OIA1003 ENZYMES

Definition of Enzymes

Biological catalysts that increase reaction rates without being consumed.

Genomic Presence

~25% of human genes encode for enzymes.

Composition

Mostly proteins; some include non-protein parts (e.g., cofactors).

Oxidoreductases

Catalyze redox reactions (e.g., Lactate dehydrogenase).

Transferases

Transfer functional groups (e.g., Transaminase, Kinase).

Hydrolases

Catalyze hydrolysis reactions (e.g., Lipase, Phosphatases).

Lyases

Add/remove groups to form double bonds (e.g., Carbonic anhydrase).

Isomerases

Catalyze isomerization within a molecule (e.g., Phosphoglycerate mutase).

Ligases

Join two molecules using ATP (e.g., DNA ligase).

Active Site

Special pocket for substrate binding and catalysis.

Catalytic Efficiency

Enzymes can accelerate reactions by 10³–10⁸ times.

Specificity

High specificity for substrate and reaction type.

Holoenzyme

Active enzyme with non-protein part (cofactor).

Apoenzyme

Inactive enzyme without its cofactor.

Regulation

Enzyme activity can be upregulated or downregulated.

Energy Barrier

Enzymes lower the activation energy (ΔG‡).

Lock-and-Key Model

Rigid specificity; substrate fits perfectly into active site.

Induced Fit Model

Active site adjusts its shape to fit substrate after binding.

pH Effect

Enzymes work best at optimal pH; extreme pH leads to denaturation.

Example: Salivary amylase pH 6.7–7.0.

Temperature Effect

Optimal around 37°C; too high → denaturation.

Substrate Concentration

Rate increases with substrate until Vmax is reached (enzyme saturation).

Michaelis-Menten Equation

Describes relationship between reaction rate and substrate concentration.

Vmax

Maximum reaction velocity when enzyme is saturated.

Km

Substrate concentration at half Vmax; indicates enzyme affinity.

Low Km

High affinity (enzyme binds substrate easily).

High Km

Low affinity (requires more substrate).

Hyperbolic Curve

Michaelis-Menten plot: curve levels off as substrate increases.

PURPOSE (Lineweaver-Burk Plot)

Linear form of Michaelis-Menten equation; easier to calculate Km and Vmax.

Key Axes (Lineweaver-Burk Plot)

X-axis: -1/Km

Y-axis: 1/Vmax

Steeper Slope (Lineweaver-Burk Plot)

Indicates lower efficiency (higher Km or lower Vmax).

Competitive Inhibition

Inhibitor binds active site, competes with substrate.

Example: Statins inhibit HMG-CoA reductase.

Effect of Competitive Inhibitor

Increases Km, no effect on Vmax.

Non-Competitive Inhibition

Inhibitor binds elsewhere on enzyme → affects catalysis.

Effect of Non-Competitive Inhibitor

Reduces Vmax, no effect on Km.

Examples of Non-Competitive Inhibitors

Heavy metals (lead, mercury, arsenic);

Cyanide inhibits cytochrome c oxidase.

Allosteric Modulators

Bind away from active site → alter enzyme shape and function.

Covalent Modifiers

Phosphorylation changes enzyme activity.

End Product Inhibition

End product inhibits the first step enzyme (feedback regulation).

ACE Inhibitors

Lisinopril, enalapril, captopril inhibit angiotensin-converting enzyme (ACE) → lower BP.

Enzymes in Diagnosis

Isoenzymes (e.g., Creatine kinase CK-MB) used to detect myocardial infarction.