Mechanisms of Regulation of Enzyme Activity
Enzyme activity in living organisms must be precisely regulated to maintain metabolic balance. Since enzymes control the rate of virtually all biochemical reactions, cells use multiple mechanisms to either increase or decrease enzyme activity depending on physiological needs.
I. Major Mechanisms of Enzyme Regulation
1. Allosteric Regulation
- Many enzymes possess a regulatory site (allosteric site) separate from the active site.
- When a specific molecule (effector) binds to this site, it changes the enzyme’s conformation—increasing or decreasing its affinity for the substrate.
- Positive effectors (activators) enhance activity, while negative effectors (inhibitors) reduce it.
- Example: Phosphofructokinase-1 (PFK-1) is activated by AMP and inhibited by ATP and citrate in glycolysis.
2. Covalent Modification
- Enzyme activity can be modified reversibly by covalent attachment of a group, most commonly phosphate.
- Phosphorylation and dephosphorylation alter enzyme conformation and therefore its activity.
- The enzymes that add or remove phosphate groups are:
- Protein kinases (add phosphate, usually from ATP)
- Protein phosphatases (remove phosphate)
- This process is often hormone-regulated (e.g., via cyclic AMP pathways).
3. Regulation by Substrate Availability and Product Inhibition
- If substrate concentration increases, enzyme activity generally rises (up to a saturation point).
- Accumulation of the product may inhibit the enzyme that catalyzed its formation (feedback inhibition).
- This prevents wasteful overproduction of metabolites.
4. Control by Enzyme Synthesis and Degradation
- The amount of enzyme in a cell can be regulated by controlling gene expression (synthesis) or proteolytic degradation.
- Some enzymes are synthesized only when needed (inducible enzymes), while others are constantly present at low levels (constitutive enzymes).
5. Compartmentation
- Enzymes are localized in specific cellular compartments (mitochondria, cytosol, lysosomes, etc.), allowing spatial separation of opposing pathways (e.g., fatty acid synthesis in cytosol vs. oxidation in mitochondria).
- This prevents futile cycles and allows specific regulation.
II. Adjustable and Inducible Enzymes
1. Adjustable Enzymes
- These enzymes change their catalytic activity rapidly and reversibly in response to small molecules or covalent modifications.
- They allow fine-tuning of metabolism according to immediate cellular demands.
- Example: Enzymes regulated by allosteric effectors or phosphorylation, such as glycogen phosphorylase and glycogen synthase.
2. Inducible Enzymes
- Their synthesis is turned on (induced) by specific substrates or hormones and turned off (repressed) when not needed.
- This is a long-term regulation, as it involves gene expression.
- Example: Liver enzymes responsible for drug metabolism (cytochrome P450) are induced by certain drugs or toxins.
- Another example: β-galactosidase in bacteria is induced only when lactose is present.