ORIGIN OF ENZYMES 367 



The fact that enzymes seem to be chemically proteins, 

 having a definite sequence of amino acid residues in their 

 polypeptide chains and a definite internal structure of their 

 molecules, determines a number of the peculiarities which 

 distinguish enzymes from all other catalysts known to us. The 

 most important of these is their intense catalytic activity. 



There are known to be a large number of inorganic and 

 organic substances which can hasten the same reactions as 

 those affected by enzymes, but there is no comparison between 

 the strengths of their catalytic activities. For example, 

 hydrogen ions can catalyse the hydrolytic reaction whereby 

 sucrose is hydrolysed to glucose and fructose, a reaction which 

 is also catalysed by the invertase of yeast, but the enzyme is at 

 least ten million times as effective. The very simple nitrogen- 

 containing organic compound, methylamine, increases the 

 rate of breakdown of pyruvic acid. So does the enzyme 

 carboxylase, but the catalytic activity of the enzyme is about 

 thirty million times as great as that of methylamine. The 

 ferric ion appreciably facilitates the breakdown of hydrogen 

 peroxide into water and oxygen. The enzyme catalase, which 

 is a combination of an iron-porphyrin complex with a specific 

 protein-* has the same effect but brings about the reaction 

 about 10^" times as fast as inorganic iron. 



The complicated structure of the protein molecule is also 

 responsible for the second important peculiarity of enzymes, 

 the high specificity of their action. Inorganic catalysts are 

 rather indiscriminate in their action. For example sucrose, 

 maltose, starch, proteins and many other substances may all 

 be hydrolysed equally well by hydrogen ions. But enzymes 

 act in a highly specific way, only catalysing particular 

 reactions. They only break the bonds between certain definite 

 groups of atoms and leave others quite intact, although these 

 may be very similar to those of their substrates. If, there- 

 fore, we have any organic substance which is capable of a 

 number of chemical changes, then, in the presence of any 

 one enzyme it will react with remarkable speed, but only in 

 one particular direction. For example, pyruvic acid in the 

 yeast cell, where the enzyme carboxylase is present, is almost 

 entirely broken down to carbon dioxide and acetaldehvde 

 and it is only the acetaldehyde which is reduced to alcohol 



