6.5 Examples of Reactions of Regulatory Enzymes

Question 1

Types of Enzyme Regulation

Answer 1

Allosteric: allosteric effectors or modulators regulate by noncovalent binding.
Reversible covalent modification.
Binding of other regulatory enzymes.
Removal of peptide segments by proteolytic cleavage.

Question 2

Allosteric Enzymes

Answer 2

Undergoes conformational change when modulator binds.
Modulator binding site (allosteric site) is different from active site.
Homotropic: substrate and modulator are identical.
Heterotropic: modulator and substrate are different.
T state: “tense”. Low affinity for substrate (inactive).
R state: “relaxed”. High affinity for substrate (active).

Question 3

Kinetics of Allosteric Enzymes

Answer 3

Differs from Michaelis-Menten behaviour.
Sigmoin curve instead of hyperbolic curve.

Question 4

Example of ATCase

Answer 4

Aspartate transcarbomyolase (ATCase) consists of 6 catalytic subunits, and 6 regulatory subunits. Regulatory subunits have binding site (allosteric) for ATP and CTP, positive and negative heterotropic modulators.. CTP is the product of the reaction and works as feed-back inhibitor. ATP increases activity.

Question 5

Reversible Covalent Modifictation (phosphoryl groups)

Answer 5

Protein kinases catalyzes attachment of phosphoryl groups of Ser, Thr, Tyr, and His residues.
(Phospho)protein phosphatases remove the phosphoryl groups

Question 6

Mechanisms of Phosphorylation Modification

Answer 6

Bulky charged group increases polarity.
Oxygen atoms may form hydrogen bonds.
Two negative charges may repel neighbouring negative charges.
This changes the conformation of the enzyme and affects the activity of the enzyme.