Pfizer Handbook of Microbial Metabolites 



48 



The net effect of the cycle is to oxidize pyruvic acid to carbon 

 dioxide and water : 



CH3COCOOH + 50 ^ 3CO2 + 2HoO 



Enzymes of the citric acid cycle occur widely among micro- 

 organisms, and it is likely that the cycle and variants of it are 

 equally ubiquitous. Its primary physiological function in micro- 

 organisms (if a primary function can be singled out) is less 

 clear, two possibilities being: (a) an energy source and (b) a 

 source of amino acid skeletons. Interruption of the cycle or im- 

 balances under certain conditions lead to accumulation of cer- 

 tain acids. Thus high yields of citric, isocitric, a-ketoglutaric, 

 fumaric and malic acids can be obtained in controlled fungal 

 fermentations. 



It was mentioned in the preceding chapter that certain micro- 

 organisms are capable of growing on a medium containing ace- 

 tate as the sole carbon source, synthesizing all their carbo- 

 hydrate requirements from it. In some of these microorganisms, 

 at least, this ability may be due to possession of a pair of en- 

 zymes (malate synthetase and isocitritase ) which permit opera- 

 tion of a cycle ancillary to the citric acid cycle or replacement 

 of the steps from isocitric acid to malic acid and commonly 

 called the glyoxylic acid cycle: 



HC— COOH 



II 



HOOC— CH — 



Fumaric Acid 



The Glyoxylic Acid Cycle 

 Acetyl CoA 



CH2— COOH 



■I 



CH2— COOH OHC— COOH 

 Succinic Acid Glyoxylic Acid 



HO— CH— COOH 



CH— COOH 



I 



CH2— COOH 

 Isocitric Acid 



HO— CH— COOH 

 I 



CH2— COOH 

 Malic Acid 



CO— COOH 



CH2— COOH 

 Oxaloacetic Acid 



CH2— COOH 

 I 

 HO— C— COOH 



I 



CH2— COOH 

 Citric Acid 



