01:694:301 Lecture Notes - Lecture 13: Conformational Change, Serca, Multiple Drug Resistance

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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
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! 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)
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