2a Membrane Structure

Membrane Proteins: Introduction

• Membrane proteins are embedded within the lipid bilayer.

• They exhibit different conformations, types, roles, and functions.

Recap of Plasma Membrane Structure

• Lipid bilayer:

  • Exoplasmic leaflet (outside) and cytosolic leaflet (inside).

  • Asymmetric composition of phospholipids.

• Cholesterol molecules:

  • Embedded within the bilayer.

  • Maintain fluidity and stability.

• Proteins:

  • Embedded within the lipid bilayer.

  • Attached to inner or outer sides.

• Cytoskeleton:

  • Attachment point on the inner side of the bilayer.

• Role:

  • Separates the inside from the outside of the cell.

• Membrane composition:

  • Varies depending on cell type and organelles.

  • Defines membrane properties: fluidity, thickness, and curvature.

• Asymmetric nature:

  • Composition differs between the two leaflets.

Membrane Protein Composition

• Proteins constitute approximately 50% of the membrane's weight.

• Ratio is roughly one protein molecule for every 50-100 lipid molecules.

• Protein percentage by mass depends on the membrane's function.

  • Membranes involved in ATP synthesis require a higher amount of membrane-bound protein.

Learning Outcomes

• Understand the structure and function of different membrane protein types.

• Describe the key roles of membrane proteins.

• Explain the fluid mosaic model of the cell membrane.

Components of Membranes

• Lipids:

  • Phospholipids

  • Sphingolipids

  • Glycolipids

  • Cholesterol

Membrane Proteins

• Integral membrane proteins:

  • Amphiphilic, like the lipids they interact with.

  • Transmembrane proteins:

    • Span the membrane from one side to the other.

  • Interact with only one of the two bilayers.

• Lipid-anchored proteins:

  • Covalently linked to lipids (glycolipids or phospholipids).

  • May not directly enter the bilayer.

• Peripheral proteins:

  • Attached via noncovalent bonds.

  • Interact with polar heads of phospholipids or integral proteins.

  • Easier to remove and study.

Membrane Protein Conformations and Structures

• Lipid bilayer represented by two gray lines.

• Protein structures represented in pink and purple.

Transmembrane Proteins

• Span from the extracellular to the intracellular (cytoplasmic) environment.

• Require different domains:

  • Extracellular part: hydrophilic (interacts with the aqueous environment).

  • Transmembrane part: hydrophobic (interacts with the lipid bilayer's fatty acid section).

  • Cytoplasmic part: hydrophilic (aqueous environment).

Types of Transmembrane Protein Structures

• Alpha helix:

  • Embedded part composed of hydrophobic amino acids.

  • Requires at least 20-21 amino acids to cross the bilayer.

• Cluster of alpha helices:

  • Penetrates the plasma membrane.

  • Composed of alpha helices.

• Beta barrel:

  • Formed by beta sheets.

  • Forms porins, allowing a pore from one side of the bilayer to the other.

• Three distinct domains:

  • Exoplasmic domain

  • Transmembrane domain

  • Cytosolic domain

Single-Layer Linkage

• Involved in cell-cell signaling.

• Found on the cytoplasmic or exterior bilayer.

• Responds to signals from the outside or inside environment.

• Undergoes structural changes that interact with proteins.

Fluid Mosaic Model

• Membrane proteins can move laterally within the leaflets of the bilayer.

Lipid-Linked Types

• Embedded and anchored within the bilayer.

• Do not cross from one side to the other.

Protein Attachment Type Proteins

• Further details will be provided.

Alpha Helix Stabilisation

• Stabilised by hydrogen bonding between the amide group of one peptide and the carbonyl oxygen of another (four amino residues upstream).

• Right-handed alpha helix is the most common secondary structure.

• Requires approximately 3.6 amino acid residues for a turn in the alpha helix.

• Transmembrane helices require several turns to cross the membrane (about 20-21 amino acids).

Beta Barrel Structure

• Beta sheet that twists and coils to form a closed structure.

• Stabilised by hydrogen bonding between peptide bonds of adjacent sequences.

• Formed from chains of the same parallel or opposite antiparallel directions.

Summary of Membrane Protein Types

• Integral/Transmembrane proteins:

  • Directly contact the lipid bilayer.

  • Run from one side to the other.

  • Function on both sides.

  • Roles: receptors, signalling, channels (molecule movement).

• Lipid-anchored proteins:

  • Covalently bound to one or more lipids.

  • Do not enter the membrane.

• Peripheral proteins:

  • Bind to the membrane surface.

  • Interact with lipid head groups or integral (transmembrane) proteins.

  • interact on the surface of the lipid bilayer.