Notes on Proteins, Enzymes, pH, pI, and Separation Techniques (Bio152 Topic 2)

Proteins recap

  • Proteins are polymers built from amino acids. Each amino acid has three components: a carboxyl group, an amino group, and a variable side chain (R group).
  • Properties of a protein are determined by its R group (size, charge, polarity, hydrophobicity) as well as the overall amino acid composition and sequence.
  • Four levels of protein structure:
    • Primary structure: the sequence of amino acids in the polypeptide chain.
    • Secondary structure: local folding patterns such as alpha helices and beta sheets stabilized primarily by hydrogen bonds.
    • Tertiary structure: the overall 3D folding of a single polypeptide, stabilized by a variety of noncovalent interactions and, in some proteins, disulfide bonds.
    • Quaternary structure: the assembly of multiple polypeptide chains (subunits) into a functional protein complex.
  • The properties and function of a protein are determined by the types and numbers of amino acids and the amino acid sequence, as well as the resulting three-dimensional structure and the environment in which the protein operates.

Protein classifications and functions (Topic 2.3)

  • Based on roles, proteins are classified into:
    • Enzymes
    • Storage proteins
    • Signalling / Hormonal proteins
    • Motility proteins
    • Defensive proteins
    • Transport proteins
    • Receptor proteins
    • Structural proteins
  • Enzymatic proteins
    • Function: selective acceleration of chemical reactions
    • Example: digestive enzymes catalyse hydrolysis of bonds in food molecules
  • Defensive proteins
    • Function: protection against disease
    • Example: antibodies inactivate and help destroy viruses and bacteria
  • Storage proteins
    • Function: storage of amino acids
    • Examples: Casein (milk) is a major amino acid source for baby mammals; ovalbumin (egg white) provides amino acids for embryo development
  • Transport proteins
    • Function: transport of substances
    • Examples: Hemoglobin transports oxygen in vertebrate blood; other transport proteins move molecules across membranes
  • Hormonal (signalling) proteins
    • Function: coordination of an organism’s activities
    • Example: Insulin controls glucose uptake and helps regulate blood sugar
  • Receptor proteins
    • Function: cellular response to chemical stimuli
    • Example: membrane receptors detect signaling molecules released by other cells
  • Structural and motor proteins
    • Function: movement and support
    • Examples: Actin and myosin drive muscle contraction; motor proteins power ciliary/flagellar movement
    • Keratin provides structural support (hair, nails, skin appendages); Collagen and elastin form fibrous networks in connective tissue
  • Note: Illustrative examples include muscle tissue, connective tissue, and bio-molecular interactions (e.g., antibody–virus, hemoglobin–oxygen). Visuals in course materials depict these roles.

Protein structure levels (reiterated)

  • Primary structure determines all higher levels and ultimately the protein’s function.
  • Secondary structure arises from hydrogen bonding between backbone atoms, creating alpha helices and beta sheets.
  • Tertiary structure results from the folding of the entire polypeptide into a unique 3D shape, driven by hydrophobic interactions, hydrogen bonds, ionic interactions, and sometimes disulfide bonds.
  • Quaternary structure involves the assembly of multiple polypeptide chains into a functional complex.

pH effects on proteins (LLO 2.4)

  • At low pH (acid): Increase in H+ concentration leads to proteins acquiring a net positive charge (more protonated groups).
  • At neutral pH: Many groups are at or near their pKa values; proteins can be uncharged for some residues, contributing to the isoelectric point behavior.
  • At high pH (alkaline/basic): Decrease in H+ concentration leads to a net negative charge (deprotonation of groups).
  • Overall, pH affects proteins by changing the protonation state of charged residues, which in turn influences solubility, interactions, and stability.

Isoelectric point (pI) (LLO 2.4)

  • Definition: the pH at which a protein carries zero net charge.
  • At the pI, the positive and negative charges are balanced.
  • The pI is also dependent on the properties of the R groups.
  • Consequences:
    • Solubility: proteins are least soluble at their pI because they interact with each other rather than with water.
    • Proteins may precipitate near their pI.
  • Conceptual view: proteins with a net zero charge at pI may aggregate when conditions favor protein–protein interactions over protein–water interactions.

Separation of proteins (LLO 2.5)

  • Separation depends on intrinsic protein properties: net charge, size, and solubility (which relates to pI).
  • Common approaches in cell biology include:
    • Electrophoresis (charge-based separation in a gel under an electric field)
    • Gel filtration (size-exclusion chromatography)
    • pI-based precipitation (solubility differences at pI)
    • Ion-exchange chromatography (charge-based binding and elution)
    • Ammonium sulfate precipitation (solubility-based precipitation)

Charge-based separation: Electrophoresis

  • Principle: separation of a mixture of proteins in a gel matrix under applied voltage.
  • Electrodes: Cathode (-) and Anode (+); opposite charges move toward the opposite electrode.
  • Factors determining separation:
    • Net charge of each protein (depends on pH and amino acid composition)
    • Size of the protein (larger molecules experience more friction in the gel)
  • Migration behavior:
    • Proteins with net negative charge move toward the anode; proteins with net positive charge move toward the cathode.
    • Mobility is influenced by the magnitude of the net charge and the size of the molecule; smaller, highly charged proteins migrate faster.
  • Visualisation: gels stained with Coomassie Blue reveal discrete bands, each corresponding to a polypeptide in the mixture.

Chromatography (overview)

  • Separation of molecules suspended in a phase based on differences in properties such as charge, affinity, and polarity.
  • Key forms relevant to proteins:
    • Ion-exchange chromatography (separation by charge)
    • Affinity chromatography (separation by specific binding interactions)
    • Thin-layer chromatography (primarily used for lipids, but included as a related technique in the notes)

Ion-exchange chromatography (LLO 2.5 C)

  • Principle: beads in a column have fixed charges and trap proteins of opposite charge; proteins with the same charge as the bead flow through.
  • Elution: to release retained proteins, change the buffer by adjusting pH and salt concentration; higher salt disrupts ionic interactions and elutes bound proteins.
  • Example from lecture: beads are negatively charged; positively charged proteins bind and are retained; negatively charged proteins pass through.
  • Practical note: selection of buffer pH is critical to control binding and elution of target proteins.

Size: Gel filtration (size-exclusion) chromatography (LLO 2.5 C)

  • Principle: separation based on molecular size via passage through a porous bead matrix.
  • Behavior:
    • Small proteins can enter the pores of the beads and migrate more slowly; elute last.
    • Large proteins are excluded from the pores, travel around the beads, and elute first.
  • Outcome: fractions collected to purify proteins; enables separation of proteins with similar charge but different sizes.

Solubility: Isoelectric point precipitation (LLO 2.5)

  • Separation by solubility differences around the pI.
  • Process: proteins are loaded or separated on a gel or in solution where their migration stops at their isoelectric point.
  • Concept: at pI, reduced net charge minimizes electrostatic repulsion, promoting aggregation and precipitation in solution.

Solubility: Ammonium sulfate precipitation (LLO 2.5)

  • Principle: ammonium sulfate ((NH4)2SO4) salts out proteins by removing the hydration shell and promoting precipitation.
  • Outcome: different proteins precipitate at different ammonium sulfate concentrations, allowing selective enrichment of a protein of interest.
  • Practical notes:
    • Increasing ammonium sulfate concentration moves the solution toward saturation; both saturated and subsaturated conditions may be used.
    • Buffer pH adjustments can influence which proteins precipitate at a given salt concentration.

Protein denaturation (LLO 2.6)

  • Denaturation: loss of the native three-dimensional structure, leading to loss of biological function.
  • Cause: disruption of non-covalent bonds (and sometimes covalent disulfide bonds) due to external agents, leading to altered or unfolded proteins.
  • Relationship: structural integrity (2°, 3°, 4°) is essential for function; disruption of structure results in loss of function.

Factors causing denaturation

  • Organic solvents: disrupt hydrogen bonds that stabilize secondary, tertiary, and quaternary structures.
  • Detergents: disrupt hydrophobic interactions that stabilize tertiary and quaternary structures.
  • Extreme pH: disrupt hydrogen bonds and ionic/electrostatic interactions (affecting 2°, 3°, and 4° structures).
  • Heat: increases molecular motion, disrupting non-covalent interactions and destabilizing 2°, 3°, and 4° structures.

Textbook readings and references

  • Chapter 21: Molecular Biology Techniques for Cell Biology.
  • Chapter 3: The Macromolecules of the Cell.
  • Becker’s World of the Cell (Hardin, et al.) 9th global edition, various figures referenced for protein functions and separation techniques.

Image and figure references

  • Protein function overview (Figure 5.13 in Campbell Biology) and figures illustrating chromatography, electrophoresis, 2D gel electrophoresis, and protein folding stability.
  • Use of visuals supports understanding of how enzymes, antibodies, and structural proteins operate in cells.

Practical implications and real-world relevance

  • Protein purification workflows rely on the separation techniques detailed here (electrophoresis, ion-exchange, gel filtration, precipitation) to isolate target proteins for research or therapeutic use.
  • Understanding pH and pI is essential for designing buffers and conditions that maximize solubility and stability during purification.
  • Denaturation studies inform storage conditions, formulation of biologics, and interpretation of folding diseases or misfolding-related issues.

Connections to foundational principles

  • Structure–function relationship is central: form dictates how proteins interact with substrates, ligands, and other biomolecules.
  • Non-covalent interactions (hydrogen bonds, ionic interactions, hydrophobic effects) govern folding, stability, and interactions; disruption alters function.
  • The collection of separation techniques exemplifies how physical principles (charge, size, affinity, solubility) are exploited to manipulate biomolecules in the lab.

Ethical and practical considerations

  • Purification and analysis methods must preserve protein function when needed and minimize denaturation; improper handling can lead to loss of activity.
  • Reproducibility and proper documentation of buffer conditions, pH, and salt concentrations are essential for scientific rigor.
  • Use of solvents and salts requires safe laboratory practices and waste management to reduce environmental impact.

Key takeaways

  • Proteins have four hierarchical levels of structure that determine function and interactions.
  • Proteins are classified by function (enzymes, storage, signalling, motility, defence, transport, receptors, structural roles).
  • pH and pI dramatically affect protein charge, solubility, and interactions; pI is the pH where net charge is zero.
  • Separation techniques exploit charge, size, and solubility to purify proteins: electrophoresis, ion-exchange chromatography, gel filtration, pI-based precipitation, and ammonium sulfate precipitation.
  • Denaturation is the loss of structure and function caused by disruption of non-covalent bonds through solvents, detergents, pH changes, and heat; understanding these processes is critical for protein handling and applications.