30 ENZYMES 



Action of amylase and phosphorylase on polysaccharides: 



Exercise V 



CH2OH 



-K)- 



I 



HO-^H 



I— o- 



CH20H 



J-O. 



-0— 



HO-f-H 



polysaccharide + water 



amylase 



maltose 



HO 



^o— ^ 



phosphorylase 



(0H)20P0— H (0H)20P0— H 



polysaccharide + phosphoric acid 



CH2OH 



-O 



HO 



OH 



I 



'— op=o + 



I 



OH 



CHoOH 



-0 



IV 



HO 



-0— 



glucose-1-phosphate + shorter chain 



These two ways of degrading polysaccharides 

 differ in fundamental ways. The difference in 

 rate has already been mentioned; the phos- 

 phorylases are among the most active of known 

 enzymes. Second, amylases end by cleaving 

 polysaccharides into maltose units, which require 

 a second enzyme, maltase, to yield glucose. The 

 phosphorylases yield instead glucose phosphate 

 units, which are hydrolyzed further to glucose 

 and phosphoric acid by the enzyme phosphatase. 



The most interesting and important difference 

 in the action of these enzymes, however, involves 

 the reversibility of the phosphorylase reaction. 

 Whereas the hydrolysis of polysaccharides by 

 amylases is virtually irreversible, their phos- 

 phorolysis goes readily in either direction. It is 

 important that you understand the reason for 

 this difference. 



Hydrolyses in general tend to be virtually ir- 

 reversible for two reasons: (1) in polysaccharides, 

 for example, the glucose-glucose bond has an 

 energy of about 3 kcal/mole. The hydrolysis of 

 the polysaccharide results in a loss of this 

 energy; and conversely one should have to add 

 this amount of energy per glucose-glucose link 

 from outside in order to resynthesize a poly- 



saccharide. In the absence of such added energy, 

 only the hydrolysis can occur. (2) Such enzy- 

 matic reactions, in and out of the cell, ordinarily 

 occur in the presence of an overwhelming con- 

 centration of one of the reactants, water. The 

 molar concentration of pure water is 55.6 M 

 (why so?), and most aqueous solutions approach 

 this concentration of water. If we write a 

 reversible equation for a hydrolysis, this enor- 

 mous concentration of water on one side of the 

 equation pushes the equilibrium very far in the 

 other direction ("mass action effect"). 



A phosphorolysis presents a very different 

 situation in both regards. On the one hand, the 

 energies of the reactants and products are fairly 

 evenly balanced : a glucose-phosphoric acid bond 

 has very nearly the same energy as a glucose- 

 glucose bond. Hence little energy is lost or need 

 be added in going in either direction. Further- 

 more the concentrations of reactants and prod- 

 ucts are more evenly balanced, since here phos- 

 phoric acid in relatively low concentration takes 

 the place that water occupies in a hydrolysis. 

 The phosphorylase reaction therefore is freely 

 reversible. In neutral solution and at room 

 temperature the equilibrium lies somewhat over 



