RESPIRATION 471 



enzymes. As in the case of glycolytic degradation this allows 

 the organism, not only to surmount the energy barrier, but 

 also to obtain the energy in separate, easily-used portions 

 rather than all at once, in an explosive form. At one end 

 of this chain stand phosphopyridine nucleotides and the corre- 

 sponding enzymes, dehydrogenases, which transfer hydrogen 

 from the system which we have referred to as the first, to the 

 second or oxidative system. At the other end of the chain 

 are the specific respiratory enzymes, oxidases and peroxidases, 

 the role of which, according to A. N. Bach,^^^ is the activation 

 of molecular oxygen and peroxides. They complete the 

 process of oxidation to water of the hydrogen which has been 

 brought into the system, and are therefore sometimes called 

 the ' terminal ' or ' finishing ' enzymes."® There is a consider- 

 able diversity in different organisms as regards the inter- 

 mediate links in the oxidative chain, but flavoproteins occupy 

 a prominent position, sometimes transferring hydrogen from 

 pyridine nucleotides to the oxidative mechanisms, and some- 

 times completing its oxidation by the oxygen of the air with 

 the formation of hydrogen peroxide. This is then broken 

 down by catalase or used for oxidising reactions by means of 

 peroxidases. 



Recently H. Mahler"'' has shown that there are to be found 

 among living things a large number of flavoproteins having, 

 in their prosthetic groups, such metals as iron, molybdenum 

 and copper. 



Mahler gives the following scheme for the part played by 

 flavoprotein enzymes in the transfer of the hydrogen liberated 

 in the first system to the oxygen of the air (Fig. 43). Under 

 the letter A we have the case in which the substance giving 

 up hydrogen to the flavine enzyme is reduced pyridine 

 nucleotide which has obtained hydrogen from the substrate 

 (from a reaction in the first system). The hydrogen is trans- 

 ferred by the flavine enzyme, either to a component of the 

 cytochrome system, or to some other oxidase mechanism, but 

 not directly to molecular oxygen. In the case designated by 

 the letter B the flavine enzymes taking part obtain hydrogen 

 directly from the substrate and transfer it to the cytochrome 

 system. Finally, the letter C refers to the case in which the 



