01:694:301 Lecture Notes - Lecture 13: Conformational Change, Serca, Multiple Drug Resistance
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14 Nov 2018
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Chapter 13 – ‘Membrane Channels and Pumps’
Membrane Permeability and Types Proteins Transport
• Permeability is conferred to three classes of membrane proteins: pumps, carriers, and
channels.
o Pumps mediate active transport, and channels are involved with passive transport.
! Pumps use a source of free energy such as ATP hydrolysis or light absorption
• Energy transducers (convert free energy into other forms)
• Two types of ATP driven pumps undergo conformational changes on ATP binding
and hydrolysis causing bound ion to be transported across membrane
o P-type ATPases – form a key phosphorylated intermediate
! ATP hydrolysis drives membrane transport by means of conformational change which
induced by +/- of phosphoryl group to asparate site of protein
o ATP-binding cassette (ABC) transporters
! Carriers mediate transport of ions and small molecules without use of ATP;
! Channels provide a membrane pore through which ions can flow
• The expression of these transporters defined many metabolic activities of a given cell type
• Two factors that determine whether or not molecules will cross membrane:
o 1) Permeability of molecules in lipid bilayer
o 2) Availability of energy source
• Understand class discussion of differences between diffusion, facilitate diffusion, primary
active transport and secondary active transport (symport and antiport pg. 380).
o Diffusion – pass through membrane down [] gradient;
! Second Law Thermo states that molecules spontaneously move from area higher []
to lower []
! Ex:
Lipophilic
molecules
such as steroid hormones can easily dissolve in lipid bilayer
• Ink in water, cigarette smoke, etc.
o Facilitated diffusion (passive) – diffusion across membrane is ‘facilitated’ by channel
! Passive due to its lack of energy use; energy driving movement is the ion’s own []
gradient
! Channels (like enzymes) display substrate ‘specificity’ in that they facilitate transport
of some ions but not others (even if related)
! Analogy – Office building and revolving door (only one exit); constraints on what can
enter and how fast can enter
! Determines the maximum rate/specificity " graphed as
hyperbolic
o Primary AT – free energy of ATP hydrolysis is used to drive ions against their [] gradient
! Pumps propel ions across membrane by ‘spending ATP ($$)
• Ex: Circus " flying human cannon
! Each pump exists in two conformations (binding site open on one side membrane
and binding site open to other side) (Fig. above referring to
P-type ATPase
)
o Secondary AT - utilize the gradient of one ion to drive another ion against its gradient;
osmotic pressure (carriers)
• Ex:
E. coli
lactose transporter
! Symport – use flow of one species to drive flow of a different species in same
direction across the membrane
• Lactose permeases also has a conformation change (Fig 13.12)
o This symporter uses H+ across
E. coli
membrane (outside higher [])
generated by oxidation of fuel molecules to drive uptake of lactose and other
sugars against [] gradient
! Antiport – couple the downhill flow of one species to uphill of another species in
opposite direction across membrane
• You should understand the math on (373)
o Osmotic free energy is a special case of the free energy equation.
o We can assume that the Keq will always be 1.0 for osmosis
o Therefore the STD free energy must be 0, and ΔG = 0 + RTln (P/R)
! ‘Product’ and ‘reactant’ are the ‘to’ and ‘from’ [] C2 and C1
o When ions are involved, there has to be a voltage term added in for the
electrochemical
potential
(membrane potential); Z is the electrical charge; ΔV is potential volts across
membrane; F is Farday constant
o Transport process
must
be active when ΔG is positive; can be passive when ΔG is neg
Two Membrane Proteins Utilizing ATP Hydrolysis
• The Ca2+ pump is a ‘P-type’ ATPase (Fig. 13.5)
o
Sarcoplasmic reticulum Ca2+ ATPase
(
SERCA
)
! Here Ca2+ ions are actively transported out of the cytoplasm (to muscle
cells) with direct participation of ATP hydrolysis
! P-type ATPase plays important role in relaxation of contracted muscle
! Muscle contraction is triggered by abrupt rise in cytoplasmic [Ca2+]
! Muscle relaxation dependent on the removal of Ca2+ to sarcoplasmic
reticulum
o Aspartate is
phosphorylated
(374)
! Phosphoryl group from ATP is attached to side chain of aspartate residue
o There are many example of similar proteins including ‘flippases’ (378)