Proteolysis

Question 1

What are pro factors or zymogens?

Answer 1

Precursors of active proteins

Question 2

What are four properties of proteolytic activation?

Answer 2

• Allows enzymes to be held in cells or in circulation in inactive form
• Allows rapid activation at action site
• irreversible
• Enzymes must be inactivated by some other mechanism (e.g. inhibitors called antiproteases

Question 3

Where are the digestive enzymes produced?

Answer 3

Salivary glands

Stomach

Pancreas

Small Intestine

Question 4

What keeps pepsinogen inactive at a neutral pH?

Answer 4

A Lysine side chain hydrogen bonds to an Aspartate pair in the active site

Question 5

What happens to pepsinogen at a lower pH? I.e. in the stomach.

Answer 5

Aspartate carboxyls in the active site become protonated, breaking salt bridges in the enzyme and changing its conformation. The now active catalytic site hydrolyzes the bond between the precursor and pepsin segments.

Question 6

What is secreted into the small intestine by pancreatic cells?

Answer 6

Zymogens and a dilute bicarbonate solution

Question 7

What is enteropeptidase and what does it do?

Answer 7

A protease that cleaves trypsinogen between Lys-6 and Ile-7, converting it to active trypsin

Question 8

What is the role of trypsin?

Answer 8

Activates more trypsinogen and other intestinal zymogens.

Chymotrypsinogen, procarboxypeptidays, proelastase

Question 9

How is the digestive system enzyme cascade regulated?

Answer 9

It is self regulating, the active proteolytic enzymes destroy each other and themselves

Question 10

What happens to trypsinogen upon cleavage?

Answer 10

The residues forming the oxyanion pocket and residue His57 move into a stable conformation, forming a complete active site.

Question 11

What is the first step in chymotrypsin activation?

Answer 11

Cleavage of a peptide bond between Arg15 and Ile16

Forming an active enzyme called (pie)-chymotrypisn

Question 12

What is the second step in chymotrypsin activation?

Answer 12

(Pie)-chymotrypsin catalyzes its own cleavage to release two dipeptides (Ser14-Arg15, and Thr147-Asn148)

Produces a-chymotrypsin, the stable form of the active enzyme

Question 13

What is pancreatic trypsin inhibitor?

Answer 13

A small protein present in the same secretory granules as the pancreatic zymogens. Binds the active site of trypsin.

Question 14

Describe the binding of PTI to the active site of trypsin

Answer 14

Tight interaction between Lys15 of PTI and Asp189 of trypsin

Carbonyl of Lys15 fits into the oxyanion pocket of trypsin, mimicking the transition state of the true substrate.

This transition state is cleaved very slowly, over several months

Question 15

What is a1-antitrypsin?

Answer 15

An elastase inhibitor that nearly irreversibly inhibits the protease.

Question 16

What does a homozygous genetic change in a1-antitrypsin result in?

Answer 16

Circulating levels of inhibitor that are 15% of normal

This means elastase is unopposed in the lungs and these individuals suffer from emphysema.

Question 17

How does smoking affect a1-antitrypsin?

Answer 17

Oxidizes a critical Met residue responsible for selectively trapping elastase.

Heterozygotes for a mutation in a1-antitrypsin are more likely to develop emphysema if they smoke.