User:Karsten Theis/Chymotrypsin

Please click on the as you read through the text and watch how the 3D picture on the right changes.

Overview

Chymotrypsin is a protease, an enzyme catalyzing the hydrolysis of peptide bonds of proteins. Chymotrypsin helps to digest proteins in our food. Other proteases are crucial for blood clotting (thrombin and other proteases), for the AIDS virus metabolism (HIV protease) and for many other processes relevant to human health and agriculture.

While chymotrypsin occurs in many organisms, the most-studied chymotrypsin is that from cows (bovine chymotrypsin). In its mature form, bovine chymotrypsin is a protein consisting of 245 amino acids. This string of amino acids folds into a

. (Can you guess where the substrate might bind? Try spinning around the molecule by dragging it with the mouse cursor. There should be a pocket somewhere on the surface of the enzyme. The active site is colored in this ). The path of the backbone is easier to see in this , which shows that chymotrypsin folds into two large beta sheets.

Active site residues

The active site of an enzyme is the location where the substrate binds and where the chemical reaction occurs. Active site residues are those amino acid residues demonstrated to have importance for catalysis or substrate binding. Chymotrypsin contains three residues, Ser 195, His 57 and Asp 102, which are known as its . Similar three-dimensional arrangements of a serine, a histidine and an aspartate are observed in many other proteases, and the role of these three residues in catalysis has been studied extensively. Serine acts as a nucleophile (contributing the electron pair for a new bond) attacking the carbonyl carbon of the peptide bond to be hydrolyzed. Histidine and aspartate turn serine into a better nucleophile by assisting in removing a hydrogen ion from serine.

Further reading

You can learn more about chymotrypsin structure, function and regulation in this publicly available chapter of the Biochemistry textbook by Berg, Tymoczka and Stryer.