enzyme kinetics and inhibition

Enzymes: Defined as 'biocatalysts' synthesized by living cells.
Catalysts: Chemical agents that change the rate of a reaction without themselves being consumed.
Composition: All enzymes are proteins except for ribozymes, which are RNA that act as enzymes.
Characteristics: Colloidal, thermolabile in nature, and specific in their action. Found in all body tissues and fluids - can be extracellular, intracellular, or membrane-bound.

Activation Energy: Enzymes lower the activation energy of reactions, facilitating the transition state.
Holoenzyme vs. Apoenzyme:
- Apoenzyme: Enzyme without its nonprotein component, inactive.
- Holoenzyme: Catalytically active enzyme with its nonprotein component.

Enzyme Commission (EC) Classification:
- A unique four-digit number preceded by "EC" that classifies enzymes.
- First digit: Class, Second digit: Subclass, Third digit: Sub-subclass, Fourth digit: Serial number.
Enzyme Classes (IUBMB):
1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases
7. Translocases
Mnemonic to remember classes: OTHLIL.

Active Site: Area on the enzyme formed by folding of the protein, where substrate binding occurs.
- Contains amino acid side chains and acts as a flexible template to bind substrate and initiate conversion to the transition state.
Characteristics:
- Responsible for substrate specificity.
- Contains substrate binding and catalytic sites.
- Coenzymes or cofactors may bind to these sites.
- Weak non-covalent bonds (hydrogen bonds) exist between enzyme and substrate.
Enzyme-Substrate Interaction:
- The substrate binds the active site forming the enzyme-substrate (ES) complex.
- ESTransition:
- $E + S ightleftharpoons [ES] ightleftharpoons [EP] ightleftharpoons E + P$

Enzyme Concentration [E]:
- Reaction velocity (v) increases linearly with enzyme concentration.
- Doubling enzyme concentration doubles the number of binding sites, thus doubling the reaction rate.
Temperature:
- Reaction velocity increases with temperature up to an optimum, after which it declines (bell-shaped curve).
- Optimum Temperature: Generally 37°C, varies for specific enzymes (e.g., urease: 55°C).
- Q10 Temperature Coefficient: Velocity doubles for every 10°C increase in temperature until optimum.
pH:
- Reaction velocity increases with increased pH until a maximum, has a bell-shaped curve.
- Optimum pH: Most enzymes work between pH 6 to 8; specific enzymes have their own ranges (e.g., pepsin: pH 1–2).
Substrate Concentration [S]:
- Graph of reaction velocity vs. substrate concentration shows three phases:
  - Linear (first order with respect to [S])
  - Curved (mixed-order)
  - Plateau (zero-order when all enzyme active sites are saturated).
Activator Effects:
- Inorganic ions enhance enzyme activity:
  - Anionic activators (e.g., chloride ions activate salivary amylase).
  - Cationic activators (e.g., calcium ions activate lipases).
Inhibitors:
- Product accumulation can slow down reactions; quick removal and feedback inhibition are necessary to maintain reaction rates. Example: HMG CoA reductase feedback inhibition by cholesterol.

Lock and Key Model (Emil Fischer, 1890):
- The active site of the unbound enzyme is complementary to the substrate shape.
Induced Fit Model (Daniel Koshland, 1958):
- The enzyme changes shape upon substrate binding, making the active site complementary to the substrate. This model allows enhanced catalysis by creating stress on the substrate.

Substrate Strain: Enzyme-substrate interactions can distort the substrate's bonds, leading to the transition state. This mechanism drives the reaction forward.
Transition State Complex: Represents the unstable intermediate before product formation. Weak interactions are optimized in this state.

Kinetics: Derived from Greek "kinetos," referring to movement; the study of reaction rates under varying conditions.
Michaelis-Menten Model:
- $ext{Vo} = rac{V{ ext{max}}[S]}{Km + [S]}$
- Where:
- Vo = Initial velocity,
- Vmax = Maximal velocity,
- Km = Michaelis constant.
Significance of Km:
- Independent of enzyme concentration; a constant characteristic of enzyme affinity for its substrate.
- Low Km indicates high affinity; high Km indicates low affinity.

Types of Enzyme Inhibition:
- Competitive Inhibition:
- Competitors for the active site resemble substrates, reversible by excess substrate.
- Effects on kinetics:
  - Increased Km.
  - Vmax unchanged.
- Example: Malonate inhibits succinate dehydrogenase.
- Non-Competitive Inhibition:
- Inhibitor binds at a site different from the active site, not relieved by substrate concentration.
- Effects on kinetics:
  - Km unchanged.
  - Decreased Vmax.
- Irreversible Inhibition:
- Inhibitor binds covalently to the active site; often toxic.
- Example: Cyanide inhibits cytochrome oxidase.

Sulphonamides: Competitive inhibitors of dihydropteroate synthetase, blocking folic acid synthesis in bacteria.
Methotrexate: Inhibits folate reductase in cancer therapy by mimicking folic acid.
Statins (e.g., Lovastatin): Compete with HMG CoA for the active site on HMG CoA reductase, decreasing cholesterol synthesis in the liver.

Enzymatic classifications, mechanisms of action, and kinetic properties are critical assessment points for understanding enzymatic functions.
Case scenarios involve clinical presentations related to enzyme activity and diagnosis, emphasizing the practical implications of enzyme kinetics in health and disease management.