Reaction Coupling versus Active Transport

[premed_noob]

I have a quick question regarding the difference between reaction coupling and active transport.

This is specifically based off of a question in TPR Practice Exam 3 (Question 16) where it asks which reaction is not an example of reaction coupling. Of the answer choices - skeletal muscle contraction with myosin and actin uses reaction coupling; GPCR's use reaction coupling for cAMP, and RNA/DNA/Protein synthesis use reaction coupling. The correct answer is however, that the sodium potassium pump (2 Na+ out and 3 K+ in) does not use reaction coupling. It is my understanding that all of these processes however use ATP (or GTP) and are examples of active transport. Is that correct? Could someone explain how the sodium potassium is NOT an example of reaction coupling while the other three each specifically are?

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[ElectricNoogie]

A coupled biochemical reaction is one where the free energy of a thermodynamically favorable reaction (e.g. ATP hydrolysis) is used to drive a thermodynamically unfavorable one, by coupling or joining the two reactions. The body has 3 uses for ATP:


1. Chemical Work - ATP supplies the energy needed to synthesize bio-molecules

2. Mechanical Work - ATP supplies the energy needed to allow muscle contraction, cilia and flagella movement, chromosome movement plus other functions

3. Transport - ATP supplies the energy cells need to pump substances across the cell membrane

For the Na/K pump, (Net movement 3 Na+ OUT of the cell, 2 K+ IN to the cell): the membrane protein can allow 3 Na+ to enter the protein. Once ATP is hydrolyzed and the phosphate is transferred to the carrier protein, this allows a change in shape to the protein that releases the 3 Na+ to the outside and allows 2K+ to enter the protein. The phosphate is released, the K+ is released into the cell as the protein changes shape and the process begin anew.

To that end, active transport like the Na/K pump can be defined as a coupled reaction because it is coupled to ATP hydrolysis. It is not a coupling reaction, though (i.e. a reaction where 2 hydrocarbon fragments are coupled with the aid of a metal catalyst).