Biochemistry Study Guide: Proteins, Enzymes, and Ligand Binding

Chapter 4: Amino Acids, Peptides, and Protein Structure

Amino Acids

Amino acids are the building blocks of proteins, each with a unique side chain that determines its properties and classification.

• Structure: Each amino acid contains an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a distinctive side chain (R group) attached to a central carbon (α-carbon).

• Identification: Be able to identify the structure, full name, three-letter, and one-letter codes for the 20 common amino acids.

• Classification: Amino acids are classified based on the chemical nature of their side chains (e.g., polar, nonpolar, acidic, basic).

Example: Glycine (Gly, G) is the simplest amino acid with a hydrogen as its side chain.

Peptide Bond

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

• Condensation Reaction: Formation of a peptide bond involves the removal of a water molecule ($\text{Amino group} + \text{Carboxyl group} \rightarrow \text{Peptide bond} + \text{H}_2\text{O}$).

• Planar Character: Peptide bonds are planar due to resonance stabilization.

Polypeptides as Polymers

Polypeptides are long chains of amino acids linked by peptide bonds.

• Charge Prediction: The net charge of a polypeptide at neutral pH depends on the side chains of its constituent amino acids.

• Sequence Identification: Be able to name amino acids in a polypeptide chain and identify key parts.

Secondary Structure

Secondary structure refers to local folding patterns within a polypeptide chain.

• Alpha Helices and Beta Sheets: Common secondary structures stabilized by hydrogen bonding.

• Hydrogen Bonding: Key to maintaining secondary structure.

• Collagen Triple Helix: A unique secondary structure found in collagen.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain.

• Globular Proteins: Compact, generally soluble proteins with diverse functions.

• Domains: Distinct functional and structural units within a protein.

• Hydrophobic vs. Hydrophilic Regions: Tertiary structure is stabilized by interactions between hydrophobic and hydrophilic side chains.

• Fibrous Proteins: Structural proteins with elongated shapes (e.g., collagen, keratin).

Protein Folding

Protein folding is the process by which a polypeptide attains its functional three-dimensional structure.

• Denaturation: Loss of structure and function due to external stress (e.g., heat, pH).

• Melting Temperature ($T_m$): The temperature at which half of the protein population is denatured.

• Protein Stability: Determined by the balance of stabilizing and destabilizing forces.

Quaternary Structure

Quaternary structure refers to the arrangement of multiple polypeptide chains (subunits) in a protein.

• Protein Purification Methods:

  • Recombinant protein expression

  • Affinity chromatography

  • Ion exchange chromatography

  • Size exclusion chromatography

  • SDS-PAGE for analysis

Chapter 5: Ligand Binding and Hemoglobin

Ligand Binding

Ligand binding is the interaction between a protein and a specific molecule (ligand).

• $K_d$ vs. $K_a$: $K_d$ is the dissociation constant (lower $K_d$ means higher affinity); $K_a$ is the association constant.

Structure and Function of Hemoglobin

Hemoglobin is a quaternary protein responsible for oxygen transport in blood.

• Heme: The iron-containing prosthetic group that binds oxygen.

• Biological Roles: Oxygen transport, carbon dioxide transport, pH regulation.

• Binding Assays: Used to study hemoglobin-ligand interactions.

• Quaternary Structure: Hemoglobin consists of four subunits (two α and two β chains).

• Cooperative Binding and Allostery: Binding of oxygen to one subunit increases affinity in others (sigmoidal binding curve).

• Allosteric Effectors: Oxygen, carbon dioxide, pH (Bohr effect), 2,3-bisphosphoglycerate.

Mechanisms of Protein Mutation

Mutations in hemoglobin can lead to genetic diseases (e.g., sickle cell anemia).

• Variants and Inheritance: Different hemoglobin variants can be inherited and cause disease.

Structural Proteins

Structural proteins provide support and shape to cells and tissues.

• Actin Filaments: Involved in cell movement and structure.

• Microtubules: Composed of alpha and beta tubulin; important for cell division and transport.

• Intermediate Filaments: Includes keratin and collagen; provides mechanical strength.

Motor Proteins

Motor proteins convert chemical energy into mechanical work.

• Actin and Myosin: Responsible for muscle contraction.

• Kinesin: Moves along microtubules to transport cellular cargo.

Chapter 6: Enzymes and Catalysis

Role of Enzymes as Catalysts

Enzymes are biological catalysts that accelerate chemical reactions without being consumed.

• Enzyme Naming: Based on the reaction catalyzed (e.g., oxidase, kinase).

Chemical Catalytic Mechanisms

Enzymes use various mechanisms to lower activation energy and increase reaction rates.

• Acid-base catalysis

• Covalent catalysis

• Metal ion catalysis

Example: Chymotrypsin uses a catalytic triad for peptide bond hydrolysis.

Transition State Theory

Enzymes stabilize the transition state, reducing the activation energy required for reactions.

Proteases

Proteases are enzymes that cleave peptide bonds in proteins.

• Chymotrypsin: A serine protease with a well-characterized mechanism.

• COVID-19 Main Protease: Target for antiviral drug development.

Cofactors

Cofactors are non-protein molecules required for enzyme activity (e.g., metal ions, coenzymes).

Chapter 7: Enzyme Kinetics and Inhibition

Michaelis-Menten Kinetics

Michaelis-Menten kinetics describes the rate of enzymatic reactions as a function of substrate concentration.

• Key Parameters: $K_m$ (Michaelis constant), $V_{max}$ (maximum velocity), $k_{cat}$ (turnover number), catalytic efficiency ($k_{cat}/K_m$).

• Michaelis-Menten Equation:

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

• Plot Interpretation: Be able to read and interpret Michaelis-Menten and Lineweaver-Burk plots.

• Lineweaver-Burk Equation:

$\frac{1}{v} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$

• Parameter Estimation: Estimate $K_m$ and $V_{max}$ from plots.

Enzyme Inhibition

Enzyme inhibitors reduce or block enzyme activity.

• Reversible vs. Irreversible Inhibition: Reversible inhibitors bind non-covalently; irreversible inhibitors form covalent bonds.

• Types of Reversible Inhibitors: Competitive, non-competitive, uncompetitive, and mixed.

Regulation of Enzymes

Enzyme activity is regulated by various mechanisms to control metabolic pathways.

• Why Inhibit Enzymes? Regulation, therapeutic intervention, and metabolic control.

Additional info:

• Some content inferred from standard biochemistry curriculum (e.g., details on protein purification and enzyme mechanisms).