Wk 3&4 lecture 3: Enzyme regulation, allostery

Enzyme Regulation

Enzyme Activity Control
- Enzymes must be regulated to respond to changing cellular environments.
- Regulation prevents uncontrolled reactions, ensuring metabolic efficiency.

Basic Regulation
- Enzymes can be regulated by controlling substrate availability.
- Without substrates, enzymes cannot produce products.
- As substrate levels rise, enzyme activity typically increases (Michaelis-Menten kinetics).
Michaelis-Menten Enzymes
- Named after researchers who characterized them.
- Work efficiently near Km (Michaelis constant), the substrate concentration at which enzyme activity is half-maximal.
- Changes in substrate concentration lead to proportional changes in enzyme activity.
Limitations
- Enzyme pathways may require multiple enzymes, limiting control by substrate concentration alone.
- Complicated pathways require more sophisticated regulation to direct substrates appropriately.

Definition and Function
- Allosteric enzymes regulate metabolic pathways at crucial decision points.
- Act like "traffic police," directing the flow of substrates through interconnected pathways.
Pathway Complexity
- Metabolic pathways can be intricate, with many branching points.
- Regulation must ensure that substrates flow through desired routes.

Mechanism
- A high concentration of product (F) inhibits the first enzyme (E1) in a metabolic pathway, effectively controlling flow to prevent overproduction.
- This is crucial for maintaining stable product levels in the cell.
Example
- If F accumulates, it slows down further synthesis by inhibiting E1, allowing resources for alternative pathways.

Enzyme Activation
- Allosteric inhibitors stabilize the less active T-state of enzymes, decreasing their activity.
- Allosteric activators stabilize the active R-state, increasing enzyme activity.
Cooperativity
- Allosteric enzymes exhibit sigmoidal kinetics due to cooperative binding, where the binding of substrate to one active site influences others.
Kinetics Comparison
- Michaelis-Menten enzymes show hyperbolic kinetics.
- Allosteric enzymes demonstrate sigmoidal kinetics with a critical substrate concentration required for activity.

Hypothetical Pathway
- Pathway begins with substrate A and goes through intermediates B, C, D, E to final product F.
- Each enzyme in the pathway converts one intermediate into another sequentially.
Commitment Steps
- E1 catalyzes the first committed step - once substrate A is converted to B, it commits to producing F.
- Feedback inhibition is used to regulate this pathway effectively.

Function
- Catalyzes the first step in producing CTP from carbamyl phosphate and aspartate.
- Exhibits allosteric regulation via feedback inhibition from CTP.
Structural Insights
- ATCase has 12 subunits (6 catalytic and 6 regulatory).
- CTP binds to regulatory subunits, stabilizing the T-state and reducing activity.
Kinetic Behavior
- Shows sigmoid activity curve; response to aspartate concentration is influenced by CTP.

Covalent Modification
- Involves adding/removing chemical groups to change enzyme activity quickly.
- Allows for reversible modification and potential amplification of enzyme activity by cascades of activation.
Hydrolytic Cleavage
- Some enzymes, like trypsin, are activated by hydrolysis of inactive precursors.
- Activates precursor enzymes without causing self-damage to tissues (e.g., pancreas).

Allosteric control is vital for enzyme activity and pathway regulation.
Enzymes can be effectively controlled through feedback inhibition and allosteric regulation to maintain metabolic homeostasis.
Understanding these processes lays a foundation for further exploration of metabolic pathways in future lectures.