Non-Enzymatic Protein Function and Protein Analysis

Non-Enzymatic Protein Functions

• Structural Proteins: These proteins provide the internal and external framework of the cell and tissues. They constitute the cytoskeleton, anchor proteins in place, and form the extracellular matrix (ECM).

  • Collagen: A triple helix fiber that is the primary component of the extracellular matrix in connective tissues.

  • Elastin: A major component of connective tissue that allows tissues to stretch and recoil, maintaining their shape.

  • Keratins: Intermediate filament proteins primarily found in epithelial cells, making up structures like hair and skin.

  • Actin: Forms microfilaments and is a key component of thin filaments in muscle tissue.

  • Tubulin: The building block of microtubules, which are essential for cell structure, intracellular transport, and chromosome separation.

• Motor Proteins: Responsible for movement, these proteins undergo conformational changes powered by ATP hydrolysis. They function as ATPases to convert chemical energy into mechanical work.

  • Myosin: The primary motor protein that interacts with actin to enable muscle contraction.

  • Kinesins: Assist in aligning chromosomes during metaphase and depolymerizing microtubules during anaphase. They are also involved in transporting cargo along microtubules.

  • Dyneins: Involved in the movement of cilia and flagella, allowing for cellular locomotion or the movement of substances across the cell surface.

• Binding Proteins: These proteins stabilize or transport specific molecules (hormones, ions, or other small molecules) without chemically modifying them. They function by holding the target in place or regulating its concentration.

• Cell Adhesion Molecules (CAMs): Surface proteins that allow cells to bind to other cells or the surrounding extracellular environment. They are categorized into three main types:

  • Cadherins: Calcium-dependent glycoproteins that mediate cell-to-cell adhesion between similar cell types, helping to keep tissues organized.

  • Integrins: Membrane-spanning proteins that connect the internal cytoskeleton to the extracellular matrix. They play critical roles in anchoring cells and sensing environmental signals to regulate growth or movement.

  • Selectins: Proteins that bind to carbohydrate molecules on the surfaces of other cells. These form weaker, temporary bonds, which are vital for the immune response (e.g., allowing white blood cells to roll along blood vessel walls to reach sites of inflammation).

Immunoglobulins (Antibodies)

• Definition: Antibodies, or immunoglobulins (Ig), are Y-shaped proteins produced by B cells to identify and neutralize foreign antigens (viruses, bacteria, or toxins).

• Structure: Each antibody consists of four protein chains:

  • Two identical heavy chains.

  • Two identical light chains.

  • These chains are stabilized by disulfide bonds and various non-covalent interactions.

• Functional Regions:

  • Variable Regions: Located at the tips of the Y-shape (the antigen-binding sites). These regions differ among antibodies to allow for specificity toward unique antigens.

  • Constant Region: This part of the antibody remains relatively consistent across a specific class. It mediates signaling to other parts of the immune system, such as immune cells or complement proteins.

Bio-Signaling Mechanisms

• Process Overview: Bio-signaling is the method by which cells interpret and respond to environmental stimuli to coordinate physiological functions.

• Ion Channels: Membrane proteins that facilitate the movement of charged ions across the membrane via facilitated diffusion (down a concentration gradient).

  • Ungated Channels: Open at all times, allowing free movement of specific ions.

  • Voltage-Gated Channels: Open or close in response to changes in the membrane potential; common in neurons and muscle cells.

  • Ligand-Gated Channels: Open or close only when a specific signaling molecule (ligand), such as a hormone or neurotransmitter, binds to the receptor.

• Enzyme-Linked Receptors: These receptors consist of three domains and initiate intracellular cascades upon ligand binding:

  1. Membrane-Spanning Domain: Anchors the receptor.

  2. Ligand-Binding Domain: Extracellular portion where the signal binds.

  3. Catalytic Domain: Becomes activated to perform an enzymatic function inside the cell.

• G-Protein Coupled Receptors (GPCRs): A large family of proteins involved in signal transduction via G-proteins.

  • G-Protein Types:

    • Gs: Stimulates adenylate cyclase, increasing intracellular levels of cyclic AMP (cAMP).

    • Gi: Inhibits adenylate cyclase, decreasing cAMP levels.

    • Gq: Activates phospholipase C, which cleaves PIP2 into DAG and IP3. IP3 subsequently opens calcium channels in the endoplasmic reticulum.

  • Subunit Mechanism: G-proteins have $\alpha$, $\beta$, and $\gamma$ subunits. In the inactive state, the $\alpha$ subunit binds GDP. Upon receptor activation, GTP replaces GDP, causing the $\alpha$ subunit to dissociate and affect target enzymes (like adenylate cyclase or phospholipase C).

Protein Isolation Techniques: Electrophoresis

• General Principle: Migration of charged proteins in an electric field through a gel matrix. The migration speed ($V$) is described by the equation:     $V = \frac{E \times Z}{F}$     where $E$ is electric field strength, $Z$ is the net charge, and $F$ is the frictional coefficient (related to size and shape).

• Polyacrylamide Gel: The standard matrix for electrophoresis, acting as a molecular sieve.

• Specific Types:

  • Native PAGE: Maintains the protein in its natural, folded state, preserving its charge and shape. This is used to analyze protein function or natural interactions.

  • SDS-PAGE: Proteins are denatured and coated with sodium dodecyl sulfate (SDS), a detergent that gives all proteins a uniform negative charge. This masks the native charge and shape, separating proteins strictly based on size (molecular mass).

  • Isoelectric Focusing: Proteins are separated based on their isoelectric point ($pI$)—the pH where the net charge is zero. In a gel with a pH gradient, a protein moves until it reaches the region where $pH = pI$ and then stops. Note that for migration to occur in other methods, the pH must differ from the $pI$.

Protein Isolation Techniques: Chromatography

• General Principle: Separation based on partitioning between two phases: a stationary phase (usually a polar solid) and a mobile phase (a liquid or gas). Molecules with high affinity for the stationary phase move slowly, while those with affinity for the mobile phase move quickly.

• Thin Layer (TLC) and Paper Chromatography:

  • Stationary phases are polar (silica gel, alumina, or cellulose paper).

  • Mobile phase is non-polar.

  • Retardation Factor ($R_f$): Used to identify substances.     $R_f = \frac{\text{Distance moved by the spot}}{\text{Distance moved by the solvent front}}$

• Column Chromatography:

  • Ion-Exchange: Beads are charged and bind compounds of the opposite charge (e.g., negative beads bind positive proteins).

  • Size-Exclusion: Beads have small pores; small molecules get trapped in pores and move slowly, while large molecules bypass pores and move faster.

  • Affinity: Beads are coated with specific receptors, antibodies, or enzymes to trap a target protein. The target is later recovered using a specific wash.

• Advanced Methods:

  • Gas Chromatography (GC): Separates vaporized compounds using an inert gas (helium or nitrogen) carrier. Separation is measured by retention time.

  • High-Performance Liquid Chromatography (HPLC): A precise, computer-controlled version of column chromatography used for small samples and highly accurate analytical results.

Protein Analysis and Concentration

• Structure Determination:

  • X-Ray Crystallography: High-resolution image generated by analyzeing X-ray scattering through a crystallized protein. This is the most common method but requires difficult crystallization.

  • NMR Spectroscopy: Used for smaller proteins in solution, utilizing the magnetic properties of nuclei.

• Amino Acid Sequence:

  • Hydrolysis: Determines total amino acid composition but not sequence.

  • Edman Degradation: Sequentially removes amino acids from the N-terminus to determine the primary sequence. Effective for peptides of $50 \text{ to } 70$ amino acids.

• Enzyme Activity: Monitored by measuring the rate of a specific reaction, often indicated by a color change in a spectrophotometric assay.

• Concentration Determination:

  • UV Spectroscopy: Measures absorbance of aromatic amino acids (sensitive to contaminants).

  • Colorimetric Assays: BCA, Lowry, and the Bradford protein assay.

  • Bradford Protein Assay: Uses Coomassie Brilliant Blue dye. The dye is brownish-green when protonated but turns blue when it binds to amino acids and loses protons. The intensity of the blue color (measured against a standard curve) indicates protein concentration. This assay is sensitive to detergents and high buffer concentrations.