fe(III). These labile ions form a wide range of complexes of general chemical and biochemical importance. As a result, there have been many studies of the kinetics and equilibrium constants for reactions of the general form... 

For biochemical reactions in which hydrogen ions (H ) are consumed or produced, the usual definition of the standard state is awkward. Standard state for the ion is 1 M, which corresponds to pH 0. At this pH, nearly all enzymes would be denatured, and biological reactions could not occur. It makes more sense to use free energies and equilibrium constants determined at pH 7. Biochemists have thus adopted a modified standard state, designated with prime ( ) symbols, as in AG°, AH°, and so on. For values determined... 

A certain enzyme-catalyzed reaction in a biochemical cycle has an equilibrium constant that is 10 times the equilibrium constant of the next step in the cycle. If the standard Gibbs free energy of the first reaction is —200. k -mol 1, what is the standard Gihhs free energy of the second reaction ... 

All enzymatic reactions are initiated by formation of a binary encounter complex between the enzyme and its substrate molecule (or one of its substrate molecules in the case of multiple substrate reactions see Section 2.6 below). Formation of this encounter complex is almost always driven by noncovalent interactions between the enzyme active site and the substrate. Hence the reaction represents a reversible equilibrium that can be described by a pseudo-first-order association rate constant (kon) and a first-order dissociation rate constant (kM) (see Appendix 1 for a refresher on biochemical reaction kinetics) ... 

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. 

Both ways of writing a metabolic reaction have value in biochemistry. Chemical equations are needed when we want to account for all atoms and charges in a reaction, as when we are considering the mechanism of a chemical reaction. Biochemical equations are used to determine in which direction a reaction will proceed spontaneously, given a specified pH and [Mg24], or to calculate the equilibrium constant of such a reaction. 

The addition of an enolate anion to C02 to form a (3-oxoacid represents one of the commonest means of incorporation of C02 into organic compounds. The reverse reaction of decarboxylation is a major mechanism of biochemical formation of C02. The equilibrium constants usually favor decarboxylation but the cleavage of ATP can be coupled to drive carboxylation when it is needed, e.g., in photosynthesis. 

A chemist suspects that manganese(III) might be involved in an unusual biochemical reaction and wants to prepare some of its compounds. Could aqueous potassium permanganate be used to oxidize manganese(II) to manganese(III) If so, what would be the equilibrium constant for the reaction ... 

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. 

In considering reactions in biochemical systems it is convenient to move the activity coefficients into the equilibrium constants. For example, the equilibrium constant expression for the dissociation of a weak acid can be written as follows ... 

Biochemical textbooks often add a H+ on the right-hand side, but this is stoichiometrically incorrect when the pH is held constant, as we will see in the next section. It is also wrong, in principle, as we will see in Chapter 4, since hydrogen atoms are not balanced by biochemical reactions because the pH is held constant. The statement that the pH is constant means that in principle acid or alkali is added to the reaction system as the reaction occurs to hold the pH constant. In practice, a buffer is used to hold the pH nearly constant, and the pH is measured at equilibrium. 

The equations and calculations described in this chapter are very useful, but so far we have not discussed thermodynamic properties other than equilibrium constants. The other properties introduced in the next three chapters provide a better understanding of the energetics and equilibria of reactions. We will consider the basic structure of thermodynamics in Chapter 2 and then to apply these ideas to chemical reactions in Chapter 3 and biochemical reactions in Chapter 4. 

Equation 4.1-18 can be used to derive the expression for the apparent equilibrium constant K for a biochemical reaction at a specified pH. If a single biochemical reaction is catalyzed, the amounts n of the pseudoisomer groups at each stage of the reaction are given by... 

K and went on to calculate A,G ° and AfH ° at pH 7 and ionic strength 0.25 M for the corresponding reactants. This made it possible to calculate apparent equilibrium constants for six biochemical reactions at 283.15 and... 

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