INTRODUCTION 339 



term substrate to the hydrogen donor, and use the term acceptor 

 (hydrogen acceptor) for oxygen or hydrogen peroxide, except in the 

 case of catalase. 



Considered from the general viewpoint of biological oxidation, there is no 

 principal difference between substrates and acceptors, both forming a ternary 

 system with the catalyst, except that the latter has a higher oxidation- 

 reduction potential. In the cases we discuss, however, there is the additional 

 difference that the substrate is bound to the protein, the acceptor to the 

 heme iron of the reactive complex. 



In general it can be observed that the catalytic activity (catalatic, 

 peroxidative, or oxidative) of a free hematin or a simple hematin 

 compound {e.g., a hemochrome) is far lower than that of the specific 

 hemoprotein enzymes found in the cell. The same holds if we com- 

 pare the catalytic activity of a hemoprotein specially adapted to one 

 kind of catalysis {e.g., catalatic destruction of hydrogen peroxide) 

 with a hemoprotein specially adapted to other reactions. Thus the 

 combination of protoheme with globin or w ith the protein of horse- 

 radish peroxidase does not increase its catalatic activity to anything 

 approaching that of catalase. 



Reaction Velocities and Thermodynamics. In previous chapters we 

 have been concerned mainly with problems of affinity. When dis- 

 cussing the catalytic activity of a substance, it must be realized first 

 that we are dealing with reaction velocities, and that we must not 

 expect to find a straightforward relationship between affinities and 

 reaction velocities, except in certain reactions {179,3016). Affinity 

 considerations will still be of importance, however, since they show 

 us whether a reaction is thermodynamically possible, and give us 

 indications in which direction it can be expected to proceed. Secondly, 

 deductions from the true equilibrium conditions cannot always be 

 applied to the false equilibrium that we find in the living cell. In a 

 chain of oxidation-reduction processes of the kind observed in the 

 cells, the system with the higher potential, E'o, will as a rule supply the 

 oxidizing component, and the system with lower E'o the reducing 

 component. In the cell, however, we find instances where the reverse 

 occurs. In two systems Oi -\- Ri and O2 + R2, where and R signify 

 oxidized and reduced forms, and the potential of system 1 is assumed 

 to be higher than that of system 2, the equilibrium : 



Oi + /?o - /?! -f O2 

 will normally lie toward the right and the reaction will proceed 



