Download LECTURES 5, 6 Membrane protein lecture

Membrane Proteins
Membrane Proteins
Protein structure
•  Primary structure
Amino acids: Polar (charged and uncharged)
Non polar
Unique side chains (Gly, Cys, Pro)
•  Secondary structure
•  Tertiary structure
•  Quaternary
Polar amino acids
* Serine
* Threonine
Side chains have partial charge
∴ participate in reactions, associate with water
Glutamine
*
Tyrosine
Asparagine
Non-polar amino acids
Alanine
Valine
Leucine
Side chains have H and C atoms; hydrophobic
Isoleucine
Associate with lipid layer
Methionine
Phenylalanine
Tryptophan
Amino acids with unique side chains
Glycine (H side chain – hydrophobic or hydrophilic
Cysteine (contains SH moiety – can form S-S bridge)
Proline (hydrophobic side chain; can create kinks and disrupt 2° structure
Secondary structure: α-helix
Opposite sides of
helix may have
contrasting properties
-hydrophobic
-hydrophilic
Secondary structure: β-sheet
Bonds which maintain protein structure
Tertiary structure
•  Stabilized by non covalent bonds
between diverse chains of protein
Some tertiary structures
Haemoglobin has a quaternary structure:
2 α-subunits and 2 β-subunits
Quaternary structure enhances O2 association
Proteins in membrane
1.  Receptors: specificity, subtypes
2.  Ion channels: specificity, selectivity
importance of pore size, and charge
3.  Ion pumps
4.  Enzymes: mainly on intracellular face
5.  GTP-binding proteins
6.  Carrier molecules: very specific
7.  Cell adhesion molecules (Glycoproteins)
MEMBRANE PROTEINS…… Communication
Receptors
Transport:
Ion channels
Pumps
Carrier proteins
Release:
Synaptic plasma membrane proteins
Integral membrane protein
Single/several hydrophobic domain
e.g. glycophorin/channels+pumps
Peripheral: anchored
by glycolipid
e.g. Receptor
Anchored to
lipid bilayer
eg G-prot
Membranes as Barriers
•  Hydrophobic interior of bilayer
is a barrier to transport (size &
charge)
•  The membrane is impermeable to
ions and large charged molecules
and requires the aforementioned
special membrane proteins to
transport across
Membrane proteins - NOTES
Transmembrane proteins
•  Protein has hydrophilic and hydrophobic portions
–  Hydrophilic will interact with the aqueous solutions on either
surface
–  Hydrophobic will be in contact with the hydrophobic interior of the
bilayer
•  Also called integral membrane proteins
Peripheral membrane proteins
•  Are attached to either surface of the bilayer
•  Those attached to lipids are covalently linked
•  Those that interact with other transmembrane proteins are attached
by noncovalent interactions, such as:
–  H-bonds, hydrophobic and hydrophilic interactions
Membrane-spanning proteins
•  Must have hydrophobic side chains in the area that
spans the membrane
•  Peptide backbone is polar
–  Not “happy” in the hydrophobic interior
Membrane Pores
•  When proteins span the membrane several
times they usually form pores that allows
molecules to move back and forth through
the membrane
•  Multiple α helices span membranes
–  Hydrophilic on the inside of the
channel
–  Hydrophobic on the outer surface of
the channel
α-Helix Span Interior (Pore)
•  Interior forces the peptide
backbone to form α helix
•  Non-polar R groups are on
the outside of the helix
•  Transmembrane proteins
usually span the membrane
once
–  Receptors – collect signal,
pass on to the inside of
cell
β Barrel Pore
•  β barrels are made of β
sheets that are curved into
a cylinder
•  The hydrophilic line the
inner side and hydrophobic
the outer surface
•  Larger pore than α helix
pore
Membrane Transport of Small Molecules
Membranes present a barrier to the movement of most
materials
Transport proteins allow movement
Transport proteins comprise 15-30% of membrane proteins.
Up to 2/3 of a cell’s metabolic energy can be used
for transport
Tight Junctions restrict transport
Proteins can diffuse within their own domains,
but are prevented from entering the other domain
by ‘tight junctions’ (specialized cell junction)
Transport
1.  Simple Diffusion
2.  Carrier-mediated
(a) Facilitated diffusion
High------Low
(b) Active transport (ATP)
Low
High
Uniport, symport antiport
Primary, secondary
Transport May be Passive or Active
Passive transport may or may not require protein transporters; active
transport always requires transporter proteins.
The Electrochemical Gradient is the Determining
Force for Ion Transport
The electrochemical gradient is the combination of concentration and
charge differences across the membrane.
Carrier Proteins Bind Solutes Tightly and Undergo Large
Conformational Changes
The tightness and specificity of carrier protein-solute binding is
much like enzyme-substrate binding.
Protein phosphorylation changes
conformation
ATP
+ ADP
Transport Via Carrier Proteins is Saturable
Simple Diffusion is Not
Active Transport Always Occurs Using Carrier
Proteins (not channels)
Coupled Carriers Function as Symports or Antiports
Coupled carriers always function in active transport; uniports function in
facilitated diffusion.
A Glucose Antiport
Driven by the Na+
Electrochemical
Gradient
The Na+ electrochemical
gradient is harnessed to
drive many transport
processes.
The Na+-K+ Pump is Abundant and Ubiquitous
The Na+-K+ pump maintains low ic [Na+] and high ic [K+]
Ionic imbalance important for intracellular pH control, osmotic control,
excitability, and transport
The Na+-K+ Pump is a P-Type Transport ATPase
This class of pumps autophosphorylates following ATP hydrolysis.
The phosphorylation is reversible and changes the conformation of the
pump, alternately exposing ion binding sites on the extracelluar and
cytosolic faces of the membrane.
About 1/3 of the Cell’s Metabolic Energy Goes to Powering
the Na+-K+ Pump
The Na+-K+ Pump
The Na+-K+ Pump is abundant and ubiquitous
The Na+-K+ pump maintains low ic [Na+] and high ic [K+]
It is actually an enzyme – ATPase (P-type transport ATPase)
The phosphorylation changes the conformation of the pump.
Ionic imbalance important for intracellular pH control
osmotic control
excitability
transport
About 1/3 of the Cell’s Metabolic Energy Goes to Powering the Na+-K+ Pump
Vesicular Transport
•  Transport of large particles and
macromolecules across plasma membranes
–  Exocytosis – moves substance from the cell
interior to the extracellular space
–  Endocytosis – enables large particles and
macromolecules to enter the cell
–  Phagocytosis – pseudopods engulf solids and bring
them into the cell’s interior
Vesicular Transport
Endocytosis of cholesterol molecule (large)
Clathrin-Mediated Endocytosis
Figure 3.13
Exocytosis
Figure 3.12a
Ion Channels
Ion Channels are the Second Major Class of Transport Proteins
Ion channels differ from carriers in always working in passive
transport, their exquisite selectivity, and their higher rate of
transport.
A single ion channel may transport up to 100 million ions per second, a
rate 100,000 times higher than the fastest carrier.
Ion channels are on all cells, but reach their highest level of
sophistication on electrically excitable cells like neurons.
Channel Opening and Closing is Regulated in Three Broad Ways
Receptors
SIGNALLING THROUGH
1. Ionotropic receptors
2. Metabotropic receptors/G-protein-linked
3.  Tyrosine kinase-linked receptors
4.  Cytokine receptors (Class I and II)
5.  Tumour necrosis factor (TNF) family
6.  Haematopoietic antigen receptors
Ionotrophic
Receptors
G-protein-linked receptors
G-protein-linked receptors
Tyrosine kinase receptors
Note steps involved:
1.  Ligand Reception
2.  Receptor Dimerization
3.  Catalysis (Phosphorylization)
4.  Subsequent Protein Activation
5.  Further Transduction
6.  Response