Protein Purification

Introduction to Protein Isolation and Purification

Challenge: Isolating a single target protein from the highly crowded and complex cellular environment.
- Cells are densely packed with various biomolecules, making specific protein extraction difficult.

Quantifying Protein Concentration

Importance: Knowing the amount of protein in a cell or solution is the first step in purification.
Methods for Protein Quantitation:
- UV Light Absorption by Proteins:
  - Certain amino acids absorb UV light, allowing for protein quantification.
  - Aromatic Amino Acids: Phenylalanine (Phe, F), Tyrosine (Tyr, Y), and Tryptophan (Trp, W) absorb ultraviolet light.
    - Tryptophan (Trp, W): Exhibits strong absorption.
    - Tyrosine (Tyr, Y): Also shows significant absorption.
    - Phenylalanine (Phe, F): Absorbs less strongly.
  - Principle: More protein in a solution leads to higher UV absorbance.
  - Sunscreen Analogy: Sunscreen works by absorbing UV light, similar to how these amino acids function.
  - Wavelengths Range: Absorption typically occurs between $200$ nm and $320$ nm, with distinct peaks for each aromatic amino acid (e.g., Tryptophan around $280$ nm).
- Chemical Methods for Protein Quantitation: Utilize chemicals that complex with proteins and produce a measurable readout.
  - Bicinchoninic Acid (BCA) Assay:
    - Principle: Proteins reduce Cu $^{2+}$ to Cu $^{1+}$ in an alkaline solution (known as the biuret reaction).
    - Readout: The resulting Cu $^{1+}$ ions form a purple-colored complex with bicinchoninic acid, which can be measured spectrophotometrically.
    - Used for total protein quantitation.
  - Bradford Assay:
    - Principle: Involves Coomassie Brilliant Blue G-250 dye.
    - Readout: The dye binds to basic and aromatic amino acid residues in proteins, causing a shift in its absorption maximum from $465$ nm (red/brown) to $595$ nm (blue), which is then measured.

Typical Protein Purification Scheme Overview

Protein purification typically employs a series of separation methods to isolate the target protein from a complex mixture.
Example Purification Scheme (for an enzyme from a cell extract):
1. Crude extract: Initial preparation from cells.
  - Volume: $3,800$ mL.
  - Total Protein: $22,800$ mg.
  - Total Activity: $2,460$ units.
  - Specific Activity: $0.108$ units/mg.
  - Percent Recovery: $100\%$
2. Salt precipitate: Separates proteins based on solubility changes due to ionic strength.
  - Volume: $165$ mL.
  - Total Protein: $2,800$ mg.
  - Total Activity: $1,190$ units.
  - Specific Activity: $0.425$ units/mg (increase in purity).
  - Percent Recovery: $48\%$
3. Ion exchange chromatography: Separates proteins based on charge.
  - Volume: $65$ mL.
  - Total Protein: $100$ mg.
  - Total Activity: $720$ units.
  - Specific Activity: $7.2$ units/mg (significant purity increase).
  - Percent Recovery: $29\%$
4. Molecular sieve chromatography (Size exclusion chromatography): Separates proteins based on size.
  - Volume: $40$ mL.
  - Total Protein: $14.5$ mg.
  - Total Activity: $555$ units.
  - Specific Activity: $38.3$ units/mg (further purity increase).
  - Percent Recovery: $23\%$
5. Immunoaffinity chromatography: Highly specific purification based on binding to an antibody.
  - Volume: $6$ mL.
  - Total Protein: $1.8$ mg.
  - Total Activity: $275$ units.
  - Specific Activity: $152$ units/mg (highest purity).
  - Percent Recovery: $11\%$
Key Metrics:
- Total Activity: Relative enzymatic activity, arbitrarily defined in units (e.g., $1$ nanomole/mL of substrate converted per minute for xanthine dehydrogenase).
- Specific Activity: Total activity of the fraction divided by the total protein in the fraction ( $Units/mg$ ). This value indicates the increase in purity; a higher specific activity means the sample is enriched for the desired enzyme.
- Percent Recovery: A measure of the yield of the desired enzyme.

Salt Precipitation: Protein Solubility and Ionic Strength

Ionic Strength Influence: Profoundly affects protein solubility.
- Salting-in: At low ionic strengths, most globular proteins become more soluble as ionic strength increases.
  - Mechanism: Abundant salt ions diminish electrostatic attractions between protein molecules, which would otherwise lead to precipitation.
- Salting-out: As salt concentration reaches high levels (greater than $1$ M), the effect may reverse, and the protein becomes less soluble (salted out).
  - Mechanism: Numerous salt ions begin to compete with the protein for waters of solvation. As salt ions win out, proteins become insoluble.

Protein Solubility and Isoelectric Point (pI)

Isoelectric Point (pI): The pH value at which the sum of a protein's positive and negative electrical charges is zero.
Solubility at pI: Proteins tend to be least soluble at their isoelectric point.
- At the pI, there is minimal net charge, leading to reduced electrostatic repulsion between protein molecules and increased aggregation.
Application in Purification: The pI of a protein can be utilized as a basis for purification, especially in methods like isoelectric focusing or differential precipitation.

Ion Exchange Chromatography

Principle: Separates proteins based on their net electrical charge.
Determining Protein Charge: The pI and molecular weight of a protein can be determined from its primary amino acid sequence using tools like ProtParam (e.g., http://web.expasy.org/protparam/).
- Example: A protein with a theoretical pI of $6.03$ and molecular weight of $59679.23$ Da has $509$ amino acids.
Types of Ion Exchange Resins:
- Anion Exchange Chromatography:
  - Uses a positively charged resin (e.g., Quaternary amine, HiTrap $^{ ext{TM}}$ Q HP).
  - Binds negatively charged proteins.
- Cation Exchange Chromatography:
  - Uses a negatively charged resin (e.g., Sulfopropyl, HiTrap $^{ ext{TM}}$ SP HP).
  - Binds positively charged proteins.
Separation Process:
1. A sample containing several proteins is applied to the column.
2. Proteins with a charge opposite to the resin bind.
3. Proteins with the same charge as the resin (or no net charge at the buffer pH) flow through.
4. Bound proteins are typically eluted by gradually increasing the salt concentration (e.g., NaCl) in the buffer. The increasing ionic strength competes with the protein for binding sites on the resin, releasing the protein.
5. Proteins elute at different salt concentrations based on their charge properties, leading to separation into discrete bands.

Molecular Sieve Chromatography (Size Exclusion Chromatography)

Principle: Separates proteins based on their size (molecular weight).
Mechanism:
- The chromatography resin consists of porous beads, similar to a