BIOC 3560

signals

1. specific - signal molecule fits binding site on it complementary receptor, other signals dont fit, some receptor are present only in certain cell types

2. amplification - when enzymes activate enzymes the number of affected molecules increase in an enzyme cascade

3. modularity - proteins with multivalent affinities form diverse signaling complexes from interchangeable parts, phphylation provides reversible points of interaction

4. desensitization/adaptation - receptor activation triggers a feedback circuit that shuts off the receptor or removes it from cell surface, when stimulus falls below certain threshold the system is active again

5. integration of signals - when 2 signals have opposite effects that outcome results from the input on both receptors, unified response

types of signal transducers

1. G protein-coupled receptor - external ligan binds to GTP-binding protein which produces second messenger

2. receptor enzymes - ligand binding activates activity by autophph

3. gated ion channel - opens or closes in response to concentration of signal ligand or membrane potential

4. nuclear receptor - hormone binding allows receptor to regulate expression of specific genes

neuron ion gradient

neuron has high [K+] and low [Na+]'

at rest inside cell is ΔΨ=-60mV

action potential carries electrical signal down axon and neurotransmitter carries signal to next cell

nicotinic acetylcholine receptor (AchR)

• example of neurotransmitter

• passage of electrical signal from motor neuron to muscle fiber at neuromuscular junction

• acetylcholine released by motor neuron diffuses to pm of myocyte → binds AchR

• conformational change in AchR → open and activated

  • inward movement of cations (ion channels go down concentration gradient)

  • trigger muscle contraction

• has 5 subunits with 4 helices in each

neural transmission

• acetylcholine (Ach) opens Ach receptors (ligand gated Na+/Ca2+ channel)

• Na+ flows in (down gradient) → depolarization

• adjacent voltage-gated Na+ channels open: Na+ rushes in → ΔΨ=+30mV

• Na+ channels inactivated

• K+ channels opens: K+ flows out (down gradient) → ΔΨ=-75mV

• K+ channels inactivated → ΔΨ=-60mV

heterotrimeric G protein-coupled receptors

• glucogon and epinephrine

• has pm receptor with 7 transmembrane helices

• heterotrimeric guanosine nucleotide-binding protein (G-protein)

• intracellular enzyme that generates second messenger

• G_s is stimulatory

• G_i is inhibitory

epinephrine signal transduction pathway

Gα has GDP bound at rest

epinephrine binding receptor promotes GTP-binding at Gα, activating it

Gα dissociates from receptor moving to adenylyl cyclase and activates it

catalyzes formation of cAMP

cAMP activates PKA which phph cellular protein

what drives epinephrine receptor (internal)

Internalization of the epinephrine receptor is induced by phosphorylation of the receptor by β-adrenergic receptor kinase (βARK) and subsequent binding of β-arrestin.

insulin receptor

1. insulin bind externally and activates tyrosine kinase activity in intracellular domain

2. β-chains are autophph which opens up active site

3. insulin receptor phph Tyr

4. SH2 domain of Grb2 binds to phphTyr of IRS1

5. Sos binds to Grp2 than Ras releasing GTP

6. activated Ras binds and activated Raf-1

7. Raf-1 phph MEK which phph ERK

what blocks insulin receptor

tyrosine kinase catalytic site is blocked by its activation loop when its inactive

loop is Tyr which H bonds with Asp to keep in position

when insulin bind Tyr kinase phph all Tyr to stabilize loop so it wont block catalytic site

insulin activates what in muscle cells

• increase glucose transport by recruiting GLUT4 to membrane

• induce synthesis of hexokinase

• activate glycogen synthase by phph of GSK3

effects of insulin

• reduced phph of glycogen synthase → increased activity and glycogen synthesis by glycogen synthase

• movement of glucose transporter to pm → brings more glucose into cell

• modulation of insulin-responsive transcription factors

nuclear hormone receptor

1. hydrophobic hormone diffuses across membrane and binds to receptor protein in nucleus

2. binding causes conformational change of receptor to form dimer - receptor has zinc finger to allow it to bind to specific DNA sequences

3. receptor attracts transcription regulating proteins to either increase or decrease mRNA formation

4. cellular response

phosphoglycerides

glycerol backbone

2 FA in ester link

alcohol head group

sphingolipids

sphingosine backbone

1 FA in amide link

glycolipids

sphingolipids with carbohydrate headgroups

what is the major lipid in all membranes? what is major component of pm?

phosphatidyl choline

cholesterol

how can membrane proteins be removed?

• pH changes or Ca2+ is removed → peripheral membrane protein removed

• detergent → integral membrane protein removed

• phospholipase → lipid anchored membrane protein removed

• amphitropic can attach and leave independently

types of integral membrane proteins

hydropathy plots

predict the number of transmembrane helices a protein might have

why dont beta barrel integral membrane proteins show up on hydropathy plots?

alternating hydrophobic, hydrophilic sheets that arent long enough

what aa are found where transmembrane protein meets headgroup of membrane

Tyr and Typ (hydrophobic and polar)

charged aa are unfavourable in hydrophobic environments → found mostly in aqueous phase

N-linked carbohydrate chain is attached to what

Asn side chain

O-linked carbohydrate chain is attached to what

Ser and Thr

what do the sugar groups of glycoproteins and glycolipids do?

contribute to cell surface recognition

function as receptors

lipid bilayer state changes

• gel phase (cold) → all motion is constrained, lipids ordered in paracrystalline state

• liquid-ordered state (physiological) → intermediate thermal motion, lateral movement in the plane of bilayer

• liquid-disordered state (fluid) → hydrocarbons chains in constant motion with no regular organization

what state do different FA favour?

• long chain, saturated FA pack into liquid ordered

• short chain, unsaturated favour liquid disorders

behaviour of cholesterol on membrane fluidity

• long saturated → cholesterol INCREASES fluidity -

• unsaturated cis → cholesterol DECREASES fluidity

• high temps → cholesterol DECREASES fluidity

• low temps → cholesterol INCREASES fluidity

cholesterol is a fluidity buffer → does opposite of what composition/temp does to maintain good fluidity levels of membrane

enzymes that help move lipids in pm

flippase - P-type ATPase that moves PE and PS from outer to cytosolic leaflet

floppase - ABC transporter that moves phospholipid from cytosolic to outer leaflet

scramblase - moves lipids in either direction toward equilibrium (no ATP)

what restricts lipid/protein motion in pm

spectrin is part of cytoskeleton and links to membrane proteins and keeps lipids from diffusing freely

lipid rafts

sections of pm that are enriched in sphingolipids and cholesterol

simple diffusion

spontaneously from high to low concentration

what does a membrane transporter help with?

decreases the amount of free energy needed to transport hydrophilic solutes across membrane

membrane channels

pore that spans bilayer

solutes flow through rapidly compared to transporters

they are gated - open/close in response to stimuli

highly selective

membrane transporter classification

• uniport → single molecule down concentration gradient

• symport → 2 different molecules across in same direction (one down concentration gradient other up concentration gradient)

• antiport → 2 different molecules across in different directions (one down concentration gradient other up concentration gradient)

passive transporters

• transport one set of molecules at a time down concentration gradient

• highly selective

• not continuous pore through membrane

• rate of transport is dependent on number of binding site for substrate

GLUT1 transporter

1. substrate binds on one side of membrane

2. conformational change takes place

3. site opens on other side of membrane and substate is released

4. conformational change takes place

active transporters

• against concentration gradient

• many powered by ATP hydrolysis

• generate ion gradients across membrane

Na+ K+ ATPase

• generates gradient

• 3 Na+ out and 2 K+ in

• net negative charge in cell

• both ions move up concentration gradient

• functions:

  • control cell volume

  • drive active transport of other species

  • render nerve cells electrically excitable

example of secondary active transporter

Na+-glucose symporter

one ion down concentration gradient drives other ion up concentration gradient

differences between channels and transporters

• rate of flux → high in channels limited only by diffusion

• saturability → binding sites on transporters

• channels are gated

• ion channels present in pm of all cells

voltage gated K+ channel

• tetramer

• outer helices interact with bilayer

• inner helices contributes to inner pore

• K+ doesnt have to pass through pm

• partially neg

• K+ are spaced apart in the channel since pos charge repel each other

voltage gated Na+ channel

• helix 6 forms pore

• helices 1-4 help with gating mechanism

• 4 pore-forming helices arranged around pore

• helix 4 has high net pos charge and is pulled into cell

• depolarization moves helix 4 outside and Na+ enter

• inactivation loop stopping ions from passing