282 ENZYMES 



ferric state by molecular oxygen under the influence of cytochrome oxi- 

 dase (Table 10-1, class F.II.a.). Tyrosinase is another example of an 

 oxidase. 



Peroxidases lead to the oxidation of substrates by hydrogen peroxide. 

 Catalase also activates hydrogen peroxide and decomposes it to water and 

 oxygen in the absence of added substrates. However, in the presence of 

 certain oxidizable substrates, catalase can act as a peroxidase. To illus- 

 trate : 



2H2O2 *■ 2H20H-02 (H2O 2 decomposition) 



H2O2 + CH3CH2OH '^^^^"'^ * 2H2O + CH3CHO (peroxidation) 



or peroxidase 



Dehydrogenases result in a removal of hydrogen ions and an equal 

 number of electrons from an organic molecule. Examples of various 

 dehydrogenases have already been encountered in the discussion of 

 pyridino and flavin coenzymes. The nature of the coenzymes of certain 

 other important dehydrogenases, e.g., succinic dehydrogenase (Table 

 10-1, class r.1.3.), has not been completely elucidated to date. When 

 the biocatalyst is concerned with the oxido-reduction of a compound from 

 which electrons, but not hydrogens, are removed, the enzyme is called a 

 reductase, e.g., cytochrome reductase (Table 10-1, class F.I.2.a.). 



The basic mechanism of all oxidation-reduction reactions involves the 

 transfer of electrons, and the enzymes or coenzymes concerned can be 

 considered as carriers of electrons. To illustrate schematically: 



(1) HOOC— CHj— CH2— COOH 



Succinic acid dehydrogenase^ 



•<- 



HOOC— CH=CH— COOH + 2H+ + 2e 

 Fumaric acid 



(2) 2Cytochrome c (Fe+++) + 2e 



reductase 2Cytochrome c (Fe+''") 



(3) Oxidase 



(a) 2Cytochrome c (Fe++) ^ 2Cytochrome c (Fe+++) -|- 2e 



(b) 2H+ + 2e + §02 > H.O 



If one considers the oxido-reduction enzymes in relation to each other, 

 a certain pattern emerges. For example, a dehydrogenase containing 

 TPN or DPN can extract hydrogen and electrons from a given substrate, 

 reduced TPN or DPN can react with flavo-protein, and this then can 

 react with cytochrome c, which can be oxidized by oxygen in the presence 

 of cytochrome oxidase. This relationship can be summarized in the 

 following series of equations: 



