The Nature and Diversity of Catalytic Proteins 75 



Some comparisons ot muscle, liver, 

 ami n«'itsl aldolases 



The experiments on carboxypeptidase degradation of aldolase have. 

 as indicated above, led to the recognition that muscle and liver 

 aldolase may have considerable structural and catalytic features in 

 common. Some of the characteristics of these two enzymes are sum- 

 marized in Table 2. The two enzymes are similar in molecular size, 

 number of — SH groups per mole, and ultraviolet absorption spectrum. 

 Carboxypeptidase removes only one tyrosyl residue from liver aldo- 

 lase, in contrast to the three COOH-terminal residues removed from 

 muscle aldolase. The liver aldolase, before carboxypeptidase digestion, 



Table 2. Comparative properties of muscle and liver aldolases 



Property Muscle Liver 



Molecular weight 150.000 159,000 



— SH per mole 28 29 



Tyrosines removed by 



carboxypeptidase 3 1 



A . . fructose-di-P 100/2.1 (Native) 100/43 (Native) 



fructose-1-P 6 2.5 (—3 tyrosines) 40 ? (—1 tyrosine) 



Ultraviolet spectra Enzymes have similar spectra 



sources: Refs. 10. 11. 



has a catalytic action on fructose 1.6-diphosphate and fructose 1-di- 

 phosphate rather similar to that shown by muscle aldolase after 

 carboxypeptidase digestion. Thus the principal differences in cata- 

 lytic behavior between the two enzymes may rest in the presence 

 of the two additional COOH-terminal tyrosines in the muscle aldolase. 

 In addition to their indication of more structural relation between 

 liver and muscle aldolase than previously recognized, these studies are 

 of interest in that they illustrate well the importance of small differ- 

 ences in fine structure of a protein to its biological activity. 



In contrast to the similarity of liver and muscle aldolase, yeast aldo- 

 lase differs markedly from muscle aldolase. Some characteristics of 

 the yeast and muscle enzymes are summarized in Table 3. The metal 

 requirements of the yeast enzyme and the inhibition by chelating 

 agents, not shown by the muscle enzyme, are striking. In addition, the 

 marked difference in the number and behavior of — SH groups, as well 



