BIOC*3560 - Part B, Regulation of Enzymes

True or false? Enzymes increase the rate of a reaction by altering the equilibrium.

• True

• False

Answer explanation

Enzymes lower the activation energy. They do not alter equilibrium.

Organize the following molecules as being activated or inactivated by phosphorylation.

Groups:

(a) Inactivated

,

(b) Activated

• Glycogen synthase

• Glycogen phosphorylase

Answer explanation

Add more enzymes to this question as needed!

Which of the following enzymes have a stronger inhibitory effect on glycogen synthase by phosphorylating it?

• GSK3 (Glycogen synthase kinase 3)

• Protein Kinase A (PKA)

Answer explanation

These enzymes phosphorylate glycogen synthase at different sites.

Match the conformation to the enzyme + substrate state.

Recall PAM is an analogue of CP while PALA is a bisubstrate analogue of aspartate.

Groups:

(a) Between R & T state

,

(b) T state

,

(c) R state

• WT D236c with CP

• Mutant D236Ac without ligand

• Mutant D236Ac with PAM

• Mutant D236Ac with CP

• WT D236c with PAM

• WT D236c without ligand

• Mutant D236Ac with PALA

• WT D236c with PALA

Answer explanation

Order the events of the ATCase reaction. Note that the 1^st step occurs to produce catalytic activity, while the 2^nd step leads to large changes in quaternary structure.

• Carbamoyl phosphate (CP) binds the active site

• N-carbamoyl-L-aspartate (CA) is released from the active site

• Aspartate binds the active site

• Condensation reaction occurs

• Pi is released

Answer explanation

As long as substrate is available, the enzyme remains stable in the more active R state (as soon as the product leaves, substrate will bind)

Which of the following methods are suitable for pushing ATCase into the R state so we can characterize its structure and properties?

• Add PALA

• Add aspartate

• Add CTP

• Adding the activator, ATP

• Add CP

Answer explanation

N-(Phosphonacetyl)-L-aspartate (PALA) is an artificial (added by experimenters) nonreactive bisubstrate analog that mimics the reaction intermediate of ATCase & binds to its regulatory subunit. True or false?

• False

• True

Answer explanation

PALA binds to the catalytic subunits, but more specifically, the active site - it is a competitive inhibitor!

However, it cannot be chemically transformed.

There are _ active sites on each catalytic trimer – totalling _ active sites in ATCase. Substrates (aspartate & CP) bind at interfaces of these c chains, which exist in the (a)   of the complex)

For the third blank, type centre or periphery

Answer explanation

In the T-state, ATCase has a more _____ conformation. In the R state, ATCase is in a more ____ conformation.

• open, closed

• closed, open

Answer explanation

This is in contrast to hemoglobin, where the T state is more open to allow for the release of oxygen in the tissues.

Multi-step metabolic pathways, contain at least one, though often more than one (a)   .

Answer explanation

Often, the first step of a pathway is the rate limiting step, but it is not a rule (inhibition can happen further down)