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#ATP-powered pumps

3 public questions tagged with this topic.

Membrane Transportation

Which of the following ATP-powered pumps does not hydrolyze ATP during transport?

Pump classification by energy coupling highlights mechanistic differences. P-type ATPases such as Na+/K+ ATPase, SERCA, PMCA hydrolyze ATP phosphorylating conserved aspartate generating E1P E2P intermediates, actively transporting ions against gradients. V-type ATPases acidifying endosomes lysosomes vacuoles comprise Vo proton pore and V1 ATPase rotary motor hydrolyzing ATP to rotate c-ring driving proton translocation without phosphoprotein. ABC transporters dimerize nucleotide binding domains upon ATP binding to expel substrates. F-type ATPases known also as ATP synthases reside in mitochondrial inner membrane cristae, thylakoid membrane and bacterial plasma membrane. Crystallography shows Fo c-ring proton turbine and F1 head alpha3beta3. Under physiological respiring conditions proton motive force of 200 millivolts drives protons through Fo causing rotation of c-ring and central stalk gamma epsilon at about 100 hertz inducing binding change in catalytic beta subunits synthesizing ATP from ADP and phosphate via rotational catalysis, not consuming ATP for transport. Hydrolysis mode exists when gradient collapses pumping protons, but primary cellular operation synthesizes ATP using proton influx. Hence during principal physiological forward operation it does not hydrolyze ATP to drive ion transport, but manufactures ATP.

Ref: Nelson and Cox, Lehninger, F-Type ATPase Does Not Hydrolyze ATP During ATP Synthesis Mode.

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Membrane Transportation

Which of the following is a characteristic of P-class ATPases?

ATP-driven pumps belong to several mechanistically distinct families distinguished by architecture, subunit composition and mechanism of coupling hydrolysis to transport. P-type family named because first discovered intermediate was phosphorylated enzyme, now understood to include Na+/K+ ATPase maintaining sodium potassium gradients, sarcoplasmic reticulum Ca2+ ATPase SERCA sequestering calcium, plasma membrane Ca2+ ATPase PMCA extruding calcium, gastric H+/K+ ATPase acidifying stomach and heavy metal copper and manganese pumps ATP7A ATP7B. Core mechanism involves conserved aspartate within DKTGT motif in cytoplasmic P domain that transiently accepts gamma phosphate from ATP forming covalent acyl-phosphate intermediate with high energy, creating E1P state occluding ions. Phosphorylation triggers large conformational rearrangement of transmembrane helices to E2P outward facing, releasing ions due to lowered affinity from micromolar to millimolar range, followed by dephosphorylation by TGES motif in actuator domain returning to E1 ready. This phosphorylation cycle detectable by incorporation of 32P, inhibition by vanadate mimicking phosphate transition state analog, and acid stability of phosphoenzyme. In contrast V, F and ABC families use noncovalent ATP binding without phosphoprotein formation. Hence hallmark characteristic defining P-class pumps is transient phosphorylation of pump protein during transport cycle enabling alternating access.

Ref: Palmgren and Nissen, P-Type ATPases Annual Review Biophys, Mechanism: Phosphorylated Intermediate Formation.

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Membrane Transportation

Which of the following ATP-powered pumps is responsible for maintaining resting membrane potential?

Resting membrane potential stability essential for excitability maintained by differential permeability and active ion distribution. Key player is Na+/K+ ATPase, P-type ion pump encoded by ATP1A genes for alpha isoforms, ATP1B for beta, FXYD for gamma regulatory. Catalytic alpha subunit roughly 112 kDa with ten membrane spans contains nucleotide N domain binding ATP, phosphorylation P domain with conserved DKTGTLT aspartate that forms phosphoenzyme intermediate, actuator A domain and transmembrane ion binding pocket coordinating ions with carbonyl oxygens. Cycle Post-Albers: E1 high Na affinity intracellular binding three Na+, phosphorylation to E1P occluding, transition to E2P outward low Na affinity releasing, high K affinity binding two K+, dephosphorylation to E2, return. Electrogenic 3 out 2 in adds minus 3 millivolts directly plus creates gradients enabling K+ leak through Kv and Kir producing minus 70 to minus 90 millivolts. V-type pumps acidify lysosomes, F-type synthesizes ATP, ABC exporters expel drugs. Only P-class Na+/K+ ATPase directly maintains gradients underlying resting potential.

Ref: Skou and Glynn, P-Type ATPases Review, Na+/K+ ATPase Role in Resting Membrane Potential Maintenance.

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