Substrate 



~J^ — [bT 



■» 



Enzyme-substrate 

 complex 



Figure 2-3. Diagram Showing the Formation of Substrate - Enzyme Com- 

 plex as the Result of Coupling Between Two Reactive Sites of the Substrate 

 and Enzyme. (From Harrow, B. and Mazur, A., 1958. "Textbook of Bio- 

 chemistry," 7th ed., W. B. Saunders Co., Philadelphia, Pa., p. 130.) 



Nucleic Acids 



Although nucleic acids have long been known to be a constant con- 

 stituent of living tissues, it is only during the past twenty years that any 

 considerable attention has been paid to their distribution and role in the 

 cell. Nucleic acids are organic polymers composed of repeating units 

 called nucleotides, each of which consists of a pentose sugar, an organic 

 nitrogenous base, and phosphoric acid bound together by sugar-phos- 

 phate linkages (Formula (2-5)). Two kinds of nucleic acids are recog- 



(2-5) 



nized on the basis of the pentose sugar component they contain and to 

 some extent by the differences in bases making up the individual nucleo- 

 tide units. The pentoses concerned are D-ribose characteristic of ribo- 

 nucleic acid (RNA) and deoxyribose characteristic of deoxyribose 

 nucleic acid (DNA). The structural difference between these two sugars 

 involves carbon 2 as is shown in Formula (2-6). This difference appears 

 to be rather small but it is quite sufficient to distinguish one sugar from 

 the other by chemical means. 



The nitrogenous bases consist of two types, namely, the pyrimidines 

 and purines. The former are monocyclic, the latter dicyclic. The most 

 commonly occurring pyrimidines are thymine, cytosine, and uracil while 



16 / CHAPTER 2 



