What are the relevant pK values on HEPES?

HEPES is 2-[4-(2-Hydroxyethyl)piperazin-1-yl]ethane-1-sulfonic acid. I have read (MA Al-Ghobashy, Bull of Faculty of Pharmacy, Cairo University 2014; 52: 71-78) that the sulfonic acid group is completely dissociated "over almost the whole pH range" and there are two basic nitrogens N1 and N2 stated to have pKa 3 and 7.5 in the mentioned paper and in [Wikipedia article](https://en.wikipedia.org/wiki/HEPES). But what is the pKa of the sulfonic acid group? And, in the opposite end we have an OH group - is this inert or does it ever give off the proton?


