ENZYMES — KILBY 283 



some modification in structure occurs. It is difficult to see how this 

 could take place smoothly and efficiently in the cell if all the enzymes 

 in a battery were scattered throughout the volume of a cell. One 

 might suspect that they would be found to be arranged in an orderly 

 sequence, like the machine tools along the production line of a factory, 

 so that the material undergoing transformation can pass easily from 

 one enzyme in the series to the next. Recent discoveries have indi- 

 cated that this may sometimes be the case. Various small granules 

 called mitochondria exist in the cytoplasm of cells (the cytoplasm is 

 the part of a cell outside the nucleus) and most of the enzymes con- 

 cerned with oxidatioti appear to be concentrated in these granules, 

 which may be thought of as the powerhouses of the cell, since the prin- 

 cipal purpose of oxidation is to release energy. Not a great deal is 

 known yet about the structure of mitochondria, or how they are 

 reproduced or of the arrangement of enzymes in them. Handcuffed 

 together, as it were, the enzymes act as a group and their collective 

 behavior may differ from the sum of that previously observed for the 

 individual enzymes that the biochemist has obtained after destruction 

 of the unit. The study of enzymes began with the living cell and then 

 progressed to the isolated enzyme. The main aim of this phase, the 

 classical period of enzyme biochemistry, has been to separate an 

 enzyme from all others that accompany it ; the isolation of crystalline 

 enzymes marks the triumph of this technique. This phase is so pro- 

 ductive of results that it will be developed for a long time yet, but at 

 the same time another approach is being fostered. The aim of this is 

 not the separation of enzymes from each other but the avoidance of 

 this in order to obtain intact teams of enzymes and study the activity 

 of the team. 



The activities of enzymes are not the concern merely of the aca- 

 demic biochemist or the brewer, for enzymes are involved in many 

 aspects of everyday life. The housewife makes junkets by using 

 rennet, a preparation containing rennin which converts caseinogen, 

 a soluble protein of milk, into casein, whose insoluble calcium salts 

 separate out as the curds. The natural function of this enzyme 

 (which occurs naturally in the fourth stomach of the calf) is prob- 

 ably to delay the emptying of the stomach by converting liquid 

 milk into a jellylike mass. 



Meat is hung to make it tender, as animal tissues after death under- 

 go self-digestion owing to the presence of enzymes which partly 

 degrade some of the structural material of the tissues. The oxidiz- 

 ing agents added by millers to flour as "improvers" and in order to 

 bleach it, inhibit the proteinases of the wheat which, if left active, 

 vvould alter the proteins present in the flour and give rise to a less 



