ENZYMES 51 



The proteolytic enzymes are a very complex group of hydrolases. In 

 view of the complexity of protein structure this is not unexpected. The 

 question of specificity of the proteolytic enzymes has been considered by 

 Bergmann (1942), who emphasizes that the specificity of a given enzyme 

 for a certain substrate may be modified by the presence of a second sub- 

 strate. Johnson and Berger (1942) have reviewed the enzymatic proper- 

 ties of the peptidases, including those produced by the fungi. 



Oxidases, hydrogenases, and desmolases. One of the central prob- 

 lems in metabolic processes is how and by what means oxidation of 

 metabolites to carbon dioxide and water is brought about. Some 

 organisms (bacteria) are inhibited or killed by free oxygen (anaerobes). 

 Others may live either in the presence or absence of free oxygen (faculta- 

 tive anaerobes), while others require free oxygen (aerobes) to carry on 

 their metabohc processes and to maintain life. Thus, one organism may 

 degrade a substrate only partially, and these intermediate oxidation 

 products become substrates for other organisms. In the end complete 

 oxidation takes place. In other instances an organism may first carry 

 out a partial degradation and complete it later. Thus, yeast produces 

 alcohol by fermentation. In the presence of oxygen, alcohol is utilized 

 for the synthesis of cellular constituents and as a source of energy. Many 

 fungi possess two ways of obtaining energy by the degradation of metabo- 

 lites: an anaerobic (fermentative) and an aerobic (oxidative) pathway. 

 Both may function in the same organism at the same time, although 

 external conditions may favor one process at the expense of the other, or a 

 substance may inhibit one without affecting the other. 



Biological oxidations are carried out in two ways: by the removal of 

 hydrogen from, or by the addition of oxygen to, substrates. The name 

 of Wieland is associated with the process of dehydrogenation, and that of 

 Warburg with the second process. 



The theory of Wieland stressed the importance of the enzyme systems 

 which activated hydrogen or removed hydrogen from substrate molecules, 

 while Warburg's theory focused attention upon the enzyme systems 

 which activated oxygen and which carried oxygen to the substrates. 

 These two theories might seem irreconcilable, but today they are con- 

 sidered as mutually complementary. Both types of enzymatic oxidation 

 are known for the same organism. For further discussion of this problem 

 the student is referred to Elvehjem and Wilson (1944) and Meyerhof et al. 

 (1942) . For a classification of the respiratory enzymes see Gortner (1949) 

 and Sumner and Somers (1947). For the electronic mechanism involved 

 in biological oxidation-reduction see Michaelis (1946). 



Some representative dehydrogenases and oxidases are aerobic dehj^dro- 

 genases (xanthine oxidase, and uricase); anaerobic dehydrogenases, 

 (succinic dehydrogenase, glucose dehydrogenase, triose phosphate dehy- 



