GENERAL ZOOLOGY 



acid. One contains desoxyribose and is known as desoxyribonucleic acid 

 (DNA); it is localized in the nucleus and is of major importance in inher- 

 itance (p. 197). The other contains ribose and is called ribonucleic acid 

 (RNA); this type is basically concerned in the synthesis of proteins, perhaps 

 determining their pattern and thus their quality of specificity. In addition to 

 the function of RNA in protein synthesis, nucleic acids may be conjugated 

 with proteins to form the nucleoproteins; nucleic acid forms the core of the 

 enormous spiraled molecule. It is known that some nucleoproteins, such as 

 those of the chromosomes which contain DNA (p. 39), and virus particles 

 which contain RNA, are capable of self-duplication; indeed, viruses are often 

 considered to be living organisms. In the chromosome and virus it is the 

 nucleic acid moiety which endows the whole with its unique functional 

 quality. 



Enzymes are proteins which act as organic catalysts. A catalyst is a sub- 

 stance, whether inorganic or organic, that accelerates a chemical reaction 

 without afiecting the character of its end products. An enzymatic protein 

 frequently has to have another compound known as a coenzyme combined 

 with it before it becomes effective as a catalyst; some metalloproteins func- 

 tion as enzymes. Enzymes are effective in very small amounts and are 

 unchanged by the reactions in which they participate. The substance that 

 is acted upon by an enzyme is known as its substrate. Enzymes catalyze 

 reactions, reactions that would otherwise proceed very slowly, by becoming 

 associated or combined with some particular region of the substrate mole- 

 cules. The enzyme is an asymmetrical molecule of such a shape that ordinarily 

 it alone can occupy a particular niche in the substrate molecule. A com- 

 pound with molecules that are quite similar to those of a particular enzyme 

 can sometimes be used to inhibit or block a reaction normally catalyzed by 

 that enzyme; the inhibitor molecules pre-empt the substrate niches in which 

 the enzyme must be present if the reaction is to occur. The association of 

 enzyme with substrate apparently facilitates the transfer of electrons involved 

 in the reaction. When the substrate molecule is changed, either broken 

 apart or combined with another molecule, the enzyme is no longer associated. 

 It is free to combine with another substrate molecule and catalyze its reaction. 

 Within the cell, the end products of one enzyme-catalyzed reaction rarely 

 accumulate; instead, they are immediately involved in other enzyme-catalyzed 

 reactions. Expenditure of energy is required to drive certain chemical reac- 

 tions, and in others energy is released. The end products of many enzyme- 

 catalyzed reactions which release energy are involved in reactions which 

 require energy. Such coupled reactions make possible the continuous se- 

 quences which characterize the metabolism of cells. 



Enzymes exhibit extreme specificity with respect to their substrates. It is 

 obvious that with the multiplicity of chemical compounds occurring in cells 

 there must be a great many enzymes. Enzymes may be thought of as ma- 

 chines carrying out specialized operations in a factory. Each is a part of an 

 integrated enzyme complex that is coming to be recognized as the essence of 



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