CARBOHYDRATE METABOLISM 



CHgOPOg^ 



CH2OH HOCHa^^O 



fructokinase - 



Mg 



+ + 



ATP 



ADP 



H 



H HO 



H 



OH H 



U -fructose-l-phosphate 



449 



l-phosphofriictokinase 



ATP 



ADP 



HO3POCH2.O. CHgOPOgH' 

 H HO 



OH H 



D-fructose-l, 6-diphosphate 



but Avith the enzyme hexokinase, and D-fructose-6-phosphate is formed, 

 at least in some tissues. On the other hand, when D-fructose-l -phos- 

 phate is formed first, another route is utilized. 



The hexosephosphates thus formed are all in equilibrium with each 

 other by way of the reactions of page 218. Subsequent metabolism fol- 

 lows either the glycolytic route (page 168) through the tricarboxylic 

 acid cycle (page 171) or the pentosephosphate pathway (page 223). 

 It is not yet possible to decide which system, if either, is preferred and 

 processes the major fraction of the material. Energy for synthetic or 

 mechanical purposes becomes available at all points that lead to the 

 formation of DPNH, TPNH, ATP, and acyl coenzyme A. 



After digestion has taken place, glucose is the sugar that is widely 

 distributed in appreciable concentration throughout the animal body. 

 Lactose, of course, is abundant in milk but not elsewhere, and traces 

 of various other sugars have been detected in particular organs or 

 tissues. Fructose seems to play one special role, for this sugar occurs 

 in seminal fluids and serves as the major energy source for the highly 

 motile and rapidly metabolizing spermatozoa. For some reason fruc- 

 tose takes the places of glucose in the nutrition of the sperm cells of 

 higher animals. 



Pentoses, ingested as such or in nucleic acids or formed by the bac- 

 terial hydrolysis of pentosans, can probably be metabolized to a con- 

 siderable extent by animals. Specific kinases catalyze phosphorylation 

 of free pentoses, and the pentosephosphates may then break down by 

 the system of page 223 involving a 3-phosphoglyceraldehyde. This last 

 compound can go via pyruvate (page 154) to acetyl coenzyme A. 



As usually written (page 171), the tricarboxylic acid cycle provides 

 for the complete conversion of the carbon of pyruvate to carbon 

 dioxide. However, operation in this way does not continuously sup- 



