Biology 2B03 Cell Biology: Protein Purification

Protein Purification: Module 3, Lecture 1

Introduction to Protein Purification

  • Module 3 focuses on biomembranes and cell architecture.

  • Lecture 1 specifically addresses how proteins are collected from cells for study.

  • The process involves rupturing cellular biomembranes to extract proteins.

  • Key objectives include:

    • Determining when and why a researcher needs to isolate a protein.

    • Understanding the underlying principles of protein isolation.

    • Explaining the chemical basis for common protein purification techniques.

Reasons for Protein Extraction and Purification

Researchers isolate and purify proteins for various purposes:

  • Structural Studies: To study the folded three-dimensional structure using techniques like X-ray crystallography.

  • Functional Identification: To determine a protein's function, including identifying interacting substrates and understanding the effects of substrate binding.

  • Sequence Analysis: To identify a protein's amino acid sequence, which can then predict the sequence of the gene coding for the protein.

  • Antibody Development: To purify a protein for generating specific antibodies.

    • These antibodies can be used in immunofluorescence microscopy to detect protein location within a cell or tissue.

    • They can also be used in Western blot analysis to visualize relative protein concentrations from cellular extracts.

Overall Approach to Protein Isolation

Given that a cell can contain tens of thousands of distinct proteins, the main challenge is to separate the protein of interest from all others.

There are seven systematic steps to isolating a protein:

  1. Identify a unique assay/experiment for detection: Establish a method to specifically detect the protein in a cell extract.

  2. Choose a source: Select the appropriate cell type or tissue where the protein is abundant and easily accessible.

  3. Extract proteins from the cell: Break open or lyse the cells to release intracellular contents.

  4. Solubilize and stabilize your protein: Ensure the protein remains soluble and in its native, functional conformation.

  5. Fractionate or separate your protein: Employ techniques to separate the protein from other cellular components.

  6. Assess/Evaluate purity: Use the unique assay to determine the purity of the isolated protein.

Protein Assay: Detecting Your Protein

A protein assay is crucial for monitoring the presence and concentration of the protein of interest throughout the purification process. The assay must be unique to the target protein.

Examples of protein assays:

  • Enzymatic Activity: If the protein is an enzyme, measure its activity by observing the release of product or the consumption of a substrate.

  • Antibody Detection: If a unique antibody for the protein exists, use it to monitor the protein's presence and concentration.

  • Specific Binding Activity: If the protein binds to a unique substrate (e.g., RNA or actin), monitor this biological activity.

Choosing a Cell Source

Careful consideration of the cell source is vital for successful protein purification.

Key considerations:

  • Abundance and Easy Obtainability: The protein should be present in large amounts and readily accessible.

    • Example: To study a muscle protein, collect it from muscle tissue.

    • Example: For hemoglobin, use red blood cells due to their high concentration of hemoglobin.

  • Low in Co-purifying Proteins: Select cell types with minimal proteins that might possess similar characteristics and co-purify with the target protein.

  • Low in Proteases: Choose sources with low levels of proteases that could degrade the protein of interest during purification.

    • Mitigation: Protease inhibitors can be added during purification steps.

  • Alternative Expression Systems: To maximize yield, express the protein in an alternate cell type (e.g., expressing a mouse protein in bacterial cells).

  • Requirement for Cell Lysis: Regardless of the source, cells must be lysed or broken open to release proteins.

    • Methods include chemical lysis, physical grinding, or ultrasonic sonicators to disrupt biomembranes.

Protein Solubilization

This is a critical step after cell lysis, aimed at getting the protein into solution.

  • Predicting Solubility: A protein's solubility depends on its inherent properties.

    • Soluble Proteins: Cytosolic and secreted proteins are generally soluble in aqueous extracts.

      • These proteins typically have hydrophilic surfaces that allow them to interact favorably with water.

    • Insoluble Proteins: Transmembrane or membrane-anchored proteins are amphipathic (possessing both hydrophobic and hydrophilic properties due to membrane interactions).

      • They are difficult to isolate as they are not soluble in simple aqueous extracts.

  • Increasing Solubility: Various conditions can be altered to increase protein solubility:

    • pH of the solution: Modifying pH can affect the charge of amino acid residues, influencing interactions.

    • Salt concentration: Adjusting ionic strength can screen charges and influence protein-solvent interactions.

    • Detergents: Essential for solubilizing insoluble proteins, particularly membrane-associated proteins. Detergents help stabilize molecular interactions and integrate hydrophobic regions into micelles.

Protein Stabilization

Maintaining the protein's native structure (optimally folded and functional form) and preventing degradation is crucial at all stages.

Parameters to consider for protein stabilization:

  • Maintain Non-covalent Interactions: Preserve the native folded conformation by controlling:

    • pH of the solution: Limits changes in ionization states.

    • Salt concentration: Ensures appropriate ionic environment.

    • Presence of co-factors: Necessary for stability and function of some proteins.

  • Temperature: Warmer temperatures can cause protein denaturation, so purification is often carried out at low temperatures (e.g., 4^{\circ}C).

  • Protease Inhibitors: Add to cell extracts to prevent enzymatic degradation by proteases.

  • Protein Concentration: Even soluble proteins can aggregate if present at high concentrations; maintaining appropriate concentration prevents this.

Protein Fractionation (Separation)

Fractionation is the process of separating proteins into different groups or fractions from a complex mixture.

  • General Principle: Each technique exploits a unique chemical or physical property of the target protein to distinguish it from others.

  • Need for Multiple Techniques: A single fractionation method is usually insufficient. Multiple techniques in series are required to achieve high purity, leveraging a unique set of characteristics.

  • Properties Used for Separation: Proteins vary in, and techniques take advantage of:

    • Charge

    • Size

    • Polarity

    • Solubility

    • Shape

Overview of Fractionation Techniques

Specific techniques are designed to exploit different protein properties for separation:

  • Separation by Overall Charge: Ion exchange chromatography, gel electrophoresis.

  • Separation by Size: Gel electrophoresis, gel filtration chromatography, ultracentrifugation.

  • Separation by Protein Polarity: Adsorption chromatography, hydrophobic interaction chromatography.

  • Separation by Specific Binding: Affinity chromatography (protein-protein or protein-substrate binding).

Details on Differential Centrifugation, Ion Exchange Chromatography, Gel Filtration Chromatography, and Affinity Chromatography are covered below.

Differential Centrifugation

This is a preliminary, step used to separate cellular components based on their size and density, allowing for the isolation of specific organelles or proteins. The process involves spinning a sample at high speeds, which causes denser components to sediment at the bottom of the tube, while less dense components remain in the supernatant. This method is crucial for preparing samples before applying more refined techniques like chromatography.