[puzzled]

I'm stuck on some questions and seeking for help.  I have a polypeptide in the sequence Tyrosine-Asparagine-Threonine-Lysine-Cysteine.  All I have is a table of the pKa values for theseamino acids.  I have an alpha-COOH pKa and an alpha-NH3+ pKa for all of them.  How do u find the pH range for this polypeptide?  Henderson-Hausselbauch equation doesn't work, cuz i don't have enough info.  I've found the pI (isoelectric point) for all of them and found the range of them, but i don't think that's the pH range...due to what my teacher posted on an online forum.
He says: Look at the pka values of the side chains. Given a pH decide protonation/deprotonation. If you are given a structure, narrow the pH range by looking at protonated/deprotonated components.

2 more problems involve this polypeptide.
*At what pH would you most likely find the highest conc. for the zwitterions?  I just averaged the pI values of all the side chains...is that right?
*If an isoleucine amino acid was added to this polypeptide, which end would it be added and what are the products?  The pI is 6.05 for Isoleucine, so I thought it would go next to cysteine, but then i have no idea how to do this problem.

I know this sounds overwhelming, but I desperately need help, and the teacher has not taught us any of this.  I would greatly appreciate some help...

[Mitch]

You can find out how to do it by reading the first chapter of any Biochemistry book. Have you taken Biochemistry, yet?

[puzzled]

I'm in high school...taking Advanced Chemistry...and our textbook isn't Biochemistry specific.