z Hydrophobic amino acids: encountered in the interior of proteins shielded from direct contact with water z Hydrophillic amino acids: generally found on the exterior of proteins as well as in the active centers of enzymes z Imidazole group: act as either proton donor or acceptor at physiological pH – Reactive centers of enzymes z Primary alcohol and thiol groups: act as nucleophiles during enzymatic catalysis – Disulfide bonds Functional significancez Hydrophobic amino acids: encountered in the interior of proteins shielded from direct contact with water z Hydrophillic amino acids: generally found on the exterior of proteins as well as in the active centers of enzymes z Imidazole group: act as either proton donor or acceptor at physiological pH – Reactive centers of enzymes z Primary alcohol and thiol groups: act as nucleophiles during enzymatic catalysis – Disulfide bonds Functional significance