Comprehensive Study Notes on Enzymes

Introduction to Enzymes

Definition and Importance

Enzymes are biological catalysts that accelerate the rate of biochemical reactions in living organisms without being consumed in the process. Most enzymes are globular proteins, though some, such as ribozymes, are RNA molecules with catalytic activity. Enzymes are essential for processes such as respiration, photosynthesis, digestion, and biosynthesis of macromolecules.

Classification of Enzymes

Enzymes are classified based on the type of reaction they catalyze. The main classes include:

• Oxidoreductases: Catalyze oxidation-reduction reactions (e.g., dehydrogenases, oxidases).

• Transferases: Transfer functional groups between molecules (e.g., kinases, phosphorylases).

• Hydrolases: Catalyze hydrolysis reactions (e.g., sucrase, lipases, proteases).

• Lyases: Remove groups from substrates without hydrolysis (e.g., decarboxylases).

• Isomerases: Rearrange atoms within a molecule (e.g., isomerases).

• Ligases: Join two molecules using energy from ATP (e.g., synthetases, ligases).

Enzyme names typically end with '-ase'.

Characteristics of Enzymes

General Properties

• Specificity: Enzymes are highly specific for their substrates due to the unique structure of their active sites.

• Efficiency: Enzymes have high turnover rates, meaning a small amount can catalyze the conversion of large amounts of substrate.

• Unchanged by Reaction: Enzymes remain chemically unchanged after catalysis and can be reused.

• Affected by Environmental Factors: Enzyme activity is influenced by substrate concentration, enzyme concentration, temperature, and pH.

• Cofactor Requirement: Some enzymes require non-protein cofactors (inorganic ions, coenzymes, or prosthetic groups) to function. The enzyme-cofactor complex is called a holoenzyme, while the protein part alone is an apoenzyme.

• Regulation: Enzyme activity is tightly regulated by activators and inhibitors.

• Equilibrium: Enzymes accelerate the attainment of equilibrium but do not alter the equilibrium position.

Types of Cofactors

• Inorganic ions (e.g., Zn2+ for carbonic anhydrase)

• Coenzymes: Organic molecules that bind loosely (e.g., NAD+ for dehydrogenases)

• Prosthetic groups: Organic molecules tightly bound (e.g., haem in catalases)

Mode of Action of Enzymes

Active Site Structure

The active site of an enzyme is a specific region formed by the three-dimensional folding of the protein, comprising the substrate-binding site and the catalytic site. The active site is responsible for substrate recognition and catalysis.

Enzyme Specificity

Enzyme specificity arises from the precise fit between the enzyme's active site and its substrate, determined by shape, size, charge, and orientation. This specificity is a direct result of the enzyme's unique amino acid sequence and three-dimensional conformation.

Enzyme-Substrate Complex

When a substrate binds to the active site, an enzyme-substrate (E-S) complex forms. The substrate is converted to product, which then leaves the active site, allowing the enzyme to catalyze further reactions.

Lock-and-Key and Induced Fit Hypotheses

• Lock-and-Key Hypothesis: The active site is perfectly complementary to the substrate, allowing precise binding (more likely for enzymes with a single substrate type).

• Induced Fit Hypothesis: The active site is flexible and molds around the substrate upon binding, enhancing the fit and catalytic efficiency (common for enzymes acting on related substrates).

Activation Energy and Catalysis

Activation energy (EA) is the minimum energy required for reactants to reach the transition state and react. Enzymes lower the activation energy, providing an alternative pathway and increasing the reaction rate without raising the temperature.

• Enzymes promote the formation of the transition state by:

  • Bringing reactants into close proximity

  • Orienting reactants correctly

  • Destabilizing bonds in reactants

  • Providing a conducive microenvironment

Enzyme Kinetics

Measurement of Enzyme Kinetics

Enzyme kinetics studies the rates of enzyme-catalyzed reactions. Reaction rates can be measured by the amount of product formed or substrate depleted over time. The initial rate is determined from the linear portion of the reaction curve.

• Product Formation: Example—catalase catalyzing hydrogen peroxide breakdown, measuring oxygen produced.

• Substrate Disappearance: Example—amylase hydrolyzing starch, measuring decrease in starch via colorimetry.

Measurement of Enzyme Affinity (Michaelis Constant, Km)

The Michaelis constant (Km) is the substrate concentration at which the reaction rate is half of its maximum (Vmax). It indicates the affinity of the enzyme for its substrate:

• Low Km: High affinity (enzyme binds substrate readily)

• High Km: Low affinity

$K_m = [S] \text{ when } V_i = \frac{1}{2} V_{max}$

Factors Affecting Enzyme Activity

Substrate Concentration

• At low substrate concentrations, the reaction rate increases proportionally with substrate concentration.

• At high substrate concentrations, the rate plateaus as all enzyme active sites become saturated (Vmax reached).

Enzyme Concentration

• At low enzyme concentrations, the reaction rate increases proportionally with enzyme concentration.

• At high enzyme concentrations, the rate plateaus if substrate becomes limiting.

Temperature

• Increasing temperature increases reaction rate up to an optimum, beyond which the enzyme denatures and activity drops sharply.

• Q10 coefficient: Rate doubles for every 10°C rise (within optimal range).

• Denaturation at high temperatures is irreversible due to loss of enzyme structure.

$Q_{10} = \frac{\text{rate at } (x+10)^{\circ}C}{\text{rate at } x^{\circ}C}$

pH

• Each enzyme has an optimum pH; deviations reduce activity due to disruption of ionic and hydrogen bonds, altering the active site.

• pH effects are often reversible unless extreme conditions cause denaturation.

Enzyme Inhibition

Reversible Inhibition

• Competitive Inhibition: Inhibitor resembles substrate and binds to the active site, competing with the substrate. Increases Km, but Vmax can be reached at high substrate concentrations.

• Non-competitive Inhibition: Inhibitor binds to a site other than the active site, altering enzyme conformation. Vmax decreases, Km remains unchanged.

Irreversible Inhibition

• Inhibitor binds permanently (often covalently), causing permanent loss of enzyme activity (e.g., heavy metals disrupting disulfide bonds).

Allosteric Regulation of Enzymes

Allosteric Activation and Inhibition

Allosteric enzymes are often multi-subunit proteins regulated by molecules binding to sites other than the active site (allosteric sites). Binding of activators or inhibitors stabilizes the enzyme in active or inactive forms, respectively. This regulation can be positive (activation) or negative (inhibition).

End-Product (Feedback) Inhibition

In metabolic pathways, the end product can inhibit an earlier enzyme, preventing overproduction. For example, ATP inhibits phosphofructokinase in glycolysis, while ADP acts as an activator.

Cooperativity

Cooperativity is a form of allosteric regulation where binding of a substrate to one active site increases the activity at other active sites in a multi-subunit enzyme, enhancing overall catalytic efficiency.