In the biosynthesis of phosphatidylethanolamine from ethanolamine and triglyceride, 3 high energy molecules are used in the first and second step.
The first steps occur in the cytosol, where ethanolamine is phosphorlated with ATP to give phosphoethanolamine and ADP by the enzyme ethanolamine kinases, and in the next reaction which is the rate-limiting step, phosphoethanolamine react with cytidine triphosphate (CTP) to form cytidine diphosphoethanolamine. In this reaction two molecules of phosphate group is given off.
The third step occurs in the endoplasmic reticulum CDP-ethanolamine react with diacylglycerol to form phosphatidylethanolamine.
The answer is Three high energy molecules. One molecule of ATP to form phosphorylethanolamine and two molecules of ATP to regenerate CTP from CMP.
