Amino Acids, Peptide Bonds, Enzymes, Enzyme Kinetics, DNA, and Protein Structure – Microbiology Study Guide

Amino Acids

Definition and Structure

Amino acids are organic compounds that serve as the building blocks of proteins, essential for cellular structure and function.

• Definition: Organic compounds containing both amino and carboxyl groups.

• Structure: Each amino acid consists of a central (α) carbon bonded to:

  • Amino group (–NH2)

  • Carboxyl group (–COOH)

  • Hydrogen atom

  • R-group (side chain that determines the amino acid's properties)

• Classes of Amino Acids: Classified based on the properties of their R-groups (e.g., polar, nonpolar, acidic, basic).

Peptide Bond

Definition and Characteristics

Peptide bonds link amino acids together to form polypeptides and proteins.

• Definition: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another via dehydration reaction.

• Characteristics:

  • Rigid and planar due to partial double-bond character.

  • Typically found in trans configuration.

  • Forms the backbone of protein structures.

Enzymes

Definition, Structure, and Properties

Enzymes are biological catalysts that accelerate chemical reactions in living organisms.

• Definition: Proteins (and sometimes RNA) that speed up chemical reactions by lowering activation energy.

• Structure: Composed of proteins; some have RNA components (ribozymes). Enzymes have specific active sites where substrates bind.

• Properties:

  1. Specificity: Each enzyme is highly specific to its substrate.

  2. Reusability: Enzymes are not consumed in reactions.

Mechanism of Enzyme Action

Steps and Models of Binding

Enzymes facilitate reactions by binding substrates and stabilizing transition states.

• Steps:

  1. Substrate binds to the enzyme's active site to form an enzyme–substrate complex.

  2. The enzyme stabilizes the transition state, lowering the activation energy.

  3. Product is released, and the enzyme is regenerated.

• Models of Binding:

  • Lock and Key Model: Substrate fits perfectly into the enzyme's active site.

  • Induced Fit Model: The active site undergoes a conformational change upon substrate binding.

Types of Enzyme Inhibition

Competitive and Noncompetitive Inhibition

Enzyme activity can be regulated by inhibitors, which affect reaction rates and substrate binding.

• Competitive Inhibition:

  • Inhibitor binds to the active site, competing with the substrate.

  • Effect on Kinetics:

    • Km increases (lower substrate affinity).

    • Vmax remains unchanged.

• Noncompetitive Inhibition:

  • Inhibitor binds to an allosteric site, not the active site.

  • Effect on Kinetics:

    • Km remains unchanged.

    • Vmax decreases.

Enzyme Kinetics

Michaelis–Menten Equation and Parameters

Enzyme kinetics describes the rates of enzyme-catalyzed reactions and their dependence on substrate concentration.

• Michaelis–Menten Equation:

$v = \frac{V_{max} \cdot [S]}{K_m + [S]}$

• Where:

  • v: Reaction velocity

  • Vmax: Maximum reaction velocity

  • [S]: Substrate concentration

  • Km: Substrate concentration at half Vmax (indicates substrate affinity)

• Key Parameters:

  • Km:

    • Low Km: High substrate affinity

    • High Km: Low substrate affinity

  • kcat (Turnover Number): Number of substrate molecules converted into product per enzyme molecule per second.

    • Formula:

    $k_{cat} = \frac{V_{max}}{[E]}$

    • Where [E] is the total enzyme concentration.

  • Catalytic Efficiency: Defined as kcat/Km.

Components of DNA

Structure and Composition

DNA (Deoxyribonucleic Acid) is a double-stranded molecule that stores genetic information in cells.

• Composed of nucleotides: Each nucleotide consists of:

  • Sugar: Deoxyribose (five-carbon sugar)

  • Phosphate Group: Provides structural support

  • Nitrogenous Base: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

Double Helix Structure

• DNA strands are antiparallel (one runs 5' to 3', the other 3' to 5').

• Bases pair through hydrogen bonds:

  • A pairs with T (2 hydrogen bonds)

  • C pairs with G (3 hydrogen bonds)

• Watson and Crick discovered the double helix structure.

DNA Function & Replication

• DNA stores genetic information for protein synthesis.

• Replication: Semi-conservative (each new DNA molecule contains one old strand and one new strand).

Protein Structure

Levels of Protein Structure

Proteins are made of amino acids and have four levels of structure that determine their function.

1. Primary Structure

• Definition: The linear sequence of amino acids in a polypeptide chain.

• Bond Type: Peptide bonds (covalent bonds between amino acids).

2. Secondary Structure

• Definition: Local folding patterns of the polypeptide chain.

• Types:

  • Alpha-helix: Coiled structure stabilized by hydrogen bonds (e.g., Keratin).

  • Beta-sheet: Pleated sheet-like structure (e.g., Silk fibroin).

• Importance: Provides initial stability and structure.

3. Tertiary Structure

• Definition: The three-dimensional shape of a polypeptide.

• Bonding & Interactions:

  • Hydrogen bonds

  • Disulfide bonds (covalent bonds between cysteine residues)

  • Ionic interactions (between charged amino acids)

  • Hydrophobic interactions (nonpolar residues cluster inside)

• Example: Enzymes like lysozyme have specific shapes for their function.

4. Quaternary Structure

• Definition: The arrangement of multiple polypeptide chains (subunits) into a functional protein.

• Bonding & Interactions: Same interactions as tertiary structure but between different polypeptides.

• Examples:

  • Hemoglobin (4 subunits, transports oxygen)

  • Collagen (triple helix structure for strength)