CEll METABOUSfA 67 



chemical reactions would occur much too slowly to permit life to con- 

 tinue. Each molecule of the enzyme catalase, extracted from beef liver, 

 will decompose 5,0U(),{)00 molecules of hydrogen peroxide (HoO^) per 

 minute at 0° C. Hydrogen peroxide is a poisonous substance produced 

 as a by-product in a number of enzyme reactions. Catalase protects the 

 cell by decomposing the peroxide. The number of molecules of substrate 

 acted upon per minute by a molecule of enzyme is called the turnover 

 number of the enzyme. The turnover number of catalase, at 0° C, is 

 5,000,000. Most enzymes have high turnover numbers, which explains 

 why they can be so remarkably effective even though present in proto- 

 plasm only in minute amounts. 



Although enzymes in general catalyze specific reactions, they do 

 differ in the number of kinds of substrates they will attack. Urease is an 

 example of an ezyme which is absolutely specific. Urease decomposes 

 urea to ammonia and carbon dioxide and will attack no substance other 

 than urea. Most enzymes are not quite so specific, and will attack several 

 closely related substances. Peroxidase, for example, will decompose 

 several different peroxides in addition to hydrogen peroxide. A few 

 enzymes are specific only in requiring that the substrate have a certain 

 kind of chemical bond. The lipase secreted by the pancreas will split 

 the ester bonds connecting the glycerol and fatty acids of a wide variety 



of fats. 



In theory, enzyme-controlled reactions are reversible; the enzyme 

 does not determine the direction of the reaction but simply accelerates 

 the rate at which the reaction reaches equilibrium. The classic example 

 of this is the action of the enzyme lipase on the splitting of fat, or union 

 of glycerol and fatty acids. If one begins with a fat, the enzyme catalyzes 

 the splitting of this to give soine glycerol and fatty acids. If one begins 

 with a mixture of fatty acids and glycerol, the enzyme catalyzes the 

 synthesis of some fat. When either system has operated long enough, 

 the same equilibrium mixture of fat, glycerol and fatty acid is reached: 



Fat ^ glycerol -f 3 fatty acids 



The equilibrium point is determined by complex thermodynamic 

 principles, which will not be discussed. Since reactions give off energy 

 when going in one direction, it is obvious that an equivalent amount of 

 energy in the proper form must be supplied to make the reaction go in 

 the opposite direction. 



To drive an energy-requiring reaction, some energy-yielding reac- 

 tion must occur at about the same time. In most biologic systems, energy- 

 yielding reactions result in the synthesis of "energy-rich" phosphate 

 esters, such as the terminal bonds of adenosine triphosphate (abbreviated 

 as ATP). The energy of these energy-rich bonds is then available for the 

 conduction of an impulse, the contraction of a muscle, the synthesis of 

 complex molecules, and so on, much as the energy of a storage battery 

 made by a generator is available for light, heat or running a motor. 

 Biochemists use the term "coupled reactions" for two reactions which 

 must occur together so that one can furnish the energy, or one of the 

 reactants, needed by the other. 



