So technically, cysteine doesn't need to have its own category. But I put it here because I realized that it could be the most confusing one. And the reason is that cysteine is the only ionizable side chain that has neither an oxygen nor a nitrogen, right? It's the only one that has a heteroatom that's different. So, you could be really confused trying to pick which one's similar. It's the same rule, but you might just have a hard time applying it. You might say, well, is the sulfur more like the nitrogen or is it more like the oxygen? I just wanted to make it clear to you guys that for the purpose of this exercise, it is more similar to the oxygen. And it makes sense because remember the oxygen is either going to have a hydrogen or a negative charge, right? Same thing with the sulfur. It's either going to have a hydrogen or a negative charge. It's never going to have 2 Hs and a positive. So this is just basically an extra little point to say that you average the sulfur and the oxygen as the ones being similar. It's basically the same thing as the acid and base rule. It's just a different application of it. Okay?
So that means that the pKas that I'm using here well, I'm going to use these two groups which means the pKas I'm using are pKa 1 and pKa 3 and I'm not using pKa 2. So now we're just going to add them together. So it's going to be 1.96+8.18 /2. So I'm going to do that in my calculator. So I'm getting 10.142 equals an isoelectric point of 5.07. Cool? Awesome, guys. So we're done with this page. Let's move on to the next.