Question

Q1: hydrolysis of an ester using a Lipase CALB enzyme.

1. Use an organic chemistry textbook to draw the mechanism of hydrolysis of the corresponding non enzymatic reaction- ester hydrolysis using acid/base catalysis. Enzymatic reactions often involve general acid or general base catalysis rather than such specific acid or specific base catalysis. explain what this means.

3. With as much detail as you can identify the catalytic residues involved in each step, and discuss their involvement in the catalytic mechanism for each of the steps involved (acid, base, covalent catalysis). Propose how the starting material and tetrahedral intermediates/transition states could be bound via non-covalent interactions to other residues at the active site and hence any influence from them.

Answer 1

1 In general acid or base catalysis means that any acid or base added to the solution will increase the rate of the reaction. It involves a molecule beside water that acts as proton donor /acceptor during enzymatic reaction. For general acid, partial transfer of proton from Bronsted Acid, lowers free energy of transition state, rate of reaction increases with decrease in pH & increase in conc. of Bronsted acid.

In Specific acid-base catalysis , specifically or accelerates the reaction. The reaction rate is dependent on pH only & not on buffer concentration. For specific acid catalysis, protonation lowers free energy of transition state, rate of reaction increases with decrease in pH. For specific base, abstraction of proton by base lowers free energy of transition state & rate of reaction increases with increase in pH.

The acid or base catalyzed ester hydrolysis mechanism is shown below :