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. And the special thing about the aspartate 61 residue is the side chain actually contains this negative charge. And the negative charge can actually grab a proton under acidic conditions. So the A subunit, this purple structure here, is positioned to interact with the C sub units.