Enzyme Mechanisms:Serine Proteases

C483 Spring 2013

Questions1. The substrate specificity of serine proteases is primarily due to

A) a specificity pocket in the protein. B) the positions of specific side chains of serine, histidine, and aspartate.C) distinct backbone conformations of the individual proteins.D) A and B. E) A, B and C.

2. The role of serine at the active site of serine proteases is to act as a(n) ________ catalyst, while the histidine residue serves as a(n) ________ catalyst.

A) strong; weak B) weak; strong C) acid-base; covalent D) covalent; acid-base E) anionic; ionic

3. Experiments on the bacterial serine protease subtilisin show that even when all three residues of the catalytic triad are mutated, the catalytic rate of the enzyme is still 3000 times the uncatalyzed reaction rate. Which mode of catalysis is likely responsible for this remaining catalytic activity?

A) Acid-base catalysis.B) Covalent catalysis.C) Induced fit.D) Hydrophobic effects.E) Transition-state stabilization.

Proteases

• Role in Digestion• Zymogens• Serine Proteases– Chymotrypsin– Trypsin– Elastase

• Homology

Selectivity of Serine Proteases

Mechanism of Serine Proteases:Catalytic Triad

Chemical Mechanism

• Substrate binding affects Histidine base strength

• Covalent catalysis• Oxyanion hole• Covalent

intermediate

Chemical Mechanism

• Read from bottom!• General acid-base

catalysis• Oxyanion hole• Return to initial state• Compare/contrast to

Main Protease

Answers

1. A2. D3. E