Membrane transport physiology

Why are membrane transport proteins important?

Maintains homeostasis and cellular function

Regulate movement of molecules between intracellular and extracellular environments

Where are membrane transport proteins present?

Plasma membranes

Nuclear membranes

Intracellular organelle membranes

What is the main problem that membrane transport proteins solve?

The plasma membrane is a barrier to most molecules, so transport proteins enable controlled movement across it

What are the lipids in plasma membranes?

Phospholipids (including sphingolipids)

Cholesterol

Glycolipids

Percentage of phospholipids in plasma membranes?

~49%

Percentage of cholesterol in plasma membrane

~49%

Percentage of glycolipids in plasma membrane

~2%

How is the plasma membrane formed?

Forms a lipid bilayer - hydrophobic interior acts as a barrier (with embedded proteins and cholesterol)

Composed of: phospholipids (~49%), cholesterol (~49%), glycolipids (~2%)

How much does cholesterol comprise of lipid in cell membranes?

25-30%

Which lipid is part of the exclusively outer plasma membrane?

Glycolipids

Which lipid is part of the predominantly outer plasma membrane?

Phosphatidylcholine

Sphingomyelin

Which lipid is part of the exclusively inner plasma membrane?

Phosphatidylinositol (-ve)

Which lipid is part of the predominantly inner plasma membrane?

Phosphatidylethanolamine

Phosphatidylserine (-ve)

Which lipid is part of both parts of plasma membrane?

Cholesterol

Why is cholesterol important in cell membranes?

Increases packing of phospholipids reducing membrane permeability

Maintains membrane fluidity

What is net flux?

Net solute movement from high to low concentration

Influx

Into cell

Efflux

Out of cell

Equilibrium

Equal movement both directions

What are the 4 main types of membrane transport?

Simple diffusion, channel diffusion, facilitated diffusion, active transport

What determines the net direction of solute transport?

The concentration of gradient (movement is from high to low concentration)

Passive (non-mediated) transport

No energy required

Movement down concentration/electrochemical gradient

What are the two passive (non-mediated) transport?

Simple diffusion across membrane bilayer and through an aqueous channel

What does simple diffusion depend on?

Non-selective

Moves down concentration gradient (high → low)

Small, nonpolar, or lipid soluble molecules

What molecules do simple diffusion permit?

Gases

Hydrophobic molecules

Small polar molecules

Passive diffusion

No ATP

Direct through lipid bilayer

Non-specific

Non-saturable

Moves nonpolar and small uncharged molecules - gases (O2, CO2), hydrophobic molecules

Less selective than channels

More on solubility and size

No gates

Passive diffusion through channel

Specific ions or charged polar molecules pass through specialised membrane-embedded proteins - highly selective for solutes

Gated

Faster

What is diffusion through channels?

Passive transport of ions or water through specialised protein channels (highly selective) that span the membrane, moving solutes down their electrochemical gradient

What are properties of diffusion though channels>

Highly selective - to certain ions

Fast conductance

Gating - channels open and close gates in response to specific stimuli

Bidirectional (both sides) access - net flux down a concentration/voltage gradient

What are 3 types of gated ion channels?

Ligand-gated

Mechano (tension) gated

Voltage gated

How do ion channels specificity work with sodium and potassium?

Na+ and K+ exist in a hydrated form

But hydrated K+ are too large to pass through Na+ channels

So potassium channels trips water molecule (dehydrates it) so K+ can pass through channel

What are aquaporins?

Very specific for water, have extremely high conductance, and are impermeable to ions like H+ and OH-

What is mediated (facilitated) diffusion? (Carrier-mediated)

Passive movement of molecules down their concentration gradient via specific carrier proteins that a undergo conformational changes to move solutes across membrane

How are solutes moved across membrane in transported mediated diffusion?

Solute binds to ‘binding’ site on either side of membrane → conformational changes → ‘release’ of solute on opposite side

What are the 3 classes of carrier proteins?

Uniport, Symport (co-transport), antiport (counter transport)

Uniport

Single solute

Symport (co-transport)

2 different solutes transported together

Antiport (counter transport)

One solute binds and is transported, another solute binds and is transported in the opposite direction

Vmax

Maximum rate when all transport proteins are ‘occupied’

A measure of total transport proteins

Km (Solute)

At which ½ binding sites are occupied

A measure of affinity of solute for carrier protein (affinity constant)

What is the GLUT transporter family responsible for?

Facilitated diffusion of glucose

What is unique about GLUT4?

Insulin-regulated and present in muscle and adipose tissue

GLUT 4 specificity

D-glucose not L-glucose

GLUT4 transport direction

Bidirectional: most cells accumulate glucose O→I, hepatocytes release glucose I→O

What does insulin do do for GLUT4?

Increases number of GLUT4 transporters in muscle and adipose tissue membranes → higher glucose uptake

GLUT4 kinetics

Km remains the same - no change in affinity of transport proteins for substrate

Vmax increases - increase in number of transport proteins

Mediated (facilitated) diffusion characteristics

Passive - no ATP used

Carrier proteins

Specific

Saturable - shows Vmax and Km

Bidirectional - depending on gradient

Active transport

Movement of molecules against their concentration or electrical gradients - from low → high concentration - requiring energy input

Primary active transport characteristics

Requires metabolic energy - couples transport to ATP hydrolysis

Highly selective and regulated

Can be Uniport or antiport

What are the small ion active transport proteins?

P-type, F-type, V-type, ATP-binding cassette (ABC) transporter

P-type

Phosphate of ATP binds to a particular aspartate residues to form an intermediate

E.g. Na+K+-ATPase, H+K+-ATPase, Ca2+-ATPases

F-type

ATP synthase in the inner mitochondrial membrane is an F0F1ATPase

Electron transport system derived H+ gradient transported back to mitochondrial matrix through ATP synthase

V-type

H+ pumps in membranes of intracellular vacuoles

Acidify vacuole lumens

Important in accumulating neurotransmitters into secretory vehicles by H+ antiports

ATP-binding cassette (ABC) transporter

Grouped by homology of ATP binding region

One of the largest and oldest protein groups

E.g. cystic fibrosis transmembrane regulator (CFTR)

Primary active transport example

Na+/K+-ATPase

Pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed

Na+K+-ATPase function

Maintains RMP (propagation of electrical signals in nerve and muscle), osmotic balance, matins cell volume, and provides energy for secondary transport

Secondary active transport

Uses energy stored in ion gradients (often Na+ or H+) created by primary transporters

Couples downhill movement of solute (usually Na+) to the up-gradient movement of another

Coupling solute movement in same direction (symport) or opposite direction (antiport)

Na+K+-ATPase mode of action

1. E1 conformation - binding sites face inside cell, high affinity for intracellular Na+ in low (Na+)

2. Binding of 3 Na+ allows binding of ATP and its hydrolysis - phosphate remains bounds causes a conformational change

3. E2 conformation - binding sites face outside the cell. Affinity for Na+ dramatically reduces Na+ released into high Na+

4. High affinity for extracellular K+ in low (K+)

5. Binding of 2 K+ results in release of phosphate group - causes conformational change

6. E1 conformation - binding sites face inside the cell. Low affinity for intracellular K+ in high (K+)