316 Enzyme Kinetics of Hydrolytic Reactions /I7 : I 



equilibrium. For example, glucose can remain in solution in the 

 presence of oxygen for years without being significantly altered. In the 

 presence of certain catalysts, however, the glucose and oxygen react 

 rapidly to form carbon dioxide and water while releasing energy. One 

 may regard the reaction as an equilibrium represented stoichiometrically 

 by the equation 



C 6 H 12 6 + 60 2 ^ 6C0 2 + 6H 2 



This equilibrium so strongly favors the carbon dioxide and water that 

 no detectable amounts of glucose are formed when carbon dioxide and 

 water are mixed. The oxidation of glucose is catalyzed by a series of 

 enzymes found in almost all living cells. These enzymes not only 

 promote equilibrium but also convert part of the energy released to 

 other forms of chemical energy used by the living cells. In the absence 

 of catalysts, however, glucose and oxygen remain in solution in the 

 nonequilibrium condition indefinitely. 



Many biological reactions involve much more complex molecules than 

 glucose. Others involving smaller, simpler molecules are easier to 

 discuss. One such reaction, discussed in greater detail in the next 

 chapter, is the dissociation of hydrogen peroxide, according to the 

 scheme 



2H 2 2 ^ 2H 2 + 2 



Here, equilibrium also strongly favors the right-hand side of the equation. 

 Nonetheless, hydrogen peroxide can be stored for years in a dark bottle 

 in the absence of metallic ions. Many metal ions act as catalysts 

 accelerating this reaction to the right. None of these is as effective as 

 the protein catalyst catalase. 



There are also examples of enzyme-catalyzed reactions whose equili- 

 brium does not favor one side of the equation so strongly. A biologically 

 important reaction of this type is the formation and destruction of 

 carbonic acid according to the scheme 



H 2 C0 3 £ H 2 + C0 2 



In the absence of a catalyst, this reaction reaches equilibrium in a 

 matter of minutes. However, this is too slow to remove the C0 2 from 

 the blood stream during its time in the vertebrate gill or lung. The 

 rate of attainment of equilibrium is increased fourfold by an enzyme 

 present in all red blood cells called carbonic anhydrase. 



In the foregoing equation, the symbols k x and k 2 have been introduced. 

 These are rate constants. The first represents the fraction of carbonic 

 acid molecules dissociating per unit of time. The second is a pro- 

 portionality factor between the product of the carbon dioxide and water 



