Proteins

zwitterion

bear charged groups of opposite polarity

pI

isoelectric point

aromatic amino acids

absorb at 280 nm

2.2

pka for cooh

9.3

pka for NH3+ side chain

• ionize readily

disulfide bonds

• oxidized to _______ in oxidizing conditions

• reversible reactions

• can happen between protein subunits

• generally, exposed cysteines are in disulfide

6

histidine pKa

closest to neutral!

• frequent acid-base catalysis because good at acid base chemistry

• at pH 6- ½ neutral and ½ imidazole.

• at physiological pH only 4% are ionized—>

free aa pkas

• In the free amino acids, the pKa values are much lower, because the positively charged ammonium group electrostatically stabilizes the COO− group, in effect making it easier for the carboxylic acid group to ionize.

• Similarly, the pKa values for α-amino groups in proteins range from 7.5 to 8.5. In the free amino acids, the pKa values are higher, due to the electron-withdrawing character of the nearby carboxylate group, which makes it more difficult for the ammonium group to become deprotonated.

exterior

buried cysteines are in disfuldies

exposed cysteins are not in disulfides because it’s energeticalyl expensive

salt bridge

form between positive and negative side chains