Module 2 Lecture 4 - Protein Separation and Enzyme Biochemistry Study Guide

Rationales for Protein Separation

• Laboratory vs. Bulk Mixtures: In laboratory scenarios, proteins often exist as a bulk mixture or crude homogenate (from tissues, bacteria, plants, etc.). Separation is required to isolate individual proteins for downstream processes.

• Identification of Specific Proteins: Separation allows researchers to identify if a targeted protein of interest is present in a sample.     * Case Study (UME Research): The lecturer describes research at UME involving the collection of rat spleen tissues. These tissues contain millions of proteins involved in various immune functions. Separation is used to extract specific immune receptor proteins related to diseases for further application.

• Differentiation Factors: Proteins are distinguished from one another based on specific physical and chemical properties:     * Size (Molecular weight).     * Net Charge (Derived from amino acid side chains).     * Three-dimensional structure.

• Functional Study: To study enzymatic activity, researchers must extract a particular enzyme from bulk structural proteins. For example, bacterial cells in culture may be spun down into a pellet to extract enzymes secreted by the bacteria.

• Quantification: Separation is necessary to accurately measure the amount/concentration of a specific protein or enzyme in a solution.

• Structural Analysis: Pure samples are required for advanced structural determination techniques such as:     * X-ray Crystallography.     * Nuclear Magnetic Resonance (NMR) techniques.

• Post-Translational Modifications: Separation helps detect modifications made to the polypeptide chain after translation, such as phosphorylation (the addition of a phosphate group), which is crucial for protein function.

Fundamental Properties Used in Separation

• Solubility: Differences in how proteins dissolve in various solvents.

• Size: Differences in the total number of amino acids and overall mass ($Daltons$).

• Charge: Based on the presence of polar, acidic, and basic amino acids.

• Isoelectric Point ($pI$): The specific $pH$ at which a protein has no net electrical charge.

• Binding Characteristic: Specificity for certain ligands or molecules.

• Thermal Stability: The ability of a protein to resist denaturation at high temperatures.

• Fractionation: A process where an extract is separated into several fractions by altering solubility.

Chromatography Techniques

Chromatography involves a Mobile Phase (the liquid carrying the protein mixture) and a Stationary Phase (the matrix, resin, or ligands that interact with the proteins).

Column Chromatography

• Separation is based on the differential interaction between the protein molecules and the stationary phase.

• Samples are added to a column filled with a matrix.

• Different molecules elute (exit the column) at different rates and are collected in separate tubes.

Ion Exchange Chromatography

• Principle: Separates proteins based on their net electrical charge.

• The Resin: Charged beads (cationic or anionic) are used as the stationary phase.

• Binding Process:     * If a resin is positively charged (Anionic Resin), it will bind negatively charged proteins.     * If a resin is negatively charged (Cationic Resin), it will bind positively charged proteins.

• Elution Mechanism:     * Unbound or loosely bound proteins are washed away first.     * To remove the bound proteins, the ionic interaction is destabilized by increasing the salt ($NaCl$) concentration or altering the $pH$ of the buffer.     * Mechanism of Salt Elution: High salt concentration alters the charge environment or ionic strength, causing the R-groups of the protein to lose their affinity for the resin, thereby releasing the protein.

Gel Filtration (Size Exclusion Chromatography)

• Principle: Separates proteins based on molecular size using porous beads.

• Pore Mechanics:     * The column is filled with beads containing pores of a defined size.     * Small molecules: Travel through the small pores within the beads, taking a long, circuitous path. Consequently, they elute last.     * Large molecules: Are too big to enter the pores and travel only through the spaces between the beads. Consequently, they elute first.

• Example Scenario: A mixture containing Protein A ($60,000\,Daltons$), Protein B ($20,000\,Daltons$), and Protein C ($45,000\,Daltons$) is loaded. The elution order will be Protein A (Largest), then Protein C (Medium), then Protein B (Smallest).

Affinity Chromatography

• Principle: The most selective method, based on specific, reversible binding between a protein and a ligand attached to the resin.

• Binding: The protein of interest binds to the ligand in the column while unwanted proteins pass through.

• Common Examples:     * Enzymes/Substrates: Alcohol dehydrogenase has a high affinity for the ligand Zinc ($Zn$).     * Antigen/Antibody: Using specific antigens to isolate antibodies from serum.     * Histidine Tag (His-tag): Recombinant proteins are tagged with multiple Histidines (negatively charged). These have a high affinity for Nickel ($Ni$) ligands in the resin.

Electrophoresis and SDS PAGE

Polyacrylamide Gel Electrophoresis (PAGE)

• Electrophoretic Mobility: An electric field pulls proteins through a gel matrix toward the Anode (positively charged electrode) because proteins are typically negatively charged.

• Gel Properties: The gel acts as a sieve. Smaller proteins move quickly through the pores to the bottom, while larger proteins move slowly and remain near the top.

• Orientation: Unlike horizontal DNA gels, protein gels (PAGE) are typically run vertically.

SDS PAGE

• SDS (Sodium Dodecyl Sulfate): A detergent used to denature proteins.

• The Globular Problem: Natural proteins are often globular and complex (tertiary/quaternary structures), making it difficult for them to pass through the gel pores.

• Denaturation: SDS breaks ionic interactions and unfolds the protein into a linearized polypeptide chain. This ensures separation is strictly based on molecular weight (size) rather than shape.

Detection and Quantification

• Spectrophotometry: Used to confirm the presence of proteins in elutions by measuring absorbance at $280\,nm$.

• Absorbance Factors: Aromatic amino acids provide high absorbance:     * Tryptophan: Highest absorbance.     * Tyrosine: Intermediate absorbance.     * Phenylalanine: Lowest absorbance.

Introduction to Enzymes

• Definition: Biomolecules, usually proteins, that catalyze specific chemical reactions, increasing the reaction rate.

• Essential Life Conditions: Organisms must be able to replicate and catalyze chemical reactions (growth).

• Etymology: Derived from the Greek word "catalim," meaning to dissolve, loosen, or unite.

• General Properties:     * Most are proteins (globular or membrane-bound).     * Highly unstable and easily denatured by breaking tertiary/secondary structures.     * Flexible molecules that can change shape.     * Not used up (consumed) during the reaction; they are recycled.     * Highly specific for their substrates.

Enzyme Energetics and Mechanisms

• Activation Energy ($E_a$): The minimum energy required for reactants to reach the transition state to undergo a chemical reaction.

• Transition State: A high-energy, unstable state where bond breaking/formation occurs.

• Enzyme Role: Enzymes lower the activation energy, enabling the reaction to proceed faster.

• Thermodynamics:     * Enzymes do not affect the position of the equilibrium.     * Enzymes do not affect the overall free energy change ($\Delta G$) of the reaction.     * Exogenic Reactions: Reactions where the product has less energy than the substrate (energy is released).

• The Shortcut Metaphor: Enzymes are like a shortcut through a mountain. They do not change the starting point (substrate) or the ending point (product), but they make the journey easier and faster.

Enzyme Nomenclature and Classification

Enzymes typically end with the suffix "-ase" and are named based on their substrate or the reaction they catalyze.

Systematic Examples

• DNA Polymerase: Synthesizes DNA polymers using ribonucleotides as substrate.

• DNA Helicase: Breaks the helical structure of DNA.

• DNA Ligase: Joins (unites) two DNA molecules.

• Urease: Acts only on Urea (Absolute specificity).

• Hexokinase: Phosphorylates glucose; an example of the Induced Fit Model.

Six Main Classes

1. Oxidoreductases: Catalyze oxidation-reduction (redox) reactions involving electron transfer.

2. Transferases: Transfer functional groups (e.g., phosphate) from one molecule to another.

3. Hydrolases: Catalyze the cleavage of bonds with the addition of water ($H_2O$).

4. Lyases: Break various chemical bonds by means other than hydrolysis or oxidation.

5. Isomerases: Catalyze the structural rearrangement of isomers (changing atomic orientation).

6. Ligases: Catalyze the joining of two large molecules by forming new chemical bonds.

Binding and Specificity

The Active Site

• A specific cleft or pocket in the enzyme where catalysis occurs. Typically, an enzyme has 1 to 3 active sites.

• Hydrophobic Pocket: A non-polar binding site that attracts non-polar parts of the substrate via Van der Waals forces or hydrophobic interactions to strengthen binding.

Models of Binding

• Lock and Key Model: High complementarity between substrate and active site.

• Induced Fit Model: The enzyme changes its conformation (shape) upon substrate binding to better accommodate the transition state. (e.g., Hexokinase closing its "cleft" around glucose to allow ATP to donate a phosphate).

Types of Specificity

• Absolute Specificity: Acts only on one substrate (e.g., Urease).

• Group Specificity: Acts on molecules with specific functional groups (e.g., Alcohol Dehydrogenase acting on ethanol, methanol, etc.).

• Linkage Specificity: Acts on specific chemical bonds regardless of the rest of the molecule.     * Trypsin: Cleaves peptide bonds after Arginine ($Arg$) and Lysine ($Lys$).     * Chymotrypsin: Cleaves after aromatic amino acids (Phenylalanine, Tryptophan, Tyrosine).

• Stereochemical Specificity: Acts only on a specific optical isomer (e.g., L-amino acids vs. D-amino acids).

Questions & Discussion

• Q: Which molecular interactions contribute to substrate specificity?     * A: Hydrogen bonds, ionic interactions, hydrophobic effect, and Van der Waals forces. These occur between the amino acids of the active site and the substrate.

• Q: Why is substrate specificity important for cellular metabolism?     * A: It prevents unintended side reactions and allows for the precise regulation of metabolic pathways. In a multi-enzyme pathway like glycolysis, if an enzyme worked at the wrong stage, it would disrupt the entire process.

• Q: What is the primary determinant of substrate specificity?     * A: The complementary shape or three-dimensional structure of the active site.

• Q: How do enzymes affect chemical equilibrium?     * A: They do not change the position of the equilibrium; they only accelerate the rate at which equilibrium is reached.

• Q: At equilibrium, what is the relationship between forward and reverse reaction rates?     * A: The forward and reverse reaction rates are equal.

• Q: How do enzymes affect activation energy?     * A: They decrease the activation energy, making the reaction proceed faster.