234 Essays in Biochemistry 



a chemical reaction is the major problem in our attempt to understand 

 the role of enzymes in the living cell. 



The mechanism of catalysis in the non-living world is little more 

 understood than that of the living cell. Here also we recognize the 

 geometric 4 and energetic aspects of the problem. 5 The most obvious 

 division of chemical catalysis is into homogeneous and heterogeneous. 

 Most cases of homogeneous catalysis in water involve hydrogen-ion or 

 hydroxyl-ion catalysis. The mechanism of the catalysis involves inter- 

 mediates which in most cases have not been isolated. 



Most examples of chemical catalysis are heterogeneous. The most 

 common catalysts are metals of the fifth, sixth, seventh, and eighth 

 columns of the periodic table, elements having vacant d orbitals. 

 Considerable evidence has accumulated in the past years which indi- 

 cates that the properties of heterogeneous catalysts depend not on the 

 interaction of individual atoms of the catalyst with the substrates but 

 on the properties of the bulk metal. The new theory ascribes catalytic 

 activity to the transition metals because of their vacant d orbitals, 

 these orbitals being used for the chemisorption. 6 Couper and Eley 7 

 have shown that alloying of gold with a palladium hydrogenation 

 catalyst reduces its catalytic efficiency. In bulk palladium approxi- 

 mately 9.4 of the outer ten valence electrons are in the 4d shell; 0.6 are 

 in the 5s. With addition of gold its 6s electron fills the 4d shell of 

 palladium and reduces the catalytic efficiency. 



Catalysis in the biological field is almost exclusively limited to pro- 

 teins. Even such simple reactions as the hydration of C0 2 , or the 

 decomposition of H 2 2 , have enzymes in the cell for their catalysis. 

 The efficiency of the biological catalyst and the importance of the 

 specific protein is strikingly shown by the activity of catalase relative 

 to the iron ion. Haldane 8 has estimated that the catalytic activity 

 for the decomposition of H 2 2 of hematin is 10 5 and of catalase 10 10 

 that of ionic iron. Similarly neither ferrous iron nor heme will combine 

 reversibly with oxygen ; the complete iron-porphyrin-globin complex is 

 necessary. Clearly the protein portion of the complete enzyme has a 

 role other than that of a mere carrier of the prosthetic group. The 

 catalytic effect of an enzyme is intimately connected with the structure 

 of the enzyme; the protein does more than supply an active center 

 displayed in proper geometrical relationship to a few positively and 

 negatively charged groups. 



The great bulk of enzymatic reactions of the cell can be divided into 

 two classes. Those involving the addition or subtraction of water (or 



