Enzyme Regulation
Metabolic pathways must be tightly regulated to meet cellular needs. Enzyme activity can be modulated at multiple levels: gene expression, covalent modification, allosteric control, and zymogen activation.
1. Allosteric Regulation
Allosteric enzymes have regulatory sites separate from the active site. Binding of allosteric effectors changes the enzyme's conformation, altering its catalytic rate.
- Positive effectors (activators): Increase enzyme activity. Example: AMP activates Phosphofructokinase-1 (signals low energy).
- Negative effectors (inhibitors): Decrease activity. Example: ATP and citrate inhibit PFK-1 (signals high energy).
- Feedback inhibition: End product inhibits first committed step (most common). Example: Isoleucine inhibits threonine deaminase.
- Concerted (symmetry) model (MWC): All subunits flip simultaneously between T (tense, low activity) and R (relaxed, high activity) states.
- Sequential (KNF) model: Induced fit; each subunit changes conformation independently upon ligand binding → cooperativity.
2. Covalent Modification
Reversible addition of functional groups to specific amino acid residues. The most important is phosphorylation/dephosphorylation by protein kinases and phosphatases.
- Phosphorylation of Ser, Thr, or Tyr residues.
- Effect depends on the enzyme — can activate or inactivate.
- Glycogen Phosphorylase: Phosphorylated (by Phosphorylase kinase) → Active form (breaks down glycogen).
- Glycogen Synthase: Phosphorylated → Inactive form. (Insulin activates a phosphatase to dephosphorylate and activate it.)
- Other modifications: Acetylation, Methylation, Ubiquitination, ADP-ribosylation.
3. Zymogen Activation (Proteolytic Cleavage)
Enzymes are synthesized as inactive precursors (zymogens) and activated by irreversible proteolytic cleavage. This ensures enzymes are only active where needed (prevents autodigestion).
- Pepsinogen → Pepsin (stomach, by HCl/pepsin autocatalysis)
- Trypsinogen → Trypsin (by Enteropeptidase/enterokinase in duodenum)
- Prothrombin → Thrombin (blood clotting cascade)
- Pro-insulin → Insulin (removal of C-peptide in β-cells)
4. Regulatory Proteins
- Calmodulin: Ca²⁺-binding protein; activated by Ca²⁺ → activates calmodulin-dependent kinases.
- G-proteins: Activated by receptor → activate adenylyl cyclase → ↑cAMP → PKA activation cascade.
5. Quantity of Enzyme (Genetic Regulation)
Induction/repression of enzyme synthesis. Example: Insulin induces glucokinase synthesis; fasting induces PEPCK for gluconeogenesis. This is slower regulation (hours) compared to allosteric (seconds).
Key Regulatory Enzymes to Know
- Glycolysis: PFK-1 (rate-limiting), Hexokinase, Pyruvate Kinase
- Gluconeogenesis: PEPCK, Fructose-1,6-bisphosphatase, Glucose-6-phosphatase
- Glycogenolysis: Glycogen Phosphorylase
- Fatty acid synthesis: Acetyl-CoA Carboxylase (ACC)
- Cholesterol synthesis: HMG-CoA Reductase