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Definition and Mechanism
- Competitive inhibitors are molecules that resemble substrates and can bind to the active site of an enzyme, preventing the true substrate from binding.
- Specificity of Active Sites
- Active sites are highly specific for their substrates; they are not generalized.
Example 1: Zulimep
- An anti-leukemia drug that inhibits the production of a specific cancer protein by hindering ATP binding to its active site, which is crucial for that enzyme's activity.
Example 2: Penicillin
- Derived from the penicillium fungus, penicillin inhibits transpeptidase, an enzyme necessary for cross-linking peptidoglycan in bacterial cell walls.
- Consequence of inhibition: Weakening of the bacterial cell wall leads to leakage and eventual cell death or improper division.

Definition
- Allosteric regulation involves the binding of molecules to sites other than the active site, leading to conformational changes in the enzyme that affect its activity.
- Types of Allosteric Changes
- Regulatory molecules can either activate or inhibit enzyme activity by binding to allosteric sites.
Example of Allosteric Inhibition: ATP
- When ATP levels are high, ATP binds to allosteric sites of certain enzymes, inhibiting their activity.
Negative Feedback Loop
- The process of an end product inhibiting an earlier step in a metabolic pathway to prevent overproduction of that product.
Phosphofructokinase Activation
- Both molecule A and B can activate phosphofructokinase, showing how the activity of one enzyme can be enhanced while another is inhibited.

Reversible Modification
- Enzymes can be regulated through reversible covalent modifications, such as phosphorylation.
Phosphate Group Binding
- Phosphate groups bind covalently to specific functional groups (such as hydroxyl) on enzymes, influencing their activity.
Activation and Deactivation
- Kinases add phosphate groups to activate enzymes, while phosphatases remove these groups to deactivate them.
- Example:
- Initial inactive protein is phosphorylated by a kinase to activate it.
- The same enzyme can be dephosphorylated by a phosphatase to revert to its inactive state.

Regulation by Nucleotide Activity
- Some enzymes are regulated through the binding and hydrolysis of nucleotides (e.g., ATP, GTP).
- Nucleotide binding can induce conformational changes, turning enzymes on or off.
Molecular Switch
- The addition of GTP activates a protein, while hydrolysis of GTP to GDP turns the protein off.
- The structural change is accompanied by a three-dimensional conformational shift.
Example
- Microtubule dynamics are regulated by GTP binding, where GTP-tubulin adds to microtubule ends, while GDP-tubulin leads to disassembly.

Definition of Metabolic Pathways
- A metabolic pathway consists of a series of enzyme-catalyzed reactions transforming substrates into products.
Structure of Metabolic Pathways
- Each dot represents a unique enzyme; each green line represents a pathway.
- Example:
- Glycolysis includes 10 steps converting glucose to pyruvate through various intermediates.
Intermediates and Their Utilization
- Intermediates (B, C, D, E) can be redirected into other metabolic pathways, such as lipid or nucleotide metabolism.

Feedback Inhibition
- Product concentration influences pathway efficiency; when product Y concentration is high, it can inhibit enzyme 1 via allosteric regulation.
- This prevents unnecessary energy expenditure in further enzyme actions when sufficient product has been formed.
Importance of Negative Feedback
- Essential in maintaining balance within biological systems; prevents overproduction, similar to regulatory mechanisms in the immune system to avoid autoimmune responses.
Illustration of Pathways
- Each enzyme acts on specific substrates to gradually transform inputs into meaningful outputs, analogous to assembly lines where raw materials are turned into finished products during metabolic processes.