Unit 21 – Zymogens, Isozymes, Allosteric Enzymes & Phosphorylation

What is a zymogen?

An inactive enzyme precursor that requires activation (e.g. by hydrolysis).

What is another term for zymogen?

Proenzyme.

Why are zymogens important?

They prevent premature enzyme activity that could damage tissues.

What activates trypsin from trypsinogen?

A proteolytic cleavage that exposes the active site.

How many cleavages are needed to activate chymotrypsin?

Three separate hydrolytic cleavages.

What are the cysteines involved in chymotrypsin disulfide bonds?

Cys1, Cys122, Cys136, Cys201.

What are isozymes?

Genetically distinct enzymes that catalyze the same reaction but differ in sequence and regulation.

Why do organisms have isozymes?

To allow tissue-specific and developmental regulation of metabolism.

What’s an example of isozymes in mammals?

Hexokinase I–IV, which differ in kinetics and tissue localization.

What are five ways to regulate enzyme activity?

Protein synthesis, zymogens, inhibitors, allosteric regulation, covalent modification.

What is allosteric regulation?

Regulation by small molecules binding to non-active (allosteric) sites.

What are the two states of allosteric enzymes?

Active R state and inactive T state.

What shape do allosteric enzymes' kinetics typically show?

Sigmoidal vo vs. [S] curve due to cooperativity.

What are homotropic effectors?

Substrate molecules that increase binding of additional substrate.

What are heterotropic effectors?

Molecules that regulate enzyme activity without being substrates.

What do activators do to allosteric enzymes?

Stabilize the R state and increase activity.

What do inhibitors do to allosteric enzymes?

Stabilize the T state and decrease activity.

What is ATCase?

A model allosteric enzyme: Aspartate transcarbamoylase.

What subunits does ATCase have?

6 catalytic (c) and 6 regulatory (r) subunits.

How does substrate binding affect ATCase conformation?

Triggers a T to R transition with large structural changes.

What are ATCase’s allosteric effectors?

CTP (inhibitor) and ATP (activator).

Where do ATP and CTP bind on ATCase?

At regulatory subunits, far from the active site.

How does CTP regulate ATCase?

It stabilizes the T (inactive) state and reduces activity.

How does ATP regulate ATCase?

It stabilizes the R (active) state and increases activity.

What type of regulation shifts ATCase’s vo curve left or right?

Allosteric regulation by CTP or ATP.

What is phosphorylation?

A covalent modification where a phosphate is added to Ser, Thr, or Tyr residues.

What enzyme adds phosphate groups to proteins?

Kinase.

What enzyme removes phosphate groups from proteins?

Phosphatase.

What enzyme is regulated by both allostery and phosphorylation?

Glycogen phosphorylase.

What is the inactive form of glycogen phosphorylase?

Phosphorylase b (non-phosphorylated, mostly T state).

What is the active form of glycogen phosphorylase?

Phosphorylase a (phosphorylated, mostly R state).