CHAPTER XXX 



ANAEROBIC RESPIRATION AND TEE MECHANISM OF 



RESPIRATION 



Oxidizing-reducing Enzymes. — A number of substances are known to 

 occur in cells which catalyze oxidations and reductions. Some of these seem 

 to possess all of the attributes of enzymes and are properly so classified. 

 Others are considered to be enzymes by some, but not by all authorities, 

 while some are clearly non-enzymatic in their action. Many of the oxidizing- 

 reducing enzymes operate simultaneously on two different substrates, one of 

 which is oxidized, while the other is reduced. The following discussion of 

 oxidizing-reducing enzymes attempts to represent the generally accepted con- 

 sensus of most workers in this field at the present time. 



Enzymes are conventionally classified into two distinct groups, the hydro- 

 lytic enzymes (Chap. XXVII), and the oxidizing-reducing enzymes at pres- 

 ent under discussion (Table 55). In a condensed classification it is possible 

 to present only the most probable interpretation of the action of these enzymes 

 in the light of present knowledge. Ideas on this subject are very much in a 

 state of flux, and future investigations may radically change present concepts 

 of the role of some of these enzymes. 



As a rule the enzymes classed in this group possess the properties as de- 

 scribed for enzymes in general in Chap. XXVII. Unlike other enzymes, 

 however, some of the peroxidases are thermostable, which has led some authori- 

 ties to the view that they are not enzymes in the strict sense of the word. 



The occurrence and general properties of these types of enzymes will 

 be discussed briefly. 



I. Zymase was long considered to be a single enzyme, but is now recog- 

 nized to be a complex of several enzymes. Zymase is the enzyme system which 

 catalyzes the well known reaction which occurs during alcoholic fermentation, 

 and results in the production of alcohol and carbon dioxide from certain 

 sugars. Most investigations have been made upon zymase extracts prepared 

 from yeast cells, but this enzyme complex is known to be widely distributed 

 through the plant kingdom. Among the enzymes of the zymase complex are 

 glycolase which converts hexoses into methyl glyoxal, and carboxylase which 

 splits out carbon dioxide from certain types of organic acids (a-ketonic acids), 



531 



