Lecture 3: Proteins and pH

Question 1

Zwitterion

Answer 1

Molecule with a net neutral charge and separated, balancing positive and negative charges

Question 2

Isoelectric point

Answer 2

pH at which a protein/AA is electrically neutral

Question 3

Calculating PI

Answer 3

For nonionizable R groups, PI is the average of the N- and C-terminal pKa’s. For ionizable R groups, pKa is the average of the two identical groups (2 carboxyls or 2 aminos)

Question 4

What does pKa mean experimentally?

Answer 4

pKa is the pH at 50% dissociation of the group/molecule

Question 5

Histidine R group pKa

Answer 5

pKa = ~6.0; only R group w/ pKa close to physio. pH and thus plays important buffering role.