Now, even though we don't exactly know what the structure of the A subunit actually looks like this is what we believe the structure looks like. So the structure of the A subunit seems to consist of two hydrophilic half channels that do not span the entire membrane of the A subunit. One of these half channels is open to the matrix side while the other one is open to the intermembrane side. So we have one of these half channels which only spans half the membrane that is open to the intermembrane side and the other half channel is open to the matrix side. And these two channels, as we'll see in just a moment will actually play an important role and allow the movement of the protons from the intermembrane space the high concentration to the matrix, the low concentration. Now, if we zoom in into the center of each one of these C subunits showed in orange, we're basically going to find an aspartate 61 residue.