252 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



succinyl-CoA formed during the Krebs cycle. The intermediate stages are 

 a-ketoadipic acid and a-aminoadipic acid (which have been found in the 

 degradative pathway of lysine in the rat). In bacteria, one of the starting 

 materials is aspartic acid and a,a'-diaminopimelic acid is an intermediate. 



B. Biosynthesis of Compounds Containing Ester, 

 OsiDE OR Peptide Bonds 



(a) Biosynthesis of Complex Lipids 



This subject has been extensively studied in the case of the lecithins 

 (phosphatidylcholines) and it has been shown that, in the presence of 

 certain enzymes of different specificity, the same mechanism is responsible 

 for the synthesis of cephalins (phosphatidylethanolamines). As with the 

 formation of glycerides, the starting point is a phosphatidic acid. The 



HoCOH 



(CH3)3 



choline' 

 phosphokinase 



NCHa — CH2OH 

 choline 



ATP 



(CH3)3 — NCH2 — CH20PO(OH)2 



phosphorylcholine 



cytidinetri- 



phosphate 



CTP 



cytidinediphosphatecholine 



HCOH 



L- a-glycerophosphoric 

 acid 



H2COPO(OH)2 



+ 2RC0A-? 



H2COR 



I 

 HCOR 



I 

 HaCOPOCOH)^ 



diacylphosphatidic acid 



Fatty Aci<j[ 

 cycle 



H2COR 



I 

 HCOR 



i 

 H — C — H 



1,2-diglyceride 



L-a-lecithin 



Fig. 72 — Biosynthesis of a lecithin. 



knowledge that we possess today about the biosynthesis of complex lipids 

 stems from observations made on the incorporation of ^-P and ^^C-labelled 

 phosphorylcholine into the lecithin fraction of isolated liver mitochondria. 

 This incorporation is activated by CoA and ATP, and inhibited by metal 

 ions, in particular by Ca++. The incorporation requires a nucleotide as a 



