Common Pathways of Cellular Metabolism - 153 



COOH 

 I 

 CH 2 



CH 2 + TPNH. + NH 3 «= 



glutamic 



c=o 



I 



COOH 



a ketoglutaric acid 



COOH 



I 

 CH, 



I 

 CH 2 + TPN + H 2 



I 

 HC— NH 2 



I 

 COOH 



glutamic acid 



Fig. 8-10. A typical amination reaction. The amino 

 acid (glutamic acid) is formed from ammonia and an 

 organic precursor (a-ketoglutaric acid). Plant cells 

 can synthesize all the various amino acids in such 

 fashion. Animal cells, however, can use this method 

 only for the synthesis of the "nonessential amino 

 acids." 



be utilized as fuels. But as a prelude to such 

 usage, the amino group must be discharged 

 from the molecule (Fig. 8-10). The nonnitrog- 

 enous organic residuum of the molecule, 

 possessing a carbon framework of varying 

 length and structural complexity, is then 

 launched upon metabolism. Either it may 

 be catabolized, yielding a certain quantity 

 of energy, or its molecular structure may be 

 modified in such a way that it can be uti- 

 lized by the cell for the synthesis of carbo- 

 hydrate or lipid materials (Fig. 8-5). 



Amination, the reverse of deamination 

 (Fig. 8-10), is also very important. All cells 

 can utilize inorganic nitrogen, in the form 

 of ammonia (NH 3 ) or ammonium salts (for 

 example, NH 4 C1), for the amination of cer- 

 tain organic derivatives of carbohydrate and 

 lipid metabolism — as is indicated in Figure 

 8-5. Animal cells, however, are limited in 



this regard. They are able to build up cer- 

 tain amino acids but not others. The amino 

 acids of the latter group, which cannot be 

 synthesized, are called the essential amino 

 acids (p. 137). These units of protein struc- 

 ture must be obtained ready-made, or pre- 

 formed, from the protein components of the 

 animal's food. 



The Krebs Cycle; Importance of Coenzyme 

 A. A cycle of oxidation-reduction reactions, 

 called the Krebs cycle (Fig. 8-5), occupies a 

 very important central position in cellular 

 metabolism. Another name for this cyclic 

 series of reactions is the citric acid cycle, 

 since citric acid, an important 6-carbon 

 compound, keeps appearing at the beginning 

 and reappearing at the end of each complete 

 "rotation" of the cycle. The Krebs cycle has 

 also been called the energy wheel of cellular 

 metabolism. It constantly generates new en- 

 ergy, conserved largely in the form of high- 

 energy phosphate (ATP), so long as it keeps 

 "turning.'' But equally important is the fact 

 that the Krebs cycle deals not only with the 

 metabolic derivatives of carbohydrate ma- 

 terials, but with the derivatives of proteins 

 and lipids as well (Fig. 8-5). The separate 

 parts of the cycle have been recognized for 

 many years. However, in 1947, Hans A. 

 Krebs, who later shared the 1953 Nobel 

 Prize with Fritz Lipmann, first succeeded in 

 demonstrating the nature and importance 

 of the cycle as a whole. 



Coenzyme A. This remarkably versatile co- 

 factor (Fig. 8-11) plays a most important role. 

 It serves to transmit a constant flow of acti- 

 vated 2-carbon derivatives (activated acetyl 

 units) into the Krebs cycle (Fig. 8-5). The ac- 

 tive 2-carbon derivative, transferred to the 

 4-carbon compound, oxaloacetic acid, pro- 

 duces the 6-carbon molecule, citric acid — 

 which marks the beginning of a new cycle 

 (Fig. 8-12). 



A molecule of coenzyme A (Fig. 8-11) may 

 be written, in abbreviation, as CoASH, 

 since it possesses a terminal sulfhydryl 

 (— SH) group, in addition to its other con- 

 stituents (adenine, pentose triphosphate, 



