CH₂OH CH₂OPO32- a + H+ b 2 2 H-X CH2 R CH₂OH HOH -OH CH₂OPO3²- H enzyme enzyme DHAP 1 CH₂O H₂C R 2 CH₂OPO3²- R = (CH2) 16 CH3 nTP 3 enzyme C CH₂O R OH enzyme 12 NMe3* 4 R NMe3 H₂C P. B. B OH OH B designates the base of the nTP in this intermediate

The Pfizer vaccine for COVID-19 encapsulates mRNA (which encodes the Spike protein of the SARS-CoV2 virus) inside a lipid nanoparticle. Without the protection afforded by the nanoparticle, the mRNA cargo would be quickly destroyed in vivo. One of the four lipids used to make these Pfizer nanoparticles is 1,2 distearoyl-sn-glycero-3-phosphocholine 12 (see below). This compound is an example of a phospholipid, specifically a phosphatidyl choline. Phospholipids form lipid bilayers.

A. Draw a schematic picture of a lipid bilayer formed from 12, and Indicate which parts of 12 are directed towards water, and which are directed to the interior of the bilayer

B. In bacteria, phosphatidyl cholines like 12 are biosynthesized from DHAP as shown below 

Step 1: What are coenzymes a and b? 

Step 2: What is X?

Step 3: Which nucleoside triphosphate NTP is used, and what is the co-product c? 
 
Step 4: What metal ion d likely offers catalysis of the reaction, and what is the coproduct e?