L16 Biological Membranes and the Chemistry of Lipids
Context and Course Roadmap
The current focus of the course is on the generation of energy.
Previous lectures covered various reactions resulting in the production of , though was not the central focus then.
(Adenosine Triphosphate) is defined as the "energy currency of the cell."
The schedule for the coming weeks includes:
Today: Structural components and membranes (lipids).
Tomorrow: Membrane proteins.
Next Week: Oxidative phosphorylation, where energy stored in energy carriers is converted into to sustain life.
The lecture emphasizes that membranes are not merely structural units for protein insertion; they possess high complexity that is critical to cellular function.
Essential Building Blocks of Biological Membranes
Theoretical and practical knowledge of membranes has evolved significantly over the last to years.
Lipids are the basic building blocks of membranes. We obtain them from our diet or synthesize them internally.
Lipid stores are tightly regulated within the body.
Pathological contexts, such as the synthesis of "good" and "bad" cholesterol, have been previously discussed.
Glycerophospholipids: Structure and Classification
Phospholipids (specifically glycerophospholipids) are built from a glycerol moiety.
Glycerol Structure: A three-carbon (, , ) molecule with alcohol groups.
Synthesis Steps:
The carbonyl group of a fatty acid must be activated by fatty acyl .
The fatty acid then reattaches to the glycerol moiety to generate triglycerides (used for energy storage) or diglycerides.
Diglycerides: These are precursors involved in the synthesis of phospholipids and signaling molecules.
Phosphatidic Acid:
Contains two acyl chains attached to glycerol.
Contains a phosphate group and an associated head group denoted as "X."
It is a minor component of cell membranes but serves as a vital precursor for more complex phospholipids.
Amphipathic Properties: Phospholipids possess both hydrophobic (tails/acyl chains) and hydrophilic (head group) regions. The tails avoid aqueous solutions, while the head group associates with them.
Molecular Composition:
Saturated fatty acids are generally located on .
Unsaturated fatty acids (containing double bonds) often occupy the position.
Diversity in phospholipids arises from various "X" head groups and the specific composition of the acyl chains (e.g., length and double bonds).
Sphingolipids: Composition and Functional Diversity
Sphingolipids represent another major class of membrane lipids.
Sphingosine Structure: A long-chain amino alcohol. The , , and molecules of sphingosine are analogous to the glycerol moiety in phospholipids.
Assembly:
A fatty acid chain associates at the position.
A polar head group attaches to the structure, providing diversity.
Synthesized from fatty acyl and serine.
Comparison: Structurally, sphingolipids are quite similar to phosphatidylcholine (a phospholipid).
The ABO Blood Group System: A Case Study in Lipid Decoration
Sphingolipids are not just structural; they play a role in cell identity via decoration with complex sugars (glycosides).
Blood Group Genetics:
Group O: Missing the enzymes required to build from the base group (fructose, galactose, and -acetylglucosamine). This results in a "base" decoration.
Group A: Encodes a glycosyltransferase that adds an -acetylglucosamine group to the base.
Group B: Encodes a glycosyltransferase that adds a galactose group.
Group AB: Possesses both enzymes and produces both modifications.
Immunological Implications:
Individuals possess antibodies against the antigens they lack.
Group A has anti-B antibodies; Group B has anti-A antibodies.
Group O has both anti-A and anti-B antibodies.
Group AB has no antibodies against these antigens.
Transfusions: Giving B-containing blood to someone with anti-B antibodies causes the antibodies to bind and aggregate red blood cells, leading to dangerous clotting.
Universal Status:
(O negative) is the universal donor because it lacks these antigens and the protein antigen.
(AB positive) is the universal acceptor.
Cholesterol: Structure and Role in Membranes
Cholesterol is an amphipathic molecule with a polar group and a highly hydrophobic sterol region.
Chemical Formula: Contains carbons.
Synthesis: Built from acetyl . While all cells can synthesize it, most synthesis occurs in the liver. It starts in the cytosol and is transferred to the endoplasmic reticulum.
Interactions: The hydroxyl moiety allows it to interact with aqueous environments or polar head groups of phospholipids.
Formation and Physical Properties of Lipid Bilayers
Micelles: Formed by single-chain fatty acids (like those in dish soap or cooking fats) where the hydrophobic tails face inward.
Liposomes and Bilayers: Due to having two acyl chains, phospholipids spontaneously form bilayers in aqueous buffers. These consist of an outer leaflet and an inner leaflet.
Cholesterol Integration: Cholesterol intercalates between phospholipids. It is often described as "poly filler," as it fills gaps and provides structural integrity.
Lipoproteins vs. Bilayers: Phospholipids in lipoproteins (which carry fats like cholesterol esters) do not form double leaflets because they are associated with highly hydrophobic environments rather than being surrounded by aqueous solution on both sides.
Biological Significance of Compartmentalization
Membranes act as barriers that enable compartmentalization, which facilitated the evolution of complex organisms.
Separation of Competing Reactions: Reactions that would interfere with each other can occur in different environments.
Example: Fat synthesis occurs in the cytosol, while fat oxidation occurs in the mitochondria.
Signal Perception: Membranes embed proteins that receive extracellular signals (e.g., hormones) and initiate intracellular signal cascades.
The Evolution of Membrane Models
The Fluid Mosaic Model (1972): Proposed by Singer and Nicholson.
Defined membranes as a "sea of lipids" where proteins could insert or span the bilayer.
Proteins were thought to float freely without restriction.
Challenges to the Old Model: Signal transduction relies on efficiency and speed. In a completely random "sea," receptors and signaling partners (like -coupled receptors and Adenylate Cyclase) would struggle to find each other quickly. Collocation is necessary for cellular response.
Modern View (2006): Membranes are highly dynamic, heterogeneous structures. They are impermeable to polar molecules and ions, which is essential for generating proton gradients.
Influence of Lipids on Protein Function
Specific lipids are required for the activity and stability of membrane proteins.
Example 1: Acyl Chain Length: Research shows that certain proteins have peak activity only when associated with phospholipids of a specific chain length (e.g., -carbon chains), which affects membrane thickness.
Example 2: ATP ADP Translocase:
Requires cardiolipins to stabilize its structure.
Cardiolipins are primarily found (% of composition) in the inner mitochondrial membrane.
If cardiolipins appear on the outer mitochondrial membrane, it serves as a signal to the cell that the mitochondria is "stuffed" (non-functional) or that the cell is entering programmed death.
Membrane Asymmetry and Dynamics
Trans-leaflet Asymmetry: The outer and inner leaflets have different lipid compositions.
Phosphatidylcholine (larger head group) is often on the outer leaflet to facilitate membrane curvature.
Active Transport (Flippases, Floppases, Scramblases): These proteins move lipids between leaflets, a process that requires energy.
Intra-leaflet Asymmetry (Lipid Rafts):
Clusters of lipids (often rich in cholesterol) that group together to facilitate signaling.
These are controversial because they are nano-sized and short-lived.
Lipid rafts are implicated in pathologies like Alzheimer’s disease, where they may promote protein aggregation.
Cytoskeletal Interactions and Protein Mobility
The inner side of the membrane interacts directly with the cytoskeleton (the scaffold of the cell).
Protein movement is not entirely random; it can be:
Confined/fenced in by the cytoskeleton.
Tethered directly to the scaffold.
Restricted by interactions with the extracellular matrix (sugars/proteins outside).
Restricted by protein-protein interactions within the lipid environment.
Questions & Discussion
The Carnitine Shuttle: A student questioned the redundancy of adding to activate a fatty acid, only to swap it for carnitine to cross the membrane, and then swap it back to . The lecturer confirmed it is redundant but necessary because the inner mitochondrial membrane is impermeable to the large fatty acyl molecule, and there is no transporter for it; carnitine is the required carrier for regulation and transport.