So we'll see exactly what that means in just a moment. So what are these two catalytic residues? Well, we have the glue 165 and the his 95. Now, the glue 165 in the first step acts as a base, and the his 195 in the first step acts as an acid. So essentially, this histidine 95 donates an H to this carbon. So essentially, this pi bond here goes on to take the H away.