Question

Deducing Lipid Structure from Molar Ratio of Components Complete hydrolysis of a glycerophospholipid yields glycerol, two fatty acids \(\left(16: 1\left(\Delta^{9}\right)\right.\) and \(\left.16: 0\right)\), phosphoric acid, and serine in the molar ratio \(1: 1: 1: 1: 1\). Name this lipid and draw its structure.

Short answer

The lipid is phosphatidylserine.

Step-by-step solution

Understanding the Components

The hydrolysis of the glycerophospholipid results in several components: one molecule each of glycerol, two distinct fatty acids (16:1(Δ9) and 16:0), phosphoric acid, and serine. Each component suggests the structural building blocks of the glycerophospholipid.

Identifying the Fatty Acids

The fatty acids 16:1(Δ9) and 16:0 describe their carbon chain length and saturation. 16:0 is a saturated fatty acid with 16 carbon atoms, and 16:1(Δ9) is a monounsaturated fatty acid with a double bond between the 9th and 10th carbon atoms.

Analyzing the Head Group

When phosphoric acid and serine are present in the molar ratio of 1:1 in the hydrolysis products, the serine is likely part of the head group of the glycerophospholipid, typically forming a phosphatidylserine group.

Naming the Lipid

The glycerophospholipid, based on its components, would be known as phosphatidylserine. This is because it contains the phosphatidyl radical (glycerol plus phosphoric acid) attached to serine as the head group.

Drawing the Structure

The structure of phosphatidylserine includes glycerol bonded to two fatty acids at the first and second hydroxyl groups, while the third hydroxyl group is linked to a phosphate group that is also bonded to serine. The 16:1(Δ9) and 16:0 fatty acids are attached to the glycerol backbone.

Key concepts

Phosphatidylserine

Phosphatidylserine is a type of glycerophospholipid, which is an essential component of cell membranes. It plays a crucial role in maintaining the integrity and functionality of these membranes. Glycerophospholipids are composed of a glycerol backbone linked to fatty acids, and a phosphate group bonded to an additional organic molecule—in this case, serine. Phosphatidylserine specifically has a serine head group, which distinguishes it from other glycerophospholipids like phosphatidylcholine or phosphatidylinositol.
The presence of serine in the head group is vital for cellular processes such as signaling pathways that regulate various cellular functions. Phosphatidylserine is involved in the coagulation of blood and protein C activation. It also aids in the apoptosis process, which is essential for removing unhealthy or damaged cells.

Fatty Acid Structure

Fatty acids are long hydrocarbon chains that are either saturated or unsaturated, playing a significant role in the structure of lipids. The two fatty acids identified in the glycerophospholipid, 16:1(Δ9) and 16:0, provide insight into the lipid's properties.

• 16:0: This is a saturated fatty acid, meaning all carbon atoms are connected by single bonds, allowing them to pack closely together, making the lipid more solid at room temperature.
• 16:1(Δ9): This is a monounsaturated fatty acid, which has a double bond between the 9th and 10th carbon atoms. Such a kink in the chain prevents the fatty acids from packing tightly, increasing fluidity in the membrane where they are present.

The combination of saturated and unsaturated fatty acids within the same glycerophospholipid contributes to the fluidity and flexibility of the cell membrane.

Lipid Hydrolysis

Lipid hydrolysis refers to the chemical breakdown of a lipid into its component molecules. It is an important reaction in biological systems, allowing for the utilization and recycling of lipids.
During complete hydrolysis of the glycerophospholipid, products obtained include glycerol, two specific fatty acids, phosphoric acid, and serine. This breakdown happens through the action of water molecules, often facilitated by enzymes. The cleavage of ester bonds in the glycerophospholipid liberates these individual components. Understanding the result of hydrolysis allows us to deduce the original structure of the lipid, as each component is a building block indicating how the original molecule was assembled.

Molar Ratios

Molar ratios are essential in determining the stoichiometry of a reaction, indicating the relative amounts of reactants and products involved. In the original exercise, the molar ratio of the hydrolysis products is stated as 1:1:1:1:1 for glycerol, two fatty acids, phosphoric acid, and serine.
This uniform ratio implies that each component originated from one molecule of the glycerophospholipid. By understanding these ratios, we can confirm the structure of the glycerophospholipid, since each mol is linked to a specific part of the lipid. For instance, one mole of glycerol corresponds with the backbone of the molecule, while each fatty acid aligns with the two esterified positions on glycerol.

Head Group Identification

Identifying the head group of a glycerophospholipid is crucial for understanding its function and classification. In our exercise, the head group is identified by examining the products of lipid hydrolysis—particularly phosphoric acid and serine.
The presence of these two components in a 1:1 ratio indicates that serine is attached via a phosphodiester bond to the phosphate group. This configuration is characteristic of a phosphatidylserine head group. The head group influences the lipid's physicochemical properties and its biological roles. In phosphatidylserine, the serine moiety provides a net negative charge, affecting interactions with other molecules and ions, thereby playing a pivotal role in cellular systems.