Membrane Proteins 1

Overview of Cell Membrane Structure and Components

Introduction to Membrane Architecture

The cell membrane is a complex, asymmetric assembly of molecules that defines the boundary of the cell and its internal organelles. According to Mathews & Van Holde, Biochemistry, the membrane is characterized by several distinct features and components:

• Asymmetry: The membrane consists of an Outer monolayer and an Inner monolayer.

• Orientation:     * Exterior face: The side of the membrane facing the extracellular environment.     * Cytosolic face: The side of the membrane facing the cytoplasm.

• Proteins:     * Integral membrane protein: Proteins that span or are deeply embedded within the lipid bilayer.     * Peripheral membrane protein: Proteins associated with the surface of the membrane but not spanning it.

• Carbohydrate Modifications:     * Glycoproteins: Proteins with attached carbohydrate chains.     * Carbohydrate side chains: These are predominantly found on the exterior face of the membrane.

Comparative Membrane Composition

Membranes vary significantly in their ratio of lipids to proteins depending on the specific organelle or cell type. The following data details the lipid and protein proportions observed across various membranes:

• Inner Mitochondrial Membrane: Highest protein content (~80%), lowest lipid content (~20%).

• Outer Mitochondrial Membrane: Roughly equal proportions (~50% lipid, ~50% protein).

• Nuclear Membrane: Approximately 60% protein and 40% lipid.

• rER (Rough Endoplasmic Reticulum): Approximately 60% protein and 40% lipid.

• Golgi apparatus: Approximately 50% protein and 50% lipid.

• Plasma Membrane: Approximately 50% protein and 50% lipid.

• Erythrocyte (Red Blood Cell): Approximately 50% protein and 50% lipid.

• Myelin: Highest lipid content (~80%), lowest protein content (~20%). This high lipid content provides electrical insulation for neurons.

Major Classes of Membrane Lipids

There are four primary classes of lipids that constitute the membrane bilayer:

1. Phospholipids: The most abundant class, providing the fundamental structural framework.

2. Cholesterol: A sterol that modulates membrane fluidity and stability.

3. Sphingolipids: Complex lipids often involved in signaling and protection.

4. Glycolipids: Lipids with attached carbohydrates, found exclusively on the extracellular side.

Detailed Phospholipid Structure and Headgroups

Phospholipids are composed of Acyl Chains, a Glycerol Backbone, and a phosphate group ($PO_4$). They are categorized based on their headgroups into neutral or negative phospholipids:

• Neutral Phospholipids:     * Choline: (Phosphatidylcholine)     * Ethanolamine: (Phosphatidylethanolamine)

• Negative Phospholipids:     * Serine: (Phosphatidylserine)     * Glycerol: (Phosphatidylglycerol)     * Inositol: (Phosphatidylinositol)

Sterols and Sphingolipids

Sterols (Cholesterol): Cholesterol is a vital component with a characteristic four-ring structure and a hydroxyl ($OH$) group that interacts with the hydrophilic heads of phospholipids. Its chemical structure is dominated by the hydrocarbon rings and tail.

Sphingolipids:

• Sphingosine: The backbone for sphingolipids, containing an amino group ($+H_3N$) and hydroxyl groups.

• Sphingomyelin:     * A major component of the neuronal membrane.     * Primarily localized in the plasma membrane.

Glycolipids

Glycolipids possess unique properties and localized distribution:

• Localization: Exclusively found on the outside of the cell.

• Synthesis: Synthesized within the Endoplasmic Reticulum (ER) and Golgi apparatus.

• Cerebroside: Composed of a Fatty Acid, a Sugar headgroup (either glucose or galactose), and a Sphingosine backbone.

• Gangliosides: More complex glycolipids containing branched oligosaccharides.

Lipid Composition of Specific Membranes

Different membrane types within a rat hepatocyte show varying lipid profiles (Source: Physical Biology of the Cell, Garland Science 2009):

• Phosphatidylethanolamine (PE): Distributed across various membranes.

• Phosphatidylcholine (PC): A primary lipid across most membrane types.

• Sphingolipids: Present in significant amounts in specific membranes.

• Cardiolipin: Found significantly in mitochondrial membranes.

• Cholesterol: Varies by membrane type, concentrated in the plasma membrane.

• Minor lipids: Present in smaller percentages across all types.

Physical Properties and Phases of Lipids

Formation of the Bilayer: Lipids form bilayers because they are Amphipathic. They possess:

• Hydrophilic Heads: Can have positive ($+ive$), negative ($-ive$), or neutral ($0$) charges.

• Hydrophobic Tails: Consist of hydrocarbon chains (CCC).

Lipid Phases and Shapes: According to Lee 2009, J. Biology 8: 86, the overall shape of the lipid molecule determines the phase it adopts in water:

• Cone Shapes: Predispose lipids to form Micelles.

• Range of Phases: Includes cubic, micellar, $HI$, and $HII$ phases.

Methods for Studying Bilayer Structure

X-ray and Neutron Diffraction: Bilayer structure is analyzed using diffraction techniques. Studies by Nagle and Nagle (2004) utilize the CHESS (Cornell High Energy Synchrotron Source) X-ray beam and a 2D CCD detector to measure intensity at various angles.

Bragg's Law: The fundamental equation used to determine the spacing within the bilayer is: $n ext{\lambda} = 2d ext{\sin}( ext{\theta})$ Where:

• $n$ is the order of reflection.

• $ext{\lambda}$ is the wavelength of the X-ray.

• $d$ is the distance between layers (interplanar spacing).

• $ext{\theta}$ is the angle of incidence.

Scattering Length Density and Bilayer Profile

Experiments produce graphs of scattering length density as a function of the distance from the bilayer center (Å):

• Neutron scattering: Reveals specific density profiles across the bilayer thickness.

• X-ray scattering: Shows distinct peaks corresponding to high-electron-density regions (like phosphate headgroups).

Molecular Distribution Probability (White, S.H. et al. 2003): Across the bilayer, different chemical groups have differing probabilities of being found at specific distances from the center:

• Hydrocarbon Core: Contains terminating methyl groups ($CH_3$) and double bonds ($-C=C-$).

• Interface Region: Contains Carbonyls, Glycerol, Phosphate, and Choline.

• Solvent Layer: Water molecules penetrate slightly into the interface but are primarily outside the headgroup region.

Functional Roles of Membrane Proteins

Membrane proteins are categorized by their function in material and information transfer:

Material Transfer (Nutrients etc.):

• Transporters: Use active transport mechanisms to move substances against gradients.

• Channels: Facilitate passive transport.

Information Transfer (Cell Signaling):

• Ligand Gated Channels: Open in response to chemical signals.

• Tyrosine Kinase Receptor: Involved in growth and differentiation signaling.

• G-Protein Coupled Receptors (GPCRs): A massive family of receptors involved in various physiological responses.

Influence of the Bilayer on Protein Structure

The lipid environment imposes specific constraints on protein architecture:

• Hydrophobic Interactions: Proteins must have domains that interact favorably with the hydrophobic core of the bilayer.

• Surface Environment: Proteins must cope with the transition from the hydrophobic interior to the hydrophilic exterior at the bilayer surface.

• Dynamics and Integrity: While the bilayer is highly dynamic, a solid and stable interface is required between the protein and the lipid to maintain the integrity of the membrane barrier.

• Functional Architecture: The bilayer provides the necessary structural support for proteins to perform complex tasks like signal transduction and molecular transport.