Product/reactant ratio

A 10-fold difference in Keq or the products/reactants ratio changes AG by 1.36 kcal/mol. 

What s important in determining how much free energy is available from a given chemical reaction is how far the reaction is from its equilibrium position. One way to decide this is to take the ratio of the prod-uct/reactant ratio at equilibrium to the actual product/reactant ratio ... 

AG° is the free-energy change for a reaction under conditions where the product/reactant ratio is l.1 Don t get confused on this point—AG° is not the free-energy change at equilibrium (that s zero), it s the free energy change when the products/reactants ratio is 1. AG° is a way to compare different reactions to decide which one is intrinsically more favorable. The comparison is made, by convention, at a product/reactant ratio of 1. Just because a reaction has a negative AG° doesn t mean that it can t be made... 

The product/reactant ratio may have units if there are more product terms than reactant terms, or vice versa. For example, if there are two products and one reactant, the product/reactant ratio will have molar units (M). In this case, a products/reactants ratio of 1 means that the products/reactants ratio is actually 1 M. The term molar standard state means that we re talking about a products/reactants ratio that has molar units. 

UNITS The products/reactants ratios may have units associated with them. For example, a reaction of the type A B + C has a products/reactants ratio that has molar units. What you do when you take the log of a products/reactants ratio with molar units is ignore the units. You ve not really made them disappear, you ve just ignored them. The way physical chemist types make this difficult is that they call ignoring the units an assumption of standard state. It does matter, though. If you assume the units are molar (M), the products/reactants ratio has one... 

With a Keq of 2.3 X 105 M, the AG0 is -1.36 log10 (2.3 X 105) = -7.3 kcal/mol. This would be the AG if the products/reactants ratio were 1, but it s not. Let s assume that the local concentration of ATP in a cell is 5 mM, [ADP] is 60 pM, and [PJ is 5 mM (these are approximately right, but they will vary from cell to cell and at any given time in a cell). Keep in mind that this also means the amount of free energy available from ATP hydrolysis will vary from cell to cell and from time to time. 

With these concentrations, the products/reactants ratio becomes... 

AG° is the free-energy change for a reaction under conditions where the product/reactant ratio is 1. Don t get confused on this point—ACf is not the free-energy change at equilibrium (that s zero), it s the free energy... 

By NMR. Time required for conversion of half of the vinyl compound to saturated products. Reactant ratios 1 1, sealed tubes, 35"C (28). 

Under standard conditions, A G° = -RT n [product]/[reactants]. Substituting +5.7 kcal mok for A G° and solving for [products]/[reactants] yields 7 x iQ-s. in other words, the forward reaction does not take place to a significant extent. Under intracellular conditions, A G is 0.3 kcal mokk If one uses the equation A G = A G° + i rin [product]/ [reactants] and solves for [products]/[reactants], the ratio is 3.7 x io-5. Thus, a reaction that is endergonic under standard conditions can be converted into an exergonic reaction by maintaining the [products]/[reactants] ratio below the equilibrium value. This conversion is usually attained by using the products in another coupled reaction as soon as they are formed. 

A rapid method for the estimation of Xeq is to set up reaction mixtures containing known concentrations of aU reactants at some ratio close to /feq, and thus bracket the equilibrium position by varying the concentration of one reactant. After the addition of enzyme to each mixture, the change in concentration of one reactant is measured, and the change is plotted against the [product]/[reactant] ratio. The change in concentration of a chosen reactant can be positive or negative and where the line in the plot crosses zero, the [product]/[reactant] ratio equals Jfeq (Fig. 2). 

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