MICROBIAL METABOLISM CONTD..

TRICARBOXYLIC ACID CYCLE AMPHIBOLIC IN NATURE: 

TCA cycle is embelished  with  the name Amphibolic because there is catabolism & anabolism. The term Amphibolic was proposed by B.Davis in 1961 to emphasize dual metabolic role of such pathway. The citric acid cycle is an good example of amphibolic pathway. In first step of TCA cycle, Oxaloacetate( 4-Carbon compound) combines with acetate to form Isocitrate(6-carbon compound) is anabolic. TCA cycle serves as catabolism of carbohydrates,fatty acids,& amino acids, also cycle provides precursors for many biosynthetic pathways, through  reactions that served the same purpose in anabolism. a-Ketoglutarate & Oxaloacetate  can for eg serves  for precursors of amino acid Aspartate & Glutamate by simple transamination. Through aspartate & glutamate, the carbons of oxaloacetate & a-Ketoglutarate  are then used for synthesis of amino acids, as well as purine & pyrimidine nucleotides. Oxaloacetate is converted into glucose by gluconeogenesis. Succinyl CoA is intermediate for the synthesis of porphyrin ring of heme groups which serve as oxygen carriers in Hemoglobin & Myoglobin & electron carriers in cytochromes. 
 Hence, TCA cycle is called as Amphibolic Cycle.

                                                         Fig: Amphibolic nature of TCA Cycle.
                          

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