236 



NUCLEIC ACIDS AND RELATED COMPOUNDS 



boiling with dilute acid, while pyrimidine nucleosides are considerably more resistant to 

 hydrolysis so that autoclaving with strong acid is required, and some decomposition may 

 occur (e.g. deamination of cytosine to form uracil). 



NUCLEOTIDES 



The nucleotides, aside from their function as components of nucleic acid, constitute 

 one of the most physiologically interesting groups of natural products. Many of the impor- 

 tant coenzymes for dehydrogenation and group transfer reactions are nucleotides. The 

 mononucleotides have one or more phosphoric acid groups esterified to the sugar portion 

 of a nucleoside. The dinucleotides may be regarded as having two nucleoside units joined 

 through a pyrophosphate bridge which esterfies their sugar units. Additional phosphate 

 groups may also be present. Nucleotides containing adenine appear to be the most com- 

 mon in nature. Three different monophosphates of adenosine are known, depending on 

 which hydroxyl group of the ribose is esterfied. Muscle adenylic acid is the 5' -phosphate, 

 while 2' and 3' phosphates have been isolated from hydrolyzed nucleic acid of both fungi 

 and higher plants. These last two compounds have not been reported to occur in a free 

 form in higher plants. Mono-, di-, and triphosphates of adenosine, guanosine, cytidine, 

 and uridine have all been found in young Vicia faba plants (10). 



(ATP): 



By far the most important adenine mononucleotide is adenosine-5' -triphosphate 



ATP 



It has been isolated from animals, fungi and higher plants where it serves as a coenzyme 

 for many phosphokinases and may be regarded as an energy storage compound since hy- 

 drolysis of the third phosphate group releases energy which can be used to drive ender- 

 gonic reactions. The role of ATP in activating amino acids for protein synthesis and ac- 

 tivating methionine for transmethylation is discussed in Chapter 9. Adenosine-5' -nucleo- 

 tides are also found as subunits of some other important coenzymes. 



Diphosphopyridine nucleotide (DPN) or nicotinamide adenine dinucleotide (NAD), 

 triphosphopyridine nucleotide (TPN) or nicotinamide adenine dinucleotide phosphate (NADP), 

 riboflavin phosphate (flavin mononucleotide, FMN), and flavin-adenine dinucleotide 

 (FAD) serve as hydrogen carriers in oxidation-reduction reactions. Their complete struc- 

 tures are given in Table 2. In the first two coenzymes the pyridine ring is involved in the 

 reversible reduction, and in the flavine coenzymes the isoalloxazine ring functions, as 

 follows: , , 



H W 



+ H^ 



