Allosteric Enzymes and Regulation: Structure, Mechanisms, and Models Ch 7

Behavior of Allosteric Enzymes

Introduction to Allosteric Enzymes

Allosteric enzymes are a class of enzymes whose activity is regulated by the binding of specific molecules at sites other than the active site. This regulation is crucial for controlling metabolic pathways in cells.

• Allosteric: Derived from Greek allo (other) + steric (shape), indicating a change in enzyme conformation.

• Allosteric enzyme: An oligomeric enzyme whose biological activity is affected by substances binding to it, altering its quaternary (4°) structure.

• Allosteric effector: A molecule that modifies the behavior of an allosteric enzyme. Effectors can be:

  • Allosteric inhibitor: Decreases enzyme activity.

  • Allosteric activator: Increases enzyme activity.

• Example: Aspartate transcarbamoylase (ATCase) is a classic allosteric enzyme.

Feedback Inhibition

Mechanism and Importance

Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

• Definition: The accumulation of product inhibits its continued production.

• Pathway Example: The product CTP inhibits ATCase, controlling pyrimidine biosynthesis.

Step	Enzyme	Effect
Initial substrate	Enzyme 1	Pathway begins
Intermediate	Enzyme 2-6	Sequential reactions
Final product (CTP)	Enzyme 7 (ATCase)	Inhibits pathway

Concerted and Sequential Models for Allosteric Enzymes

Sigmoidal Kinetics and Regulation

Allosteric enzymes often display sigmoidal (S-shaped) kinetics, reflecting cooperative substrate binding and regulation by effectors.

• Sigmoidal curve: Indicates cooperative binding; reaction velocity increases sharply after a threshold substrate concentration.

• Effect of effectors: Activators (e.g., ATP) shift the curve left (increase activity), inhibitors (e.g., CTP) shift it right (decrease activity).

• K0.5: Substrate concentration at half-maximal velocity for allosteric enzymes.

System Type	Effect of Effector
K system	Effector alters K0.5
V system	Effector alters Vmax

Organization and Structure of ATCase

Subunit Composition and Separation

ATCase is a multi-subunit enzyme with distinct catalytic and regulatory units, allowing complex regulation.

• Catalytic unit: 6 subunits, organized into 2 trimers.

• Regulatory unit: 6 subunits, organized into 3 dimers.

• Separation: Catalytic and regulatory subunits can be separated by compounds reacting with cysteine (e.g., p-hydroxymercuribenzoate).

Allosteric Effects and Effector Types

Homotropic and Heterotropic Effects

Allosteric enzymes are regulated by both homotropic and heterotropic interactions, affecting their activity and substrate affinity.

• Homotropic effects: Occur when identical molecules (e.g., substrate) bind to the enzyme, promoting cooperative binding (e.g., aspartate binding to ATCase).

• Heterotropic effects: Occur when different molecules (e.g., inhibitors or activators) bind, modulating enzyme activity (e.g., CTP inhibition, ATP activation).

Concerted Model (Monod-Wyman-Changeux Model)

Mechanism of Cooperative Binding

The concerted model explains how allosteric enzymes switch between active and inactive conformations in a coordinated manner.

• Two conformations: R (relaxed, active) and T (tight/taut, inactive).

• Substrate binding: Shifts equilibrium from T to R form; all subunits change conformation simultaneously (concerted).

• Equilibrium: In absence of substrate, T form is favored; substrate binding promotes R form.

• Mathematical representation:

$L = \frac{[T]}{[R]}$ (ratio of T to R forms)

Higher L means T form is favored; effectors shift equilibrium.

Sequential Model

Induced-Fit and Cooperative Binding

The sequential model proposes that substrate binding induces conformational changes in individual subunits, leading to positive cooperativity.

• Induced-fit: Substrate binding causes local conformational change, increasing affinity for additional substrate molecules.

• Positive cooperativity: Each binding event makes subsequent binding more likely.

• Allosteric inhibition: Can also occur via induced-fit mechanism.

Control of Enzyme Activity by Phosphorylation

Regulation via Covalent Modification

Phosphorylation is a common mechanism for regulating enzyme activity, often switching enzymes between active and inactive forms.

• Target residues: Serine, threonine, and tyrosine side chain -OH groups can be phosphorylated.

• Mechanism: ATP donates phosphate group, converting inactive precursor into active enzyme.

• Example: Glycogen phosphorylase is activated by phosphorylation.

Zymogens

Inactive Precursors and Activation

Zymogens are inactive enzyme precursors that require proteolytic cleavage to become active, ensuring spatial and temporal control of enzyme activity.

• Example: Chymotrypsinogen is activated to chymotrypsin in the digestive tract.

• Activation: Trypsin cleaves chymotrypsinogen, resulting in active chymotrypsin with altered primary, secondary, and tertiary structure.

Nature of the Active Site

Structure and Function

The active site of an enzyme is a specialized region where substrate binding and catalysis occur, often involving critical amino acid residues.

• Proximity of active amino acids: The arrangement of residues in the active site facilitates substrate binding and transition state stabilization.

• Example: Chymotrypsin's active site contains serine, histidine, and aspartate residues forming a catalytic triad.

Chemical Reactions Involved in Enzyme Mechanisms

Mechanistic Principles

Enzyme mechanisms often involve nucleophilic attack, formation of covalent intermediates, and stabilization of transition states.

• Nucleophilic attack: Oxygen from serine attacks the carbonyl group of the peptide bond in chymotrypsin.

• Transition state stabilization: Enzymes lower activation energy by stabilizing high-energy intermediates.

Coenzymes

Role and Examples

Coenzymes are non-protein molecules required for enzyme activity, often derived from vitamins and involved in group transfer reactions.

• Types: Metal ions (e.g., Zn2+), organic compounds (e.g., NAD+, derived from niacin).

• NAD+: Functions as a two-electron oxidizing agent, reduced to NADH in redox reactions.

• Pyridoxal phosphate: Derived from vitamin B6, involved in transamination reactions for amino acid biosynthesis.

Coenzyme	Function	Vitamin Source
NAD+	Redox reactions	Niacin
Pyridoxal phosphate	Amino group transfer	Vitamin B6

Equation for transamination:

$\text{Amino acid}_1 + \text{α-keto acid}_2 \rightarrow \text{α-keto acid}_1 + \text{Amino acid}_2$

Review Exercises

Recommended exercises for further study: 25-29, 32, 33, 35, 40-45, 47, 58.

Additional info: These notes expand on the provided outline and slides, adding definitions, examples, and equations for clarity and completeness.