Glycerophospholipids are formed when a phosphatidic acid (PA) reacts with another compound. The generic structure of PA is shown below, where two fatty acid chains are covalently bound to a glycerol molecule along with a phosphate group. Examples include serine reacting with PA to make phosphatidylserine and choline reacting with PA to make lecithin (phosphatidylcholine). The list of compounds that can react with PA is varied, but they all have one common feature.
Which of the following features must be present in a compound so that it will react with PA to produce a glycerophospholipid?