Biology Unit 1.4 - Biological Reactions are Regulated by Enzymes

Metabolism

• Metabolism: Series of enzyme-controlled reactions
• Combination of anabolic (build-up) and catabolic (break-down) reactions

Enzymes

• Function: Biological catalysts that speed up metabolic reactions
• Structure: Tertiary structure proteins with a specific 3D globular shape
  • Active site formed by folding of globular proteins
  • Active site held together by peptide, disulphide, hydrogen, and ionic bonds
  • Enzymes are specific to substrates forming an enzyme-substrate complex

Properties of Enzymes

• Biological catalysts
• Not changed or used up in reactions
• High efficiency and turnover rate
• Catalyse natural reactions
• Essential for cell reactions due to slow natural rates without them
• Large molecules with specific shapes, few amino acids form active site

Types of Reactions

• Catabolic: Breakdown of large molecules into smaller ones; exothermic
• Anabolic: Build larger molecules from smaller ones; endothermic

Enzyme Theories

Lock and Key Theory

• Substrate fits exactly into the active site
• Supported by X-ray diffraction studies

Induced Fit Theory

• Active site changes shape to fit substrate
• Allows broad specificity, e.g., lysozyme enzyme

Enzyme Activity Factors

• Temperature:
  • Low: low kinetic energy, slow reaction
  • Increase: increases reaction rate up to optimum (40°C)
  • Beyond optimum: denaturation
• pH:
  • Narrow optimum range
  • Outside range can cause reversible or permanent changes
• Substrate Concentration:
  • Initial increase increases rate, eventually plateaus
• Enzyme Concentration:
  • Increase leads to more active sites and faster reactions

Enzyme Inhibitors

• Competitive: Compete with substrate for active site
  • Reversible, structurally similar to substrate
  • Increasing substrate concentration decreases inhibitor effect
• Non-competitive: Bind elsewhere, changing active site shape
  • Not reversible, not structurally similar

Industrial Use of Enzymes

• Immobilised Enzymes: Fixed in place, used in industries
  • Examples: lactose-free milk, biosensors
  • Used for stability and reusability

Unit 1.5 - Nucleic Acids and Their Functions

Nucleotide Structure

• Components: Phosphate group, pentose sugar, nitrogenous base

Types of Nucleotides

• ATP: Energy currency, high energy bonds
  • Hydrolysis releases energy
  • Reversible reaction can reform ATP from ADP

DNA Structure

• Double-stranded polymer of nucleotides
• Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
• Complementary base pairing (A-T, C-G)
• Anti-parallel strands

DNA Replication

• Semi-conservative method
• DNA helicase and polymerase involved
• Meselson and Stahl experiment support

RNA

• Single-stranded, shorter than DNA
• Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

Types of RNA

• mRNA: Carries genetic code from DNA to ribosomes
• rRNA: Forms ribosomes
• tRNA: Transfers amino acids during protein synthesis

Genetic Code

• Triplet code, codons for amino acids
• DNA transcribed into mRNA, translated into polypeptides

Protein Synthesis

• Transcription: DNA to mRNA
• Translation: mRNA decoded to polypeptides

Further Modifications

• Polypeptides modified by addition of carbohydrates, lipids, or phosphates
• Some proteins consist of multiple polypeptides