Question

pK of the side chains of glutamate and lysine is 4.25, and 10.53, respectively. Circle the answer: A protein containing 3 glutamate residues and 4 lysine residues is (negatively, positively) charged at neutral pH, and its isoelectric point is in the (acidic, basic) region of pH.

Answer 1

pK of glutamate side chain: 4.25

pK of lysine side chain : 10.53

at neutral pH that is 7 charge on one glutamate side chain will be: -1

And charge total charge of 3 glutamate residues : -1+(-1)+(-1) = -3-------1

At neutral pH :7 charge on side chain of lysine residue: +1

And total charge of 4 lysine residue: +4--------------------------------------2

Total charge on protein due to 3 glutamate and 4 lysine residue: (-3)+ (+4)= +1

But we have to consider charge on amino end and carboxyl end of protein while calculating total charge on protein so, at pH 7 charge on amino group = +1 and charge on carboxyl end would be: -1 , they cancel each other . Hence total charge on protein would be +1

Now, In proteins the isoelectric point (pI) is defined as the pH at which a protein has no net charge. When the pH > pI, a protein has a net negative charge and when the pH < pI, a protein has a net positive charge.

At little higher pH above pH 9.8, peptide N-terminus will lose the proton and will carry net 0 charge. Then total charge on protein will be,

= charge on 3 glutamic acid side chain + charge on 4 lysine side chain+ charge on amino terminus of protein +charge on carboxyl terminus of protein = (-3) +(+4) + (0) +(-1) = 0

From above calculations we can conclude that iso-electric point for this protein will be in basic region of pH.