a defective na /k atpase pump is isolated from a plasma membrane and studied. careful measurements in the laboratory determine that the pump is able to hydrolyze atp and transports 3 na ions across the plasma membrane. the pump is also able to bind 2 k with high affinity. however, the pump is unable to shuttle k ions across the plasma membrane. based on these observations, what is the most likely cause of the defect in this pump?