ANTIGENS AS BIOCATALYSTS 27 



Schultz and Stacey, 1944); yeast hexokinase (Kunitz and McDonald, 

 1946); lif oxidase (Theorell, Holman and Akeson, 1947); bacterial 

 a-amylase (Meyer, Fuld and Bernfeld, 1947); pancreatic desoxyriho- 

 nuclease (Kunitz, 1948). 



The fundamental characteristic of all catalyzed processes is that they 

 are reactions which, in the thermodynamic sense,* are classed as 

 spontaneously occurring processes. That is, they are reactions which oc- 

 cur with diminution of free energy. Any added substance which can 

 accelerate such a reaction is known as a catalyst. A catalyst accelerates 

 slowly progressing reactions and enables them to attain the equilibrium 

 condition in a very much shorter time. The equilibrium point is not 

 changed but the time necessary to attain this point is shortened. 



To illustrate this point we may use the following example. We 

 know that iron can combine with oxygen to form various iron oxides. 



*The themiodjTiamic criterion for a reaction to occur spontaneously is that it do so 

 with a loss in free energy of the system, change in free energy being the difference of 

 the free energies of a system in its initial and final states. Hence, any system, left to 

 itself, will change in such a way as to approach a point of equilibrium where the 

 change in free energy will equal zero. Thus, for example, in a system consisting of 

 two bodies at different energy levels there will be transfer of energy from one to the 

 other at different rates. The body at the higher energy level, transferring energy at a 

 greater rate, tends to raise the energy level of the second body until a condition is 

 reached where both bodies are at the same energy level. No energy is thereby lost, 

 but the capacity for spontaneous change has vanished, and the system is said to have 

 less free energy. Tree energy is a function which indicates the direction in which 

 chemical or other processes take place, and is of great theoretical value in fixing the 

 conditions of equilibrium. The fundamental conception is that of a reversible cycle 

 of operations. The condition of reversibility is that the state of the system at any time 

 does not differ sensibly from equilibrium, for then the slightest variation in the con- 

 ditions will determine the occurrence of the process in the one direction or the other. 



Kinetic theory postulates that chemical reactions take place only when molecules 

 collide. However from chemical kinetics it has been demonstrated that only those 

 collisions between reactants are effective in which the joint energy contributed by the 

 molecules is equal to or greater than a certain minimum energy value termed 

 energy of activation. 



A catalyst does not affect the point of equilibrium but accelerates the rate at which 

 the equilibrium state is attained. Thermodynamically this may be expressed by saying 

 that the change of free energy involved in a chemical reaction is the same whether 

 a catalyst is present or not. Kinetically, this means that the catalyst accelerates the rate 

 of the reverse reaction to the same extent as that of the forward reaction, so that the 

 equilibrium constant, equal to the ratio of these rates, is the same for the catalyzed 

 and uncatalyzed reactions. The function of the catalyst is to bring about the desired 

 reaction with a smaller energy of activation. A lower energy of activation gives a 

 more rapid reaction because more molecules have the necessary amount of energy to 

 react. The high energy requirement is avoided by some by-pass. Usually the by-passing 

 consists in forming an unstable activated cofwplex with less energy consumption and 

 then decomposing this intermediate compound in such a way as to regenerate the 

 catalyst. In this way the catalyst is used over and over again. 



