BIOSYNTHESIS 253 



cofactor, cytidine-triphosphate. The enzyme is phosphorylcytidyl- 

 transferase and catalyses the combination of cytidine triphosphate with 

 phosphorylchoUne to form cytidine-diphosphate-chohne which is the 

 activated form of phosphorylchoKne. A molecule of cytidine monophos- 

 phate is released in this reaction, and in the presence of ATP is reconverted 

 to cytidine triphosphate. The phosphorylcholine itself is formed by a 

 direct phosphorylation of choline by ATP in the presence of the specific 

 enzyme choline-phosphokinase. But what is the acceptor of the activated 

 phosphorylcholine? The acceptor is a 1, 2-diglyceride, thus explaining 

 why natural lecithins are all of the a-type. 



The 1, 2-diglyceride arises by a dephosphorylation of a diacyl-phospha- 

 tidic acid (a glycerophosphatide without the base) in the presence of a 

 specific phosphatase. The phosphatidic acids which are intermediates in 

 the biosynthesis of lecithins are the result of a combination, in the presence 

 of ATP and a specific enzyme, of a-glycerophosphoric acid with two 

 molecules of fatty acid activated by CoA. These compounds of aliphatic 

 acids and CoA are provided by the fatty acid cycle functioning in the direc- 

 tion of biosynthesis. The enzyme catalysing acyl transfer has a special 

 specificity for the acyl-CoA derivatives of the C^g and Cig acids. 

 Example: 

 palmityl-CoA + a-glycerophosphate 



-> monopalmitylphosphatidic acid + CoA 

 palmityl-CoA monopalmitylphosphatic acid 



-> dipalmitylphosphatidic acid + CoA 



(b) Biosynthesis of Glycerides 



It has long been recognized that the biosynthesis of glycerides occurs 

 via the intermediate formation of complex lipids. 



The 1, 2-diglycerides acting as intermediates in lecithin biosynthesis 

 can also give triglycerides by the operation of an enzyme system present 

 in liver. 



The following reaction has been observed in vitro in the presence of an 

 enzyme present in liver: 



1, 2-diglyceride + palmityl-S-CoA -> triglyceride + Cox\-SH 



This establishes an intimate relationship between the biosynthesis of 

 complex lipids and that of ternary lipids, in the first case, the reaction 

 being between a 1, 2-diglyceride and cytidinediphosphatecholine and in 

 the second case between the 1, 2-diglyceride and a coenzyme A. activated 

 fatty acid. 



{c) Biosynthesis of Pentose Phosphates 



As we have seen, D-ribose-5-phosphate is formed from G — 6 — P in the 

 hexosemonophosphate shunt. 



