Question

what is the function of H57, S195, and water in each step for a serine protease. describe the movement of electrons shown by the arrows in each step as well.

Mechanism Homework Consider the following mechanism for a serine protease: His-67 M-57 Ser-195 Asp-10 NY Ber-195 0 H-N. O HN, ROX HAHN His-57 Ser-195 OHN Ser-195 H-N Asp Asp-102 HAN "HAN HAHN EA His-57 Ser-195 H Ser105 Asp-102 H-N -N WHEN For each of the five steps determine: 1. The function of H57 2. The function of 5195 3. The function of water 4. Describe the movement of electrons shown by the arrows.

Answer 1

The H57 role is to position the serine residue and polarize the hydroxyl group , so it can be deprotonated to the alkoxide ion. In the presence of substrate it act as a base

The role of S195 is to cleave the peptide bond in protein in which define serve as a nucleophillic amino acid at the active site as it is clearly seen in TI1 .

As already said the H57 along with substrate acts as base it deprotonate water and give hydroxide ion which acts as an nucleophile to displace serine residue

The H57 deprotonate serine residue to give alkoxide ion which serve as nucleophile and attack at the carbonyl carbon of amino acid leaving carbonyl carbon bonded to four atom . Out of these four group the stable amine group act as a leaving group and leaves behind an ester group . In the next step when water is added to the reaction it get deprotonated by again free H57 and produces hydroxide ion which act as nucleophile to attack at the carbonyl carbon of the ester and O-serine group leaves and takes the hydrogen deprotonated from water leaving behind an amino acid completely oxidized to acid group