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GENERAL CONCEPTS 





Enzyine— substrate complzx. 



This -molecule is 

 not a. Substra.tc. 

 It does not fit on 

 tHc enzyme- 

 Surface. 



A » B 



Split products 



Figure 4.1. Diagram illustrating the concept of a specific enzyme surface which per- 

 mits the formation of a specific enzyme-substrate complex. 



Enzymes generally work in teams in the cell, with the product of 

 one enzyme-controlled reaction serving as the substrate tor the next. We 

 can picture the inside of a cell as a factory with many different assembly 

 lines (and disassembly lines) operating simultaneously. Each of these 

 assembly lines is composed of a number of enzymes, each of which 

 catalyzes the reaction by which one substance is converted into a second. 

 This second substance is passed along to the next enzyme, which con- 

 verts it into a third, and so on along the line. From germinating barley 

 seeds one can extract two enzymes that convert starch to glucose. The 

 first, amylase, splits starch to maltose and the second, maltase, splits the 

 double sugar maltose to two molecules of the single sugar glucose. Eleven 

 different enzymes, working in a series or "cycle," are required to 

 convert glucose to lactic acid. The same series of eleven enzyines is found 

 in human cells, in green leaves and in bacteria. 



Some enzymes, such as pepsin and urease, have been found to consist 

 solely of protein. Many others, however, consist of two parts, one of 

 which is protein (called the apoenzyme) and the other (called a co- 

 enzyme) is some smaller organic molecule. Coenzymes can usually be 

 separated from their enzymes and, when analyzed, have proved to con- 

 tain some vitamin— thiamine, niacin, riboflavin, etc.— as part of the mole- 

 cule. This finding has led to the generalization that all vitamins function 

 as parts of coenzymes in the cell. Neither the apoenzyme nor the coen- 

 zyme alone has catalytic properties; only when the two are combined is 

 activity evident. Certain enzymes require for activity, in addition to a 

 coenzyme, the presence of one or more ions. Magnesium (Mg++) is 

 required for the activity of several of the enzymes in the chain which 

 converts glucose to lactic acid. Ptyalin, the starch-splitting enzyme of 

 saliva, requires chloride ion as an activator. Most, if not all, of the ele- 

 ments required by plants and animals in very small amounts— the so- 



