ADVERTISEMENTS:
In this article we will discuss about the free energy of activation and the role of catalysts.
An exergonic reaction is a spontaneous reaction; it can therefore yield energy usable for chemical, mechanical or electrical work. However, it should be remembered that spontaneous does not mean rapid, but only without external energy input. A spontaneous exergonic reaction can be extremely slow.
The reaction of glucose with oxygen is exergonic. However, glucose can be left in contact with the oxygen of air for months or years without any appreciable oxidation. The reaction is slow. To accelerate it, one or more catalysts must be used.
ADVERTISEMENTS:
This is naturally what takes place in living organisms which must, in certain physiological conditions, utilise in few minutes the energy available after the oxidation of glucose.
Figure 2-1 introduces the notion of free energy of activation. To pass from A + B to C + D the reactive path must pass through a “privileged state” of reactivity called activated complex. To transform the molecules A and B into activated complex, free energy must be supplied to the system: this quantity of free energy to be supplied is called free energy of activation.
At a given temperature, the greater the concentration of activated complex, the higher the reaction velocity (which is proportional to this concentration). If temperature is raised, more energy is supplied to the system by increasing molecular agitation; the concentration of activated complex increases and the reaction becomes more rapid. Any chemical reaction, with or without catalyst, becomes faster when temperature rises.
At a given temperature, the lower the amount of free energy of activation to be supplied, the greater the concentration of activated complex. Catalysts, and among them enzymes which are biological catalysts, act by considerably decreasing the free energy of activation. The smaller the quantity of free energy of activation to be supplied, the more effective the catalyst (see fig. 2-1).
The main properties of a catalyst may be briefly summarised as follows:
1. At the end of the reaction the catalyst is found intact, although during the reaction it was combined with the molecules which reacted. The catalyst can also accelerate the velocity of numerous successive cycles of a given reaction and that is why it acts in small doses.
2. The catalyst, does not in any manner, alters the final equilibrium of a reversible reaction; it simply allows this equilibrium to be reached more rapidly by accelerating in the same manner the velocities of the 2 reactions proceeding simultaneously in opposite directions.
Enzymes or biochemical catalysts, also possess the above properties; they differ from chemical catalysts by the following properties:
1. They further decrease the energy of activation (see fig. 2-1). For example, the decomposition of H2O2 requires 18 kcal/mole without catalyst and 11.7 kcal/mole in presence of colloidal platinum, but only 2 kcal/mole in presence of the enzyme, catalase. (One molecule of catalase allows the decomposition of 5 X 106 molecules of H2O2 in one minute, in optimal conditions of temperature and pH).
This decrease is due to the fact that the enzymatic reaction proceeds in a path other than the non-enzymatic pathway comprising in most cases, several successive steps where the energy barriers — which the molecules must cross in order to form an activated complex — are smaller.
2. They are all of protein nature. As will be seen later, this gives them a very high specificity. There are hundreds of different enzymes to catalyze the diverse reactions taking place in cells of living organisms, even if a particular reaction is considered (dehydrogenation for example), the enzyme which catalyzes it can differ according to the compound (or substrate) undergoing the transformation.
3. Their activity can be controlled which — is of very great importance for cell metabolism.
