A2 Membrane proteins

How are biological membranes formed?

Through self-assembly of amphipathic lipids into bilayers, stabilized by hydrophobic and hydrophilic interactions.

What are Type I membrane proteins?

Integral proteins with the N-terminus outside and C-terminus inside the cell.

What are Type II membrane proteins?

Integral proteins with the N-terminus inside and C-terminus outside the cell.

What are Type III membrane proteins?

Integral proteins that cross the membrane multiple times with helical domains (e.g., bacteriorhodopsin).

What are transmembrane segments typically made of?

α-helices or β-strands with ~20 hydrophobic amino acids.

What is the “positive inside rule”?

Positively charged residues (Lys, Arg) are more common on the cytoplasmic face of membrane proteins.

What do Tyr and Trp residues often do in membranes?

Act as interface anchors at the membrane-water boundary.

What is the hydropathy index?

A measure of free energy change when moving an amino acid from hydrophobic to aqueous environment.

• ΔG < 0 = hydrophilic

  • ΔG > 0 = hydrophobic

What is a hydropathy plot used for?

To predict transmembrane segments by plotting average hydropathy index vs residue number.

What are lipid anchors?

Covalent lipid modifications (e.g., fatty acids, prenyl groups, GPI anchors) that tether proteins to membranes.

Where are GPI-anchored proteins found?

Exclusively on the outer leaflet of the plasma membrane.

Why are lipid rafts important?

They are highly specialized, compartmentalize cellular processes, cluster proteins, and participate in signaling.

How do transporter proteins reduce the energy barrier for diffusion?

By forming noncovalent interactions with solutes and providing hydrophilic transmembrane pathways.

What is facilitated diffusion?

Passive transport via proteins that help specific molecules cross membranes without energy input.

What is active transport?

Energy-driven movement of solutes against their gradient, often powered by ATP or ion gradients.

What distinguishes channels from transporters?

Channels form continuous pores for rapid ion passage

transporters undergo conformational changes for each cycle.

How do transporters compare to enzymes?

They share kinetic concepts (affinity, saturation), but transporters move solutes, not catalyze reactions.