Stoichiometry of phosphatidylserine (PS) formation During the biosynthesis of phosphatidylserine in mammals, how many water (H2O) molecules are directly consumed by the head group formation reaction?

Introduction / Context:
Phosphatidylserine (PS) is a major anionic phospholipid in eukaryotic membranes. In mammals, PS is primarily formed by a base exchange reaction rather than by the CDP diacylglycerol pathway used in many bacteria. Knowing the reaction chemistry clarifies whether water is consumed during PS head group formation.

Given Data / Assumptions:

• Mammalian PS synthase 1 and 2 (PSS1, PSS2) catalyze base exchange on existing phospholipids.
• Substrates are phosphatidylcholine (PC) or phosphatidylethanolamine (PE) plus serine.
• Reaction swaps the head group; no hydrolysis by water is required for the exchange itself.

Concept / Approach:

In the base exchange mechanism, serine displaces choline (from PC) or ethanolamine (from PE) to generate PS and release the corresponding amine. The reaction proceeds without direct consumption of water. In bacterial pathways, PS can form from CDP diacylglycerol + serine → PS + CMP, again not requiring water for the bond forming step. Therefore, the number of water molecules consumed for PS formation is zero in the canonical reaction.

Step-by-Step Solution:

Verification / Alternative check:

Enzymology of PSS1/PSS2 shows direct displacement; water involvement is not part of the net reaction. PS decarboxylation to PE (mitochondrial PSD) releases CO2, not water.

Why Other Options Are Wrong:

Options 1, 2, 3, and 4 introduce water where it is not required by the reaction mechanism; they confuse hydrolysis steps elsewhere in lipid metabolism with base exchange.

Common Pitfalls:

Assuming that all head group transformations are hydrolytic or require water; conflating PS synthesis with phospholipase mediated hydrolysis.

Final Answer:

0