CELL METABOLISM 59 



called "trace elements," manganese, copper, cobalt, zinc, iron, and 

 others— serve as enzyme activators. 



Enzymes may be present in the cell either dissolved in the liquid 

 part of protoplasm or bound to, and presumably an integral part of, 

 one of the cell particles. A water extract of ground liver contains all 

 of the eleven kinds of enzymes necessary to convert glucose to lactic acid. 

 The respiratory enzymes, which catalyze the metabolism of lactic acid 

 and the carbon chains of fatty acids and amino acids to carbon diox- 

 ide and water, are integral parts of the mitochondria. The microsomes 

 have been shown to contain a number of enzymes involved in the syn- 

 thesis of proteins, cholesterol and other complex molecules. 



The Mechanism of Enzyme Catalysis. Many years ago Emil Fischer, 

 the German organic chemist, suggested that the specificity of the rela- 

 tionship of an enzyme to its substrate indicated that the two must fit 

 together like a lock and key (Fig. 4.1). The idea that an enzyme combines 

 with its substrate to form a reactive intermediate enzyme-substrate com- 

 plex, which subsequently decomposes to release the free enzyme and the 

 reaction products, was formulated mathematically by Leonor Michaelis 

 more than forty years ago. By brilliant inductive reasoning, he assumed 

 that such a complex does form, and then calculated what relationships 

 shoidd hold between enzyme concentration, substrate concentration, and 

 the velocity of the reaction. Exactly these relationships are observed 

 experimentally, which is strong evidence that Michaelis' assumption, 

 that an enzyme-substrate complex forms as an intermediate, is correct. 

 Direct evidence of the existence of an enzyme-substrate complex was 

 obtained by David Keilin of Cambridge University and Britton Chance 

 of the University of Pennsylvania. Chance isolated a brown-colored 

 peroxidase from horseradish and found that when this was mixed with 

 the substrate, hydrogen peroxide, a green-colored enzyme-substrate com- 

 plex formed. This in turn changed to a second, pale red complex which 

 finally split to give the original brown enzyme and the products of the 

 reaction. By observing the changes of color. Dr. Chance was able to 

 calculate the rates of formation and of breakdown of this complex. 



It is clear that when it is part of an enzyme-substrate complex, the 

 substrate is much more reactive than it is when free. It is not clear, 

 however, ivhy this should be true. One explanation postulates that the 

 enzyme unites with the substrate at two or more places, and the substrate 

 molecule is held in a position which strains the bonds and renders them 

 more likely to break. 



21. Factors Affecting Enzyme Activity 



Temperature. The velocity of most chemical reactions is approxi- 

 mately doubled by each ten degree increase in temperature, and, over 

 a moderate range of temperature, this is true of enzyme-catalyzed reac- 

 tions as well. Enzymes, and proteins in general, are inactivated by high 

 temperatures; the higher the temperature, the more rapidly the enzyme 

 activity is lost. Native protein molecules are believed to exist as spiral 

 coils, or helices, and the denaturation process is believed to involve the 



