Review Question - QID 210819

A scientist is studying a protein that is present on the plasma membrane of cells. He therefore purifies the protein in a lipid bilayer and subjects it to a number of conditions. His investigations show that the protein has the following properties:

1) It is able to change ion concentrations across the membrane without addition of ATP to the solution.

2) Its activity increases linearly with substrate concentration without any saturation even at mildly supraphysiologic conditions.

3) In some states the protein leads to an ion concentration change; whereas, it has no effect in other states.

4) Changing the electrical charge across the membrane does not affect whether the protein has activity.

5) Adding a small amount of an additional substance to the solution reliably increases the protein's activity.

These findings are consistent with a protein with which of the following functions?