Lecture 8 - carriers

Question 1

Valinomycin function

Answer 1

It acts as a mobile K+ carrier: it picks up a K+ ion on one side of the membrane, diffuses across, and releases it on the other side.

It is extremely selective for K+ over Na+ because:

K+ has a lower free energy of dehydration — it’s easier to remove its water shell to fit into the ring.

This selectivity is crucial for maintaining ion gradients across membranes in biological systems.

Question 2

Valinomycin structure

Answer 2

Valinomycin is a small cyclic polypeptide

It forms a ring structure made from a repeating cycle of D- and L-valines, lactic acid, and hydroxyvaleric acid.

The centre of the ring is hydrophilic (water-attracting) and can bind a single K+ ion very tightly.

The outer surface of the ring is hydrophobic (lipid-loving) — allowing the entire K+-valinomycin complex to freely dissolve in and move through lipid membranes.

Question 3

Mechanism of carriers

Answer 3

1. Carrier protein, facing outside the membrane, binds to the substrate (e.g., glucose, an ion).
2. The carrier changes shape to re-orient its binding site toward the inside of the membrane.
3. The carrier releases the substrate into the inside of the cell.
4. The now-empty carrier re-orients back to face the outside, ready to bind another substrate.

Question 4

Physiologically Important Examples of Carriers: Neurotransmitter Sodium Symporters (NSSs)

Answer 4

Function: Reabsorb neurotransmitters like dopamine, serotonin, and GABA back into neurons after release.

Driving Force: Uphill transport of neurotransmitters is powered by the Na+ gradient (and sometimes Cl-).

Na+ binding to the transporter increases its affinity for the neurotransmitter — a form of cooperative binding.

Malfunctions in NSS function are linked to depression, epilepsy, Parkinson’s disease, etc.

Question 5

Physiologically Important Examples of Carriers: GLUT 1

Answer 5

Main glucose transporter for the brain (critical at the blood-brain barrier).

Clinical links:

Mutations → GLUT1 deficiency syndrome (severe epilepsy).

Overexpression → marker for aggressive cancers.

Question 6

Physiologically Important Examples of Carriers: GLUT 2

Answer 6

Mediates glucose movement between liver, blood, and gut.

Key component of glucose sensing in pancreatic β-cells (regulates insulin secretion).

Dysfunction of GLUT2 → contributes to diabetes.

Question 7

Mechanism of GLUT-mediated glucose uptake:

Answer 7

GLUT1’s binding site faces outside → glucose binds.

Conformational shift → flips binding site inward.

Glucose released into the cytoplasm.

Empty carrier re-orients to face outside again.

Question 8

Special Case: Yeast Glucose Transport

Answer 8

Yeast like Saccharomyces cerevisiae evolved in glucose-rich environments:

Can uptake glucose passively or using facilitated diffusion.

Other fungi, adapted to nutrient-poor environments, use H+-coupled symporters to actively transport glucose using the proton gradient.

🧠 This shows how nutrient availability shapes transporter evolution.