Which of the following is (are) characteristic of the (na+–k+)–atpase?
i. it binds na+ and k+ simultaneously prior to binding atp
ii. it is covalently modified by phosphorylation at an asp residue when na+ is present.
iii. transport of na+ and k+ occur in a 1: 1 ratio, thus maintaining a balanced gradient until opening of the gated portion of the channel.
iv. it is essential for excitation of nerve cells.

II, IV

Explanation: II) Na+–K+–AT is covalently modified by phosphorylation at Asp residues which causes a conformational change in the pump.

IV). It contributes to the excitation (action potential) produced by nerve cells through the transport of three sodium ions out of the nerve cell into the extracellular space and two potassium ions into the intracellular space from the extracellular space.

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b is the correct answer