Membrane Proteins Lecture

Introduction

• Membrane proteins constitute up to 75% of the cell membrane.
• Average cell membrane protein composition: ~50% or less.
• Proteins are essential for various cell membrane functions.

Cell Membrane Composition

• Phospholipids: Building blocks forming a lipid bilayer.
• Lipid Bilayer: Comprised of two layers of phospholipids.

Types of Membrane Proteins

1. Integral Proteins

• Embedded throughout the entire membrane.
• Difficult to remove due to their deep integration into the membrane.

2. Peripheral Proteins

• Located on the surface of the membrane or on integral proteins.
• Easier to remove; attach and detach for cell processes (e.g., hormones).
• Can be present on both the outer and inner sides of the membrane.

3. Lipid-bound Proteins

• Rare and found inside the membrane.
• Limited interaction with cellular surroundings, hence less functional.

Integral Proteins: Key Types

Channel Proteins

• Form channels to allow substances (e.g., ions) to pass through the cell membrane.
• Functionality:
  • Allow substances like sodium ions to move in/out of cells.
  • Operate with the concentration gradient (high -> low concentration).
  • Do not require energy (ATP).

Carrier Proteins

• Transport substances across the cell membrane.
• Functionality:
  • Similar to a baseball glove; carries molecules into/out of the cell.
  • Can move substances against the concentration gradient (requires energy/ATP).

Glycoproteins

• Composition: Proteins with attached sugar chains.
• Function: Cell signaling and recognition.
• Can be found on any type of membrane protein (integral, peripheral, channel, carrier).

Summary

• Peripheral Proteins: Easily removable, on the outside.
• Integral Proteins: Embedded within, hard to remove.
• Lipid-bound Proteins: Rare, less functional.
• Channel Proteins: Facilitate movement along concentration gradients without energy.
• Carrier Proteins: Transport against concentration gradients, sometimes using ATP.
• Glycoproteins: Aid in cell signaling and recognition.