144 



PHYSIOLOGY OF THE FUNGI 



The mechanism of acetate oxidation by yeast is postulated by these 

 authors to follow a modified Krebs citric acid cycle (scheme VIII). The 

 oxidation of acetate is thus the result of a rather complex cyclic process. 

 While the four-carbon dicarboxylic acids of the Krebs cycle are poor 

 sources of carbon for most fungi, they are important in intermediary 

 metabolism. These acids are readily interconvertible. The role of the 

 keto acids in amino-acid synthesis was noted in Chap. 6. Lewis (1948) 

 studied the metabolism of mutants of Neurospora crassa which were 

 unable to synthesize either aspartic or glutamic acids. These amino 



Scheme IX. A Generalized Krebs Isocitric Acid Cycle Proposed to 



Illustrate the Pathways of Conversion of Carbohydrate into 



Aspartic and Glutamic Acids by Neurospora* 



Carbohydrate 



It 



Pyruvate -^ 



(CHg-CO-R) 



Cis-aconitate 

 n 



Isocitrate 

 A 

 A 



oi -Ketoglutarate 



Oxalacetate 



Succinate 



* Courtesy of Lewis, Am. Jour. Botany 36: 294, 1948. 



acids could be replaced by a-ketoglutaric, succinic, malic, or fumaric 

 acids. A generalized Krebs cycle was proposed by Lewis which indicates 

 the pathway of synthesis of aspartic and glutamic acids from glucose 

 (scheme IX). Compounds utilized by the Neurospora mutants are 

 printed in italics. The probable location of the genetic block which 

 prevents the biosynthesis of aspartic and glutamic acids is indicated by A. 



SUMMARY 



Organic compounds are utilized by fungi for the synthesis of structural 

 and functional compounds and as sources of energy. The fungi utilize a 

 wide range of natural organic compounds including those of great com- 



