Download Chem*3560 Lecture 28: Active Transport

Chem*3560
Lecture 28:
Active Transport
Active transport refers to any process in which an energy yielding
reaction such as ATP hydrolysis is coupled to transport so that the
transported substrate is forced to move against the direction of its
electrochemical gradient (Lehninger p.415).
Simple uniport occurs in the direction of the electrochemical gradient,
because if the transporter is randomly flipping orientation, it is more
likely to become occupied on the side with higher concentration.
ATP coupled transport only need add a process where ATP hydrolysis
flips the transporter into one particular orientation.
If the vacant transporter is made to face inwards after ATP hydrolysis,
it is more likely to become occupied by [S] from the inside, even when
the inside now has the lower concentration of substrate [S].
Primary and secondary active transport
Processes that are directly coupled to an energy source are
classified as primary active transport. The primary active process
may then create a gradient of solute S, for example high on the
outside and low inside.
If a different transporter now accepts the original solute S for
symport with another solute such as glucose, then S will flow down
its concentration gradient. Because of the strict stoichiometry of
symport, glucose can then be carried in from low to a higher
concentration inside the cell.
Sout + glucoseout ‡ Sin + glucosein
high
low
[S] in [Glc] in
Keq = [S] out[Glc] out
low high
Because the symport transporter is not directly coupled to an energy reaction, but is still forcing
glucose against its concentration gradient, it is classified as secondary active transport.
Transport reactions which are not primary active transport generally have Keq = 1,
so transport will occur as shown provided that [S]out×[Glc]out > [S]in×[Glc]in.
The glucose gradient can build up to the point that it is slightly less that the opposing S gradient.
Energetics of transport
The free energy change for simple uniport of a neutral solute is given by
[S] dest
∆G = RT ln [S ]
source
(Lehninger p. 415-416)
Transport occurs in the direction that gives –ve ∆G, so [S]dest < [S]source.
For active transport involving ATP hydrolysis (∆Go 'ATP = –30.3 kJ/mol)
[S] dest
[ADP][Pi]
∆G = RT ln [S ]
+ ∆Go 'ATP
+
RT ln [ATP]
source
transport term + ATP hydrolysis bonds + ATP hydrolysis concentrations
Transport can make [S]dest >> [S]source up to the point where the positive contribution to ∆G from
transport is just less than the negative contribution from ATP hydrolysis.
One advantage of ∆G is that all terms are simply additive, allowing a complex process to be put
together by adding each contributing term.
For a charged solute, the interaction of charge and potential contributes free energy zF∆ψ,
where z is the charge, e.g. –1 for Cl– and ∆ψ is the potential in the direction of travel.
(A negative ion has –ve contribution to ∆G if it moves towards the positive end of the potential).
Simple uniport of a charged solute becomes
[S] dest
∆G = RT ln [S ]
+ zF∆ψ
source
transport term
electrical term
For symport of substrates whose charges cancel, e.g. H+ and pyruvate –
H+out + pyruvate – in ‡ H+in + pyruvate – in
[H + ] in [pyruvate − ] in
∆ G = RT ln [H + ] [pyruvate − ]
out
out
(–ve ∆G if [H+]in / [H+]out < [pyruvate– ]out / [pyruvate– ]in )
For antiport of substrates whose charges cancel
malate 2– in + Pi 2– out
‡ malate 2– out + Pi 2– in Keq = 1
[malate 2− ] out [P 2−
i ]in
∆ G = RT ln [malate 2− ] [P 2− ]
in i
out
(–ve ∆G if [malate]out / [malate]in < [Pi ]out / [Pi ]in )
Transport systems in action: plasma membrane
The plasma membrane of animal has two important ATP driven
ion transporters or pumps. The ATP reaction occurs entirely on
the inside as an energy soucre, and ATP itself is not transported
(Lehninger p.420-421):
3 Na+in + 2 K+out
2 Ca2+in
+
Na+/K+ ATPase
+ ATP → ADP + Pi + 3 Na+out + 2 K+in
Ca2+ ATPase
ATP → ADP + Pi + 2 Ca2+out
Another transporter ATPase pumps Ca2+ from cytoplasm into the endoplasmic reticulum. The two Ca2+
pumps keep the cytoplasmic Ca2+ very low, normally about 10–7 M, so that a rise to as little as 10–5 M
is a strong signal for certain events such as muscle contraction.
Ionic environment in cells
Na+
K+
Ca2+
Mg2+
Pi2–
12 mM in, 145 mM out
140 mM in, 4 mM out
10–7 M in, 10–3 M out
5 mM
1 - 2 mM
gradient maintained by Na+/K + ATPase
gradient maintained by Na+/K + ATPase
concentration maintained by Ca2+ ATPases
required by many enzymes
+ about 50 mM of phosphate esters
In addition to the sodium and potassium concentration gradients, there is an inward directed membrane
potential of about –50 to –60 mV. The negative potential
contributes –5 kJ/mol to ∆G when Na+ ions reenter the
cytoplasm.
Group translocation in bacteria
Bacteria use process for import of certain sugars in which the
transport substrate is chemically modified. The PEP acts by
transferring phosphate to a histidine side chain (phosphohistidine
is a high energy phosphate), which then phosphorylates the
glucose as it enters:
Glucoseout
PEP dependent phosphotransferase
+ PEP → Glucose-6-phosphate in + pyruvate
The PEP and pyruvate react entirely inside, and are not themselves transported.
Na+ drives secondary active transport in the plasma membrane of animal
cells
The sodium gradient is used by animal cells to drive secondary transport of glucose and amino acids in
intestinal and kidney cells (Lehninger p.423):
2 Na+out
Na+/glucose symporter
+ glucoseout
→
2 Na+in + glucosein
H+ drives secondary active transport in inner membranes of mitochondria
and bacteria
Redox reactions of electron transport make up the primary active transport, to pump H+ outwards
across mitochondrial or bacterial inner membranes. The H+ gradient then drives uptake of lactose.
Secondary transporters for a variety of other substrates also use the H+ gradient:
Lactoseout
Lactose permease
→
H+in + lactosein
+
out
+
H
In mitochondria, transporters for pyruvate and phosphate act by H+ symport:
Pyruvate – out
H2 PO4 – out
+
+
H+/pyruvate– translocator
H+out
→
H+in + pyruvate – in
H+/phosphate– translocator
H+out
→
H+in + H2 PO4 – in
Electrogenic transport
A transport process is electrogenic if there it produces a a net change in charge across the membrane,
e.g. the ATP:ADP translocator of the mitochondrial membrane is an electrogenic antiport because
ATP carries one unit more negative charge than ADP, so there is a net movement of one negative
charge outwards for each transport reaction:
ATP:ADP Translocator
ATP in + ADP3– out → ATP4– out + ADP3– in
4–
∆Ψ = 150 mV (+ve out) due to the proton gradient generated by electron transport, so this contributes
–15 kJ/mol to ATP export from mitochondria (but consumes one H+ worth of ∆ψ).
Electrogenic transport makes use of membrane potential created by a primary transport process, in this
case the membrane potential due to H+ pumping by the lectron transport system.