Question

The mechanism of chymotrypsin can be viewed as a two-step process, acylation of the enzyme active site followed by a deacylation reaction. The observation of "burst" kinetics in rapid kinetic studies of the hydrolysis of p-nitrophenylphosphate by chymotrypsin is due to:_______
A. the rate of the acylation reaction being faster than the deacylation reaction.
B. the rates of acylation and deacylation being equal.
C. the rate of the acylation reaction being slower than the deacylation reaction.

Answer 1

Answer:

A. the rate of the acylation reaction being faster than the deacylation reaction.

Explanation:

Chymotrypsin belongs to a class of enzymes known as proteases; enzymes that catalyse the cleavage of peptide bonds by hydrolysis.

The mechanism of chymotrypsin catalysis occurs in two distinct phases; (1) an acylation phase where the peptide bond is cleaved and an ester linkage is formed between the peptide carbonyl carbon and the enzyme, (2) a deacylation phase where the ester linkage is hydrolyzed and the non-avylated enzyme is regenerated.

In studies by B.S. Hartley and B.A. Kilby in 1954 of chymotrypsin hydrolysis of the ester p-nitropheylacetate, as measured by the release of nitrophenol, it was discovered that it proceeded with a burst before leveling of to a slower rate. This burst was due to a rapid acylation of all the enzyme molecules with a slow deacylation limiting the turnover of the enzyme.

Similarly, the observation of burst kinetics in rapid kinetic studies of the hydrolysis of p-nitrophenylphosphate by chymotrypsin is due to the initial phase of acylation proceeding much faster than the later phase of deacylation of the enzyme.