Membranes, cell signaling, and regulation of fuel Metabolism

Lecture 15

Membrane Proteins

1. Integral

2. Peripheral

3. Anchored

Integral membrane proteins

Traverse the lipid bilayer. Hydrophobic amino acids face the acyl chains of lipids

Peripheral membrane proteins

Can interact with other proteins or lipids. Lipid binding proteins have specific interaction, through electrostatic interaction or amphipathic domain

Non-Covalent ionic interaction and H-bonds with lipid head groups

Lipid-anchored membrane proteins

lipids anchored through a covalently attached lipid molecule

Covalently

Long Saturated Fatty Acids

Low Fluidity

Strong Van Der Waals

Unsaturated Fatty Acids

More Fluidity

Bend in hydrocarbon chain

Loose Packing

Cholesterol

Buffers Fluidity.

Interacts with the acyl chain of other lipids. Stronger Interaction with Saturated FA.

Membrane Curvature

• Cylindrical

• Conical

• Inverted-conical

Cylindrical Membrane Curvature

Flat Membrane

PC, PS

“Battery”

Conical Membrane Curvature

PE, DAG, phosphatic acid

Negative Curvature

Inverted-conical

Positive Curvature

Membrane lipid assymmetry

Keeps PS on the inside: PS is a signal for cell damage

Keeps signaling lipids on the inside: PS and PI interact with cytoplasmic proteins to carry out cell function

Lateral diffusion

a lipid moves sideways within its own leaflet

Very Rapid

Transverse Diffusion

a lipid moves from one leaflet to another “flip-flop”

extremely slow

Flippase

Moves PS and PE into Cytosolic leaflet

Out → Inner

Uses ATP

Creates and maintains asymmetry

Floppase

Inner → Outer

Uses ATP

Creates and maintains asymmetry

Scramblase

Both Directions

Any Lipid

Activated by Ca²⁺

Destroys asymmetry

Simple Diffusion

Down the Gradient

No Protein

No Energy

Not Saturable, the rate rises with the gradient

Facilitated Diffusion

Down the Gradient

Protein Needed

No Energy

Yes (has Vmax and Km)

Specific and Saturable (behaves like an enzyme)

Primary Active Transport

Against the Gradient

Energy from ATP hydrolysis or light

Secondary active Transport

Against the gradient

Energy indirectly from primary transport

PLA1 cleavage site and products

cleavage site: ester at sn-1

Products: Free fatty acid + 2-acyl lysophospholipid

PLA2 cleavage site and products

Cleavage site: Ester at sn-2

Products: Free fatty acid + 1-acyl lysophospholipid.

PLC cleavage site and products

Cleavage: Between glycerol and phosphate

Products: DAG + IP₃

PLD cleavage site and products

Cleavage: Between phosphate and head group

Products: Phosphatidic acid + free head group (choline)

What is cotransport?

One transporter moves two solutes at the same time, where one solute’s gradient powers the other’s movement

Which pump creates the Na⁺ gradient that cotransport depends on?

The Na⁺/K⁺-ATPase

Where is Na⁺ concentration high and where is it low?

High outside the cell (~145 mM), low inside (~12 mM)

ATP driven ion transport

ATP runs the Na⁺/K⁺ pump, the pump builds the Na⁺ gradient, and SGLT1 rides that gradient to pull glucose in (a symport). Glucose then exits to the blood through GLUT2.

Symporter

It moves two solutes in the SAME direction. Example: SGLT1 (Na⁺ and glucose both go in).

What is an antiporter?

It moves two solutes in OPPOSITE directions. Examples: the Na⁺/Ca²⁺ exchanger, the Na⁺/H⁺ exchanger, Cl⁻/HCO₃⁻ (band 3).

Put the 3 steps of intestinal glucose absorption in order.

1) The Na⁺/K⁺-ATPase pumps Na⁺ out (basolateral). 2) SGLT1 brings Na⁺ + glucose in (apical). 3) GLUT2 lets glucose out into the blood (basolateral).

Three stages of signal transduction

1. Reception of extracellular signal by cell → conformational (shaper changes)

2. Transduction of signal from outside of cell to inside of cell

3. Cellular Response - occurs entirely in the receiving cell

Chemical (Intracellular) Signaling

1. Cytoplasmic connection

2. Cell-to-cell

3. Free Diffusion

How do the 3 receptor types differ in how they pass on a signal?

• RTK: has its own built-in kinase. Ligand binding makes two receptors pair up and phosphorylate each other’s tyrosines.

• Cytokine receptor: has no kinase of its own, so it borrows one (JAK). The JAKs phosphorylate the receptor, which recruits STATs, and the STATs go to the nucleus to turn on genes.

• GPCR: crosses the membrane 7 times and has no kinase at all. It activates a G protein by making it swap GDP for GTP.

What do protein kinases and phosphatases do, and why do cells need both?

Kinases add a phosphate (taken from ATP) to Ser, Thr or Tyr.

Phosphatases remove it.

A phosphate carries a big negative charge, so adding or removing one changes the protein’s shape and therefore its activity.

What do phosphorylated tyrosines on an activated RTK actually do?

They become docking sites. Proteins with an SH2 domain (Grb2, IRS-1, PI3K) recognize phospho-Tyr and bind there.

How does Ras work as an on/off switch?

Ras bound to GTP is ON and Ras bound to GDP is OFF.

Walk through the RTK → Ras → MAPK pathway.

Growth factor binds → the RTK autophosphorylates → Grb2 docks by its SH2 domain → Grb2 brings SOS to the membrane → SOS activates Ras (GDP → GTP) → Ras activates Raf → MEK → ERK → ERK enters the nucleus and turns on growth genes.

Walk through the insulin → IRS-1 → PI3K → Akt pathway.

Insulin binds its receptor → the β subunits autophosphorylate → the receptor phosphorylates IRS-1 → IRS-1 recruits PI3K → PI3K converts PIP₂ → PIP₃ → PIP₃ brings Akt to the membrane, where it gets activated → Akt drives GLUT4 to the cell surface (more glucose uptake) and inactivates GSK3 (more glycogen synthesis).

Why does PI3K making PIP₃ matter?

PIP₃ is a lipid that stays in the membrane and works as a “come here” flag. Akt and PDK1 have PH domains that bind PIP₃, so they gather at the membrane, and there PDK1 activates Akt.

Walk through the GPCR → cAMP → PKA pathway (e.g., glucagon or epinephrine).

The hormone binds the GPCR → Gαs swaps GDP for GTP and separates from βγ → Gαs activates adenylyl cyclase → adenylyl cyclase converts ATP → cAMP → cAMP binds PKA’s regulatory subunits and releases its active catalytic subunits → PKA phosphorylates targets (e.g., it turns on glycogen breakdown).

How does the cell shut off the cAMP signal?

Three ways:

1. Gα hydrolyzes its own GTP → GDP, which turns it off.

2. Phosphodiesterase (PDE) breaks cAMP → AMP. Insulin activates PDE; caffeine inhibits it, so cAMP lasts longer.

3. Receptor desensitization: the receptor gets phosphorylated, and β-arrestin binds and blocks it.

Walk through the GPCR → IP₃/DAG → Ca²⁺ pathway.

The hormone binds the GPCR → Gαq activates phospholipase C → PLC cuts PIP₂ into IP₃ + DAG:

• IP₃ is water-soluble, so it diffuses to the ER and opens Ca²⁺ channels, releasing Ca²⁺ into the cytosol.

• DAG is a lipid, so it stays in the membrane and, together with Ca²⁺, activates PKC.

• Ca²⁺ binds calmodulin, which activates enzymes such as phosphorylase kinase.