Storage vs Structural Lipids

Storage vs Structural Lipids

• Storage lipids and structural lipids differ in function and structure

Storage Lipids

• Triacylglycerols (triglycerides) consist of a glycerol backbone and three fatty acids
• They primarily function in energy storage
• The glycerol backbone is attached to three fatty acids
• Used for long-term energy storage in adipocytes (fat cells)

Structural Lipids

• Phospholipids have a glycerol backbone, two fatty acid tails, and a phosphate group attached to a variable polar head group
• The glycerol backbone consists of two fatty acids and a phosphate group with a head group such as choline, serine, or ethanolamine
• Major components of biological membranes form selectively permeable bilayers
• Sphingolipids comprise a sphingosine backbone, one fatty acid, and a polar head group
• The sphingosine backbone consists of one fatty acid and a polar head group (like choline or sugar)
• Important for cell recognition, signaling, and membrane structure
• Glycolipids resemble sphingolipids but have carbohydrate groups instead of phosphate
• Sphingosine backbone has one fatty acid and a sugar head group
• Glycolipids are involved in cell-cell communication and membrane integrity

Summary of Lipid Structures

• Triglycerides consist of glycerol and 3 fatty acids
• Phospholipids consist of glycerol, 2 fatty acids, a phosphate, and a polar head group
• Sphingolipids consist of sphingosine, 1 fatty acid, and a polar head group
• Glycolipids consist of sphingosine, 1 fatty acid, and a carbohydrate head group

Fatty Acid Notation

• Fatty acids are long-chain carboxylic acids
• General notation: CnH2n+1COOH, where n is the number of carbon atoms

Palmitic Acid Example

• Carbon skeleton: CH3-(CH2)14-COOH
• Has 16 carbon atoms and is saturated (no double bonds)

Oleic Acid Example

• Carbon skeleton: CH3-(CH2)7-CH=CH-(CH2)7-COOH
• Includes a double bond between carbons 9 and 10, making it an unsaturated fatty acid

Fatty Acid Classifications

• Fatty acids are classified as saturated, unsaturated, or polyunsaturated (PUFAs)

Saturated Fatty Acids

• Contains no double bonds between carbon atoms
• All carbon atoms are hydrogen saturated
• Palmitic acid (C16:0) is an example

Unsaturated Fatty Acids

• Contains at least one double bond between carbon atoms
• Monounsaturated: Contains one double bond e.g. oleic acid (C18:1)
• Polyunsaturated (PUFA): Multiple double bonds e.g. linoleic acid (C18:2) or alpha-linolenic acid (C18:3)

Omega Notation

• Describes the position of the first double bond relative to the methyl (omega) end of the fatty acid

Omega-3 Fatty Acids

• First double bond is located between the third and fourth carbon from the omega end
• Alpha-linolenic acid (C18:3w3) has three double bonds, with the first at position 3 from the methyl group

Omega-6 Fatty Acids

• The first double bond is located between the sixth and seventh carbon
• Linoleic acid (C18:2w6) consists of two double bonds, with the first at position 6

Saturation, Unsaturation, and Lipid Properties

• Saturated fatty acids are straight chains that pack closely together, leading to a higher melting point
• Solid at room temperature (e.g., butter)
• Unsaturated fatty acids possess kinked structures due to double bonds, which prevents tight packing, resulting in a lower melting point and being liquid at room temperature (e.g., olive oil)
• Polyunsaturated fatty acids (PUFAs) have even more kinks and lower melting points, leading to oils that are more fluid

Common Lipid Head Groups

• Phosphatidylcholine: Neutral head group (choline + phosphate) and common in cell membranes
• Phosphatidylserine: Negatively charged head group (serine + phosphate) that plays a role in cell signaling
• Phosphatidylethanolamine: Neutral head group (ethanolamine) and important in cell membrane structure
• Sphingomyelin: Neutral head group (choline) found in nerve cell membranes

Net Charge of Lipid Head Groups

• Depends on the nature of the functional group
• Phosphatidylserine and similar molecules with acidic groups carry a negative charge
• Phosphatidylcholine and sphingomyelin have a neutral charge
• Influences membrane properties such as fluidity, interactions with proteins, and cellular signaling

Cholesterol

• Modulates membrane fluidity
• At high temperatures, stabilizes the membrane by reducing fluidity
• At low temperatures, prevents the membrane from becoming too rigid

Lipid Rafts

• Cholesterol-rich microdomains in the membrane are more ordered/thicker than the surrounding membrane
• Serve as platforms for signaling molecules and protein interactions

Membrane Proteins: Structural Features

• Hydrophobic Regions: Membrane proteins often have hydrophobic amino acid residues in their transmembrane segments, allowing them to associate with the hydrophobic interior of the lipid bilayer
• Common hydrophobic residues; Leucine (Leu), Isoleucine (lle), Valine (Val)
• Hydrophilic Regions: Extracellular and intracellular regions typically contain hydrophilic residues that interact with the aqueous environment
  • Common hydrophilic residues: Glutamine (Gln), Asparagine (Asn), Serine (Ser)

Integral Membrane Proteins

• Span the lipid bilayer and are stabilized by hydrophobic interactions between their membrane-spanning domains and the lipid tails

Peripheral Membrane Proteins

• Associate with the membrane surface through electrostatic interactions or interaction with lipid head groups

Summary

• Storage lipids (like triglycerides) are for energy storage, whereas structural lipids (like phospholipids and sphingolipids) are key components of biological membranes.
• Fatty acids can be saturated, unsaturated, or polyunsaturated, with omega notation indicating the position of the first double bond.
• Saturated fats are solid at room temperature, and unsaturated fats are liquid due to preventing tight packing.
• Lipid head groups and their charge (neutral or negative) influence membrane properties and interactions.
• Cholesterol modulates membrane fluidity, and lipid rafts are important for signaling.
• Membrane proteins associate with lipid bilayers via hydrophobic and hydrophilic interactions, with common amino acid residues facilitating these associations.