TERMINAL RESPIRATION 



229 



Rhizopus nigricans (86) and Neurospora crassa (276). We have already 

 noted the proposal that the light-stimulated carbon dioxide fixation in 

 Blastocladiella emersonii reflects the reductive carboxylation of suc- 

 cinate and a-ketoglutarate (45). The pathway of fixation can presum- 

 ably only be detected with finality in experiments with active cell-free 

 preparations. 



12. TERMINAL RESPIRATION 



In aerobic organisms the ultimate oxidizing agent is molecular 

 oxygen; the reaction or sequence of reactions by which the electrons 

 withdrawn from the substrate are transferred to oxygen is denoted 

 terminal respiration. Conventionally the process is visualized as 

 transfer of hydrogen from substrate to oxygen. The transfer is 

 mediated by a system in which substances of increasingly positive 

 oxidation-reduction potential form a chain of carriers. 



The simplest system involves only one carrier, the flavoprotein 

 prosthetic group of an enzyme; examples are the amino acid oxidases 

 and glucose aerodehydrogenase. More usually, the pyridine nucleo- 

 tides — diphosphopyridine nucleotide or triphosphopyridine nucleotide 

 — are the first hydrogen carriers of the sequence. Perhaps the most 

 common terminal pathway involves a pyridine nucleotide, a flavo- 

 protein, and the cytochrome system in succession, that is, the flow of 

 hydrogen (electrons) is over the pathway: 



Substrate— > pyridine nucleotide —> flavoprotein — > cytochromes 



— > oxygen (16) 



The cytochrome system is complex and there is considerable doubt 

 whether what is known of animal systems can be applied directly to 

 microorganisms. The flavoprotein is probably always a metallo- 

 enzyme (141). 



Under some conditions, e.g., a high partial pressure of oxygen, flavo- 

 proteins may react directly with oxygen without the intervention of 

 the cytochrome system (295). 



The cytochrome system appears to be general in the fungi, as judged 

 in the first instance by spectroscopic observations (32, 33, 56, 117, 122, 

 197, 227, 291, 300). However, it should be noted that experience with 

 bacterial cytochromes indicates that it is not possible from simple 

 spectroscopic measurements to state with precision whether the com- 

 ponents of a new system are the same as those of the more familiar 

 yeast and animal systems (266, 321). Consequently, it is well to re- 

 serve judgment for the time being as to whether the fungal cyto- 



