222 RESPIRATION 



way, as described earlier for other fungi and actinomycetes and as is 

 common in yeast and bacterial pentose metabolism (4, 99). 



The oxidative breakdown of pentose by fungi has not been inten- 

 sively investigated. It has been mentioned in the preceding chapter 

 that there is one report of the oxidation of pentoses to pentonic acids; 

 the more recent and better documented report (175) that Pscudomonas 

 spp. form pentonic acids from pentose suggests that a re-examination 

 of the problem in fungi would be of interest. Streptomyces coelicolor 

 extracts oxidize xylose and ribose only if the cells have been grown 

 on the particular sugar; indirect evidence suggests that these pentoses 

 must be phosphorylated and that the kinase responsible is an in- 

 ducible enzyme not present in glucose-grown cells (66). 



9. THE CITRIC ACID CYCLE 



In animal tissues the functioning of a respiratory cycle, the citric 

 acid cycle (also called the tricarboxylic acid cycle or the Krebs cycle) 

 is well established. The historical development of the concept has 

 been reviewed by Krebs (165); other valuable general reviews are 

 those of Wood (326) and Ochoa (222). 



In Figure 5 the principal reactions of the citric acid cycle are out- 

 lined. It can be seen, first, that the net effect of one turn of the cycle 

 is the complete oxidation of a 2-carbon fragment, CH.CO — , to 

 carbon dioxide and water. This acetyl fragment is known to be 

 attached to coenzyme A, forming acetyl-coenzyme A. The first reac- 

 tion in the cycle is the transfer of the acetyl fragment to oxaloacetic 

 acid, forming citric acid. In subsequent reactions, two molecules of 

 carbon dioxide are split off and oxaloacetic acid is regenerated. In 

 principle, therefore, only one molecule of oxaloacetate is required, to 

 initiate the cycle, after which the reaction becomes self-sustaining. In 

 fact, of course, carbon is drawn off, especially for the synthesis of amino 

 acids, so that steady state operation of the cycle under normal condi- 

 tions presumably requires that 4-carbon dicarboxylic acids be formed 

 de novo continually. Mechanisms are known in bacteria by which 

 pyruvic acid, phosphoenolpyruvic acid, and propionic acid can be 

 carboxylated by carbon dioxide fixation (118); in addition, the pos- 

 sibility of malate synthesis from acetate has been raised by work with 

 bacteria (p. 227). 



Viewed strictly as a respiratory system, the net effect of the cycle 

 shown in Figure 5 is the oxidation of pyruvic acid: 



CH a — CO— COOH + 2.5 O, -* 3 CO. + 2 HoO (11) 



