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. At the center of each one of these C subunits is an aspartate residue that can readily bind protons under acidic conditions. So now that we know what the structure of what the structure of the f one region is, we can actually deduce what the mechanism of proton movement looks like. So let's take a look at the following three steps.