Enzyme Kinetics

Study Notes

Most biochemical reactions require enzymes to occur at a rate that sustains life
Enzyme activity can be controlled to meet an organism’s needs
Enzymes are specific: they bind to a specific substrate and catalyze a specific reaction
For example: Enzymes like alcohol dehydrogenase or lactase can only catalyze reactions involving alcohol or lactose, respectively.

Catalysts accelerate the rate of a chemical reaction without being consumed in the process
Catalysts influence the rate, not the equilibrium of a reaction, meaning they speed up the time it takes to reach equilibrium
Enzymes lower the activation energy needed for a reaction to occur.
Example: Hydrogen peroxide breaks down into water and oxygen, which is considerably faster in the presence of iron.

Isoenzymes are enzymes that catalyze the same reaction but differ in their structure and properties
Different tissues can have different forms of the same enzyme
Different forms of the same enzyme can have different substrate affinities and sensitivity to inhibitors
Example: Hexokinase 1 is found in red blood cells and is involved in fast energy production, while glucokinase is found in the liver and pancreas and is involved in storage processes.

The cytochrome P450 system is a superfamily of microsomal enzymes that play a key role in drug metabolism, particularly in the liver.
Different isoforms of cytochrome P450 have various substrate specificities and can be induced or inhibited by different compounds.
Example:
- CYP2B6 is involved in metabolizing cyclophosphamide, a drug used in chemotherapy.
- CYP2C8 is involved in the metabolism of diclofenac, a nonsteroidal anti-inflammatory drug.
- CYP2E1 is involved in the metabolism of ethanol, paracetamol, and isoniazid.

Enzyme activity is typically measured in terms of "specific activity" and units:
- Specific activity: refers to the amount of product formed per unit time per unit of enzyme protein (e.g., μmol/min/mg).
- International Unit (U): one unit of enzyme activity corresponds to the amount of enzyme that converts 1μmol of substrate per minute under optimal conditions.
- Katal (kat): one katal is the amount of enzyme catalyzing the conversion of 1 mole of substrate per second.

Enzyme inhibitors are substances that decrease the rate of enzyme activity. Different types of inhibitors have different modes of action.
Competitive inhibition: occurs when an inhibitor binds to the active site of an enzyme, competing with the substrate.
- Competitive inhibitors increase Km (the Michaelis constant, which indicates the substrate concentration for half-maximal velocity) but do not change Vmax (the maximum reaction velocity).
Noncompetitive inhibition: occurs when an inhibitor binds to a site other than the active site, altering the enzyme's shape and reducing its catalytic activity.
- Noncompetitive inhibitors do not change Km, but they do decrease Vmax.

Drugs can compete for the active site of the same enzyme, potentially affecting their metabolism and therapeutic effects.
Example: Cimetidine, a drug used to inhibit stomach acid production, can competitively inhibit the activity of various cytochrome P450 enzymes, leading to decreased metabolism of other drugs like steroids.

The process of enzyme induction involves an increase in the synthesis of enzymes due to exposure to certain substances, primarily drugs.
Induction occurs over a longer period (weeks), is slowly reversible, and its underlying mechanisms aren't fully understood.
Example: Chronic ethanol consumption can induce the expression of CYP2E1, a cytochrome P450 isoform involved in the metabolism of various substances, including ethanol, acetaminophen (paracetamol), and isoniazid.