Discussion Slides

Overview

• Ion channels
  • Selectivity
  • Electrical properties
  • Regulatory mechanisms
• Electrical properties of voltage-gated ion channels

Ion Channels

Three properties: Selectivity, Conductivity, Gating

There are Na+, Ca 2+, K+ and Cl- channels

Modeling ion channel selectivity based on size

• Effective size: Ion size + water size
• The size of a sodium ion is smaller than a potassium ion
• However, the charge density of sodium is greater than potassium, so the water shell size is greater
  • The shell of hydration, greater bonds with water
• Hence, the effective size of Na+ is greater than K+
• Practice problem: Model 1: ion channel selectivity only based on effective size?
  • No, then K+ could go through Na+ channels since K+ size < Na+ size

Based on energy

• Ion channels have high or low-energy amino acid binding sites
• The energy released from the bond between the binding site and the ion
• The energy required to shed water of hydration
• If the energy released is greater than the energy required, then energetics favorable
• For example, Na+ has higher energy than K+ (since water is more tightly bound to Na+)
• Practice problem: Model 2: Free energy = Na+ and binding site, and water shedding energy = Na+ and water
  • Passing through happens when free energy > shedding energy
  • Na+ requires high energy to break, so it cannot go through K+
  • K+ requires low energy to break, so can go through Na+