How the Cell Lives 25 



that one molecule of catalase can decompose over two million mole- 

 cules of hydrogen peroxide every minute. 



All these enzymes turned out to be proteins. In this way it became 

 evident that besides their other functions, Nature makes use of 

 proteins as 'biocatalysts'. Enzymes are indeed highly speciahzed 

 agents. There is usually a separate enzyme for each distinct chemical 

 process. Sometimes an enzyme will bring about the change in a 

 single substance only; but others are capable of changing a number 

 of closely related substances. 



Very little is known about how enzymes bring about these 

 chemical actions. It has been shown that the first step is the com- 

 bination of the enzyme with the substance undergoing change. This 

 combination is very specific. The enzyme will combine with one, or 

 perhaps a few similar molecules, but with no others. There must 

 thus be a close fit between the enzyme and the molecule or mole- 

 cules it acts on. The molecules must fit exactly into some part of the 

 surface of the enzyme molecule. 



What happens then is largely a matter of conjecture, but it is not 

 difficult to see that if the enzyme is combined firmly with the two 

 halves of a molecule, the bond joining them may be weakened so 

 that the molecule easily divides into the two halves. The binding of 

 the molecule with the enzyme will necessarily weaken the strength 

 of the bonds holding the molecule together. In this way the enzyme 

 can facilitate the rupture of a molecule into two parts. Readers who 

 are interested will see the idea worked out in more detail in Fig. 4. 



The synthesis of a molecule from its parts will also be facilitated 

 if they fit closely into adjacent parts of the enzyme surface, so that 

 they are brought into juxtaposition under conditions in which they 

 will then unite. The enzyme provides a kind of *jig' which holds the 

 bits together while a bond between them is formed. 



In either case the important thing is that the enzyme must provide 

 a structure which is exactly adapted to the molecules it has to deal 

 with. 



Life is largely a matter of enzymes. All organisms contain large 

 numbers of them — all working together and bringing about the 

 extraordinary changes which life produces in its environment. Many 

 kinds of them exist in living things. Not only is it found that every 

 organism has a considerable number of enzymes, but it also appears 

 that an enzyme of one species is not exactly the same as an enzyme 

 with similar functions in another. The enzymes are therefore not 

 only highly adapted to the chemical changes they have to bring 

 about; they are also stamped in some way with the individuality of 

 the species they belong to. We do not know how this is achieved. 



