218 THE BIOCHEMISTRY OF B VITAMINS 



are enzymatically hydrolyzed extracellularly to yield simpler sugars 

 which can be absorbed, and the subsequent intracellular processes 

 by which these absorbed compounds are converted into polysaccharides 

 in which form they are stored until utilized. A number of enzymes 

 hydrolyzing carbohydrates have been well characterized. Of these, only 

 a single specific enzyme, pancreatic amylase, has been shown to contain 

 a coenzyme (inositol) (p. 125). 



The intracellular synthesis of glycogen and starch from simple hexoses 

 cannot be carried out directly since an input of energy is required for 

 the formation of the acetal bonds. The energy for the synthesis is intro- 

 duced by a reaction in which glucose is initially converted to a phosphate 

 ester 8 by adenosine triphosphate, a transformation that uses up an 

 energy-rich phosphate bond generated previously in some metabolic 

 reaction. The phosphate ester initially formed, glucose-6-phosphate, is 

 in equilibrium with its isomer, glucose- 1 -phosphate, due to the presence 

 of an enzyme which catalyzes this intramolecular transesterification. The 

 glucose- 1-phosphate molecules polymerize to form the polysaccharide by 

 a reaction in which the phosphate ester linkage is cleaved (liberating 

 inorganic phosphate), but an acetal bond is created. The energy trans- 

 formations involved in this reaction are small and the hexose-phosphate 

 and polysaccharide are usually in equilibrium. 



x glucose-1-phosphate ^ *** polysaccharide +2H3PO4 



When an organism expends energy, the phosphoric acid anhydrides, in 

 which energy has been stored, are hydrolytically cleaved and the in- 

 organic phosphate concentration increases. This increase in inorganic 

 phosphate upsets the equilibrium between the hexose phosphate and 

 glycogen and causes the breakdown of glycogen to glucose- 1-phosphate. 

 This, in turn, initiates the glycolytic process in which the glycogen is 

 metabolized. The energy liberated during this process is utilized through 

 the resynthesis of the phosphoric acid anhydrides from the inorganic 

 phosphate. The formation of energy-containing phosphate bonds con- 

 tinues until the cell reaches a state wherein the inorganic phosphate 

 concentration will have been reduced to such a level that the equilibrium 

 shifts to favor the formation of glycogen instead of its breakdown. As 

 far as is now known, the reactions involved in the intracellular formation 

 of polysaccharides require only adenylic acid and its phosphorylated 

 derivatives as coenzymes. The B vitamins are involved only indirectly; 

 they are needed for producing the energy units used in the synthetic 

 process — the phosphoric acid anhydrides. 



Glycolysis. When the second phase of carbohydrate utilization, the 

 glycolytic process, is initiated by the phosphorolysis of glycogen (or 



