MODES OF LENKAGE BY COVALENT BONDS 67 



(/) Thiamine Pyrophosphate 

 [Cocarboxylase, Diphosphothiamine, DPT) 



Thiamine, being basic, gives a series of salts and esters. 



CHs O- OH 



! I I 



C=C— CHo— CHoO— P— O— P=0 



\ II I 



N=rC— NHo S O OH 



I I / 



CH3-C C CH N=C 



nil + H 



N-CH 



The most important ester is the product of the reaction between pyro- 

 phosphoric acid and the hydroxyl of the thiazole ring. This ester is the 

 coenzyme for carboxylase and for the decarboxylation of a series of a-keto- 

 acids. 



(g) Nucleotides 

 These are the phosphoric esters of nucleosides. 



1. Mononucleotides 



(a) Adenosine mo?io- and poly- phosphates — In most cells these com- 

 pounds act as coenzymes in the transport of phosphate groups. 



Adenosine triphosphate (ATP) was isolated from muscle by Lohmann in 

 1928. The three terminal phosphate groups of ATP are joined by two 

 anhydride bonds. Removal of the terminal phosphate gives adenosine 

 diphosphate (ADP), and of the next phosphate leaves adenosine mono- 

 phosphate (AMP) or adenylic acid. 



The adenosine phosphates (AMP, ADP, ATP) are relatively unstable 

 in solution. At 100° in dilute acid the two anhydride bonds of ATP are split 

 but the ester linkage remains intact. In AMP the esterification of the 

 adenosine is on C-5' of the sugar whilst in other nucleotides (coenzyme A, 

 triphosphopyridine-nucleotide) other carbon atoms are involved. 



In the molecules of adenosine polyphosphates, one of the phosphoric 

 acid residues is linked to the nucleoside by an ester bond but the phos- 

 phoric acid residues among themselves are joined by an anhydride bond 

 much less stable than the ester bond. As we shall see the hydrolysis of 

 anhydride linkages plays an important part in biochemical energetics on 

 account of their strongly exergonic nature. 



(b) Uridine phosphates— These mononucleotides have been demonstrated 

 in yeast and in animal liver. They are coenzymes for the reaction galac- 

 tose- 1-P ±;: glucose- 1-P, and for the formation of sucrose from glucose and 

 fructose. As with the adenosine phosphates, a uridine-5 '-triphosphate 



