Chemical Reactions and Enzyme Function in Biological Systems

Chemical Reactions

Dehydration Synthesis vs Hydrolysis

Biological macromolecules are assembled and broken down through two fundamental types of chemical reactions: dehydration synthesis (also known as condensation reaction) and hydrolysis. Dehydration synthesis forms covalent bonds between monomers by removing a water molecule, while hydrolysis breaks these bonds by adding water.

• Dehydration Synthesis: Joins two molecules by removing H2O; important in forming polymers like proteins and nucleic acids.

• Hydrolysis: Splits polymers into monomers by adding H2O; essential for digestion and cellular metabolism.

• Example: Formation of a peptide bond between amino acids (dehydration synthesis); breakdown of starch into glucose (hydrolysis).

Catabolic vs Anabolic Reactions/Pathways

Metabolic pathways are classified as catabolic or anabolic based on their function. Catabolic pathways break down complex molecules into simpler ones, releasing energy. Anabolic pathways build complex molecules from simpler ones, requiring energy input.

• Catabolic Pathways: Degradative; e.g., cellular respiration.

• Anabolic Pathways: Biosynthetic; e.g., protein synthesis.

• Example: Glycolysis (catabolic); photosynthesis (anabolic).

Endergonic (Endothermic) vs Exergonic (Exothermic) Reactions/Pathways

Chemical reactions are also categorized by their energy profiles. Endergonicexergonic (exothermic) reactions release energy.

• Endergonic Reactions: Require energy input; products have more energy than reactants.

• Exergonic Reactions: Release energy; products have less energy than reactants.

• Equation: $\Delta G = G_{products} - G_{reactants}$ (where $\Delta G$ is positive for endergonic, negative for exergonic)

• Example: ATP synthesis (endergonic); ATP hydrolysis (exergonic).

Enzymes

Characteristics of Enzymes

Enzymes are biological catalysts that accelerate chemical reactions without being consumed. They are typically proteins, though some RNA molecules (ribozymes) also exhibit catalytic activity.

• Highly specific for their substrates.

• Reusable after each reaction.

• Efficient in lowering activation energy.

Role of Enzymes

Enzymes facilitate metabolic reactions by lowering the activation energy required for the reaction to proceed, thus increasing the rate of biological processes.

• Example: DNA polymerase catalyzes DNA synthesis.

Definition of Activation Energy and How It Is Impacted by Enzymes

Activation energy is the minimum energy required to initiate a chemical reaction. Enzymes lower this threshold, making reactions more likely to occur under physiological conditions.

• Equation: $E_a$ (activation energy) is reduced in the presence of an enzyme.

• Example: Catalase lowers activation energy for hydrogen peroxide decomposition.

Interaction Between Enzyme and Substrate

Enzymes bind to their substrates at the active site, forming an enzyme-substrate complex. This interaction is often described by the induced fit model, where the enzyme changes shape to accommodate the substrate.

• Specificity: Only certain substrates fit the active site.

• Example: Sucrase binds specifically to sucrose.

Specificity of Enzyme

Enzyme specificity is determined by the shape and chemical properties of the active site. Each enzyme typically catalyzes only one type of reaction or acts on a specific substrate.

• Example: Lactase acts only on lactose.

Competitive vs Noncompetitive (Allosteric) Inhibition

Enzyme activity can be regulated by inhibitors. Competitive inhibitors bind to the active site, blocking substrate access. Noncompetitive (allosteric) inhibitors bind elsewhere, changing the enzyme's shape and reducing activity.

Type	Binding Site	Effect
Competitive	Active site	Blocks substrate
Noncompetitive	Allosteric site	Alters enzyme shape

Denaturation

Denaturation is the loss of an enzyme's three-dimensional structure, usually caused by extreme pH, temperature, or chemicals. Denatured enzymes lose their catalytic activity.

• Example: Boiling an egg denatures its proteins.

Impact of Environmental Variables on Enzyme Activity

Enzyme activity is influenced by several environmental factors:

• pH: Each enzyme has an optimal pH; deviations can reduce activity or cause denaturation.

• Temperature: Higher temperatures increase activity up to a point, but extreme heat denatures enzymes.

• Enzyme Concentration: More enzyme increases reaction rate, up to substrate saturation.

• Substrate Concentration: More substrate increases rate until all enzyme active sites are occupied.

• Example: Pepsin works best at pH 2 (stomach); amylase at pH 7 (mouth).