72 GENERAL CONCEPTS 



support lile are limited in number and in the course ot evolution other 

 methods have been tried but have not been able to persist. 



Glucose, to be metabolized within a cell, must first be converted by 

 the enzyme glucokinase to glucose phosphate. Other sugars, Iructose, for 

 example, are also converted to their respective phosphates belore any 

 further metabolism can occur. The glucose phosphate is converted by 

 one enzyme to fructose phosphate, and by a second enzyme to fructose 

 diphosphate (a fructose molecule with two molecules of phosphate at- 

 tached). The fructose diphosphate is then cleaved in the middle of the 

 molecide to yield two molecules, each containing three carbons and one 

 phosphate group (Fig. 4.2). Just as a sugar with six carbons is known 

 as a hexose, one with three carbons is called a triose, and these substances 

 are known as triose phosphates. A series of enzyme reactions converts 

 the triose phosphate into pyruvic acid. In the course of these reactions 

 two energy-rich phosphate compounds are produced as each molecule of 

 triose phosphate is converted to pyruvic acid. The phosphate group and 

 its associated energy is transferred to adenosine diphosphate to convert 

 it to adenosine triphosphate. The latter compoinid is the major currency 

 of biologically available energy, and is available for any of the many 

 energy-requiring reactions of the cell. The energy derived in this con- 

 version of triose phosphate to pyruvic acid represents only about 5 per 

 cent of the energy that is ultimately obtainable when the triose phos- 

 phate is metabolized to carbon dioxide and water. 



The other 95 per cent is obtained in the oxidation of pyruvic acid, 

 which is mediated by a series of enzymes many of which are located in 

 the mitochondria. The series of reactions was postulated by the English 

 biochemist, H. A. Krebs, and is known as the Krebs citric acid cycle, for 

 citric acid (which accumulates in the tissues of citrus fruits) is the first 

 substance in the series. To enter the citric acid cycle, pyruvic acid must 

 first be converted to an acetic acid-coenzyme A compoimd. The acetyl 

 coenzyme A (which contains two carbons) unites with oxaloacetic acid 

 (four carbons) to form citric acid (six carbons). The successive enzymes 

 of the citric acid cycle then break citric acid down stepwise through 

 eight different intermediate compounds to oxaloacetic acid, which is 

 then ready to combine with another molecule of acetyl coenzyme A and 

 continue the cycle. In this cycle (Fig. 4.2) carbon dioxide is given off by 

 decarboxylases, hydrogen atoms are removed by dehydrogenases, and 

 the electrons of the hydrogen atoms are transferred by the electron- 

 transmitting enzymes, the cytochromes, to oxygen, which then unites 

 with the hydrogen ions to form water. As the two molecules of pyruvic 

 acid derived from each molecule of glucose are metabolized in the Krebs 

 cycle and cytochromes, about 36 additional energy-rich phosphate com- 

 pounds are formed. In this way much more of the energy originally in 

 the glucose molecule is made available, as adenosine triphosphate, to 

 run the many energy-requiring processes of metabolism. The Krebs cycle 

 has been called the "intracellular energy wheel"; it takes in molecules 

 of acetyl coenzyme A, spews forth carbon dioxide and hydrogen, and 

 traps, in the form of ATP, the energy released. 



The idea that we breathe in oxygen and breathe out carbon di- 



