Question

13. How to calculation standard free energy change of a given biochemical reaction according to its equilibrium constant, K.? What does it mean for a biochemical reaction when K'>,, or <1? 20. Use an example to explain addition-elimination reactions.

Answer 1

Biological systems are generally held at constant temperature and pressure, it is possible to predict the direction of a chemical reaction by using a measure of potential energy called free energy, or G. our concern is with relative, rather than absolute, values of free energy — in particular, with the difference between the values before and after the change. This free-energy change ΔG, where Δ stands for difference, is given by

AG - Gproducts - Greactants

Gibbs showed that free energy can be defined as

G = H - TS

where H is the bond energy, or enthalpy, of the system; T is its temperature in degrees Kelvin (K); and S is a measure of randomness, called entropy. If temperature remains constant, a reaction proceeds spontaneously only if the freeenergy change ΔG in the following equation is negative:

∆G = ∆H - T∆S

Most biological reactions differ from standard conditions, particularly in the concentrations of reactants. However, we can estimate free-energy changes for different temperatures and initial concentrations, using the equation:

In a reaction A + B ⇌ C, in which two molecules combine to form a third, the equation for ΔG becomes

[C] AG = AGⓇ' + RT InQ = AGⓇ' + RT In '[A][B]

where R is the gas constant of 1.987 cal/(degree · mol), T is the temperature (in degrees Kelvin), and Q is the initial ratio of products to reactants.

The ΔG°′ of a Reaction Can Be Calculated from Its Keq

A chemical mixture at equilibrium is already in a state of minimal free energy: no free energy is being generated or released. Thus, for a system at equilibrium, we can write

0 = AG = AGⓇ + RT In Q

At equilibrium the value of Q is the equilibrium constant Keq, so that

AGⓇ' = - RT In Keg

Expressed in terms of base 10 logarithms, this equation becomes

AGⓇ' = -2.3RT log Ke

=> Keq = 10(-∆G°/2.3RT)

• If Keq = 1, then the position of equilibrium is in the center, the amount of products is roughly equal to the amount of reactants at equilibrium.

If Keq is smaller than 1, then the position of equilibrium is to the left; more reactants are present at equilibrium.

If Keq is larger than 1, then the position of equilibrium is to the right; more products are present at equilibrium.

20. Addition-elimination reaction is a two-stage reaction process of an addition reaction followed by an elimination reaction. This gives an overall effect of substitution, and is the mechanism of the common nucleophilic acyl substitution often seen with esters, amides, and related structures.

Example:

ОН О + Гон — NO2 NO2

Mechanism:

èè : X- addition + OH -:0 elimination