Human insulin consists of two polypeptide chains: A chain with $21$ amino

acids, $B$ chain with $30$ amino acids. Part of amino acid sequence in A chain is Gly$-$lle$-$Val$-$

Glu$-$GIn$-$Cys$-$Cys$-$Thr$.$

Based on the pK values provided here, complete the following two tasks:

a$.$ Determine the net charge of this polypeptide at $pH1,7$ and $11.$

b$.$ Estimate isoelectric point $($pl$)$ of this polypeptide.