Protein Structure and Function Flashcards

Overview of Protein Roles and Functions

• Main Building Blocks: Proteins represent the principal structural and functional building blocks of the cell.
• Diverse Biological Roles:
  • Enzymes: Catalyze specific biochemical transformations.
  • Structural Proteins: Provide physical framework and mechanical support (e.g., making up the cytoskeleton to enable movement within cells and movement of cells through their environment).
  • Transport Proteins: Carry small molecules or ions across membranes or throughout tissues (e.g., Bacterial transport protein HPr facilitating sugar uptake into cells).
  • Motor Proteins: Generate physical forces responsible for muscle contraction, intracellular organelle transport, and chromosome segregation during cell division.
  • Storage Proteins: Store amino acids or ions for cellular metabolism.
  • Signalling Proteins: Carry extracellular signals from cell to cell.
  • Receptor Proteins: Detect signals and transmit them to the cell's response machinery.
  • Transcription Regulators: Bind DNA to switch specific genes on or off.

Fundamental Chemical Structure of Polypeptides

• Polypeptide Backbone: Formed by joining amino acids together in a linear chain via covalent peptide bonds. The repeating core sequence along the backbone is $-N-C-C-$ (nitrogen-alpha carbon-carbonyl carbon).
• Structural Termini:
  • N-terminus (Amino terminus): The end carrying a free amino group ($-NH_3^+$ or $-NH_2$).
  • C-terminus (Carboxyl terminus): The end carrying a free carboxyl group ($-COO^-$ or $-COOH$).
• $ \alpha$-Carbon: The central carbon atom situated between the amino group and carboxyl group of each amino acid residue.
• Side Chain ($R$ group / Variable group): Attached to the $\alpha$-carbon; confers unique identity, structural properties, and chemical reactivity to each amino acid.

• Classification of Amino Acid Side Chains:
  • Charge Characteristics: Charged negative (acidic), charged positive (basic), or neutral.
  • Polarity & Water Interaction:
  • Polar (Hydrophilic): Interact favorably with water molecules. Even if formal full charges are absent, polar functional groups (e.g., oxygen-containing hydroxyl groups) impart hydrophilicity.
  • Nonpolar (Hydrophobic): Uncharged and nonpolar; cannot form favorable interactions with water.
  • Representative Examples:
  • Histidine (His): Basic amino acid, positively charged ($+$-charge), polar, and hydrophilic.
  • Aspartic Acid (Asp): Acidic amino acid, negatively charged ($-$-charge), polar, and hydrophilic.
  • Leucine (Leu): Nonpolar, uncharged, hydrophobic side chain.
  • Tyrosine (Tyr): Polar amino acid containing an $-OH$ group, hydrophilic.

Forces Driving Protein Folding and Conformation

• Chain Flexibility: Polypeptide backbones are inherently flexible due to free rotation around the single covalent bonds connecting adjacent atoms.
• Conformational Constraints: The actual 3D folding pattern is constrained by numerous weak noncovalent interactions occurring within the molecule:
  • Electrostatic Attractions (Ionic Bonds): Formed between fully charged acidic ($-$) and basic ($+$) side chains (e.g., interaction between Aspartate/Glutamate and Lysine/Arginine).
  • Hydrogen Bonds: Formed between polar atoms. Three main hydrogen-bonding arrangements occur in folded proteins:
  • Backbone-to-backbone: Hydrogen bond between atoms of two peptide bonds.
  • Backbone-to-side-chain: Hydrogen bond between an atom of a peptide bond and an amino acid side chain.
  • Side-chain-to-side-chain: Hydrogen bond between atoms of two amino acid side chains.
  • van der Waals Attractions: Weak, short-range attractive forces that develop between nonpolar atoms in close contact (e.g., methyl groups of alanine, leucine, or valine).

• Hydrophobic Forces in Aqueous Environments:
  • Nonpolar (hydrophobic) side chains are forced together toward the interior/core region of the protein to minimize contact with surrounding water molecules.
  • Polar (hydrophilic) side chains position themselves toward the outer surface of the folded protein where they can form hydrogen bonds with water or ionic interactions inside the cell.
  • Polar amino acids can only exist in the hydrophobic interior if they are paired in hydrogen bonds with other polar amino acids or backbone atoms.

Protein Stability, Denaturation, and Renaturation

• Conformation: The final, stable 3D folded shape adopted by a polypeptide chain.

• Thermodynamic Principle: Protein folding is energetically favorable because it minimizes free energy ($\Delta G < 0$).

• Denaturation:

  • The process in which a protein loses its functional 3D conformation due to the disruption of noncovalent interactions.
  • Causes: Exposure to high concentrations of denaturants (e.g., urea), extreme changes in pH, elevated heat, or altered salinity/salt concentrations.
  • Denaturing agents like urea disrupt hydrogen bonds and ionic interactions, overriding the weak forces that hold the native structure together.

• Renaturation:

  • The spontaneous refolding of a denatured polypeptide back into its original native conformation once the denaturing agent or unfavorable environment is removed.
  • Demonstrates that all the information required to direct proper 3D protein folding is contained entirely within its primary amino acid sequence.

Chaperone-Assisted Protein Folding

• Role of Chaperone Proteins:
  • Special class of proteins that assist newly synthesized or partially folded polypeptides in reaching their most energetically favorable conformation efficiently.
  • Accelerate folding processes and prevent misfolding pathways.
• Mechanisms of Action:
  • Direct Binding: Chaperones bind directly to exposed hydrophobic patches on newly synthesized, partially folded polypeptide chains, preventing improper aggregation and guiding proper folding.
  • Isolation Chambers (Chaperonins):
  • Act as physical barrel-shaped chambers equipped with a removable cap.
  • A single unfolded polypeptide chain is sequestered inside the neutral chamber cavity, completely isolated from other cellular components to prevent aggregation.
  • The polypeptide folds correctly inside the protected space, and the cap dissociates to release the folded protein.

Structural Representation Models of Proteins

• Protein Sizes: Range from 30 to 10,000 amino acids (average protein length is 50–2,000 amino acids).
• Structural Visualization Models:
  1. Backbone Model: Traces the path of the polypeptide backbone; ideal for comparing overall structures of related proteins and highlighting global structural differences.
  2. Ribbon Model: Illustrates key secondary structural elements such as $\alpha$-helices and $\beta$-sheets, emphasizing basic folding motifs.
  3. Wire Model: Includes all amino acid side chains along with the backbone; useful for locating specific active-site amino acids and interactions.
  4. Space-filling Model: Displays a contour surface representation showing the total volume occupied by the protein in the cell, revealing surface topography, active-site cavities, and pockets.

Levels of Structural Organization

• Primary Structure: The unique linear sequence of amino acids linked together by covalent peptide bonds.
• Secondary Structure: Regular, repeating local folding motifs formed by hydrogen bonding along the polypeptide backbone (principally $\alpha$-helices and $\beta$-sheets).
• Tertiary Structure: The complete 3D conformation formed by a single polypeptide chain, including all helices, sheets, loops, and unstructured regions.
• Quaternary Structure: The complex assembly formed when a protein molecule is composed of more than one interacting polypeptide chain (each individual chain is called a subunit).

Secondary Motifs: The $\alpha$-Helix

• Structural Features:
  • Hydrogen bonds form between every 4th amino acid (specifically between the $C=O$ of one peptide bond and the $N-H$ of another 4 residues along the chain).
  • Forms a rigid, cylindrical right-handed helix (coiling in a clockwise direction).
  • A complete turn of the helix occurs every $3.6$ amino acid residues, corresponding to a translation length of $0.54\,nm$
  • Highly abundant in membrane-spanning transport proteins and receptors.

• Coiled-Coil Structures:
  • Formed when two or three $\alpha$-helices wrap around each other to form an exceptionally stable elongated rod.
  • Driven by hydrophobic interactions: Helices possess a continuous stripe of nonpolar/hydrophobic amino acid side chains (often at position "a" and "d" of repeated heptad sequences) that wrap inward together to avoid the surrounding aqueous environment.
  • Helical repeat distance is approximately $11\,nm$
  • Common in fibrous structural proteins such as $\alpha$-keratin (found in hair, horns, nails) and myosin.

Secondary Motifs: The $\beta$-Sheet

• Structural Features:
  • Rigid planar structures formed at the core of many globular proteins.
  • Held together by hydrogen bonds connecting neighboring polypeptide segments lying side-by-side ($0.7\,nm$ repeat distance between adjacent pleats).
• Orientations:
  • Parallel $\beta$-Sheets: Adjacent polypeptide strands run in the same $N$-to-$C$ terminal direction.
  • Antiparallel $\beta$-Sheets: Adjacent strands run in alternating, opposite $N$-to-$C$ terminal directions.

Amyloid Structures, Misfolding, and Prion Diseases

• Amyloid Motifs:

  • Formed when identical $\beta$-sheets stack together extensively with side chains interdigitating tightly like the teeth of a zipper.
  • Physiological Role: Can be used for functional storage and efficient concentrated packaging of peptide and protein hormones.

• Pathological Amyloids:

  • Misfolded proteins aggregating into insoluble amyloid fibrils cause severe cellular damage and tissue neurodegeneration.
  • Associated with human neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's disease.

• Prions:

  • Infectious misfolded proteins capable of propagating abnormal conformations.
  • Mechanism: A misfolded prion protein binds to a normal protein, forming a heterodimer and forcing the normal protein to adopt the abnormal prion conformation. These misfolded units propagate and aggregate into destructive amyloid fibrils.
  • Diseases: Scrapie (sheep), Bovine Spongiform Encephalopathy (BSE / Mad Cow Disease), Creutzfeldt-Jakob Disease (CJD in humans), Chronic Wasting Disease (CWD in deer/elk).

Protein Domains and Higher-Order Units

• Protein Domain:
  • Any segment of a polypeptide chain that folds independently into a compact, stable, discrete 3D structure.
  • Typically consists of 40 to 350 amino acids.
  • Connected by short, flexible unstructured polypeptide linkers.
  • Example - Bacterial Transcription Regulator (CAP Protein):
  • Small domain binds directly to DNA; large domain binds cyclic AMP (cAMP).
  • Binding of cAMP to the large domain triggers an allosteric conformational shift in the small domain, allowing it to recognize and bind DNA target sequences.

• Protein Families:
  • Groups of evolutionary related proteins displaying similar amino acid sequences, overall structures, and active-site geometries, yet performing distinct biological functions.
  • Example - Serine Proteases: Includes elastase and chymotrypsin, which share matching structural core domains but exhibit distinct substrate cleavage specificities.

Quaternary Assemblies and Binding Interfaces

• Binding Sites: Specific surface cavities or regions formed by precise arrangements of amino acid side chains that interact noncovalently (via ionic bonds, van der Waals forces, and hydrophobic interactions) with other molecules.
• Multimeric Protein Assemblies:
  • Subunits: Individual polypeptide chains within a multi-chain protein complex.
  • Homodimers: Two identical polypeptide chains bound symmetrically (e.g., dimer of CAP protein joined by a single identical binding site on each monomer).
  • Homotetramers: Four identical subunits assembled into a complex (e.g., tetramer of neuraminidase formed by interactions between two nonidentical binding sites on each monomer).
  • Heteromers: Assemblies containing distinct, nonidentical subunits (e.g., Hemoglobin containing two $\alpha$-globin and two $\beta$-globin subunits).

• Large Macromolecular Assemblies:
  • Filaments & Tubes: Chains of identical proteins forming extended helical polymers (e.g., actin filaments, microtubules).
  • Spherical Shells: Protein coats forming protective viral capsids.
  • Ribonucleoprotein Complexes: Combinations of multiple proteins and RNA/DNA molecules (e.g., ribosomes, spliceosomes, intact viruses).
  • Self-Assembly: Many dissociated subunit complexes can spontaneously reassemble into their native structures when returned to physiological conditions.

Fibrous Structural Proteins

• $\alpha$-Keratin: Extended coiled-coil dimer forming stable, long-lived structures in hair, horns, and nails.
• Intermediate Filaments: Cytoskeletal rope-like proteins providing immense mechanical tensile strength to animal cells.
• Collagen:
  • Major extracellular matrix protein binding cells together into tissues.
  • Composed of three polypeptide chains wrapped in a triple helix ($300\,nm \times 1.5\,nm$ dimension).
  • Characterized by a strict sequence repeat with nonpolar glycine at every third position ($Gly-X-Y$).
  • Covalently cross-linked via lysine residues to assemble into massive collagen fibrils ($50\,nm$ diameter), preventing cell and tissue tearing under physical stress.

• Elastin:
  • Highly flexible, unstructured polypeptide chains covalently cross-linked into an extensible elastic network.
  • Uncoils under tension and recoils spontaneously when released, allowing skin, blood vessels, and lungs to stretch without tearing.

Covalent Cross-Linkages: Disulfide Bridges

• Disulfide Bridges (Disulfide Bonds):
  • Covalent $-S-S-$ linkages formed between two thiol ($-SH$) groups of cysteine side chains.
  • Produced via an oxidation reaction and reversed via reduction.
  • Tie together residues within the same polypeptide chain (intrachain) or connect distinct chains (interchain).
• Compartmental Distribution:
  • Cytosol: Disulfide bonds do not form in the reducing environment of the cytosol, where noncovalent interactions and cellular homeostatic conditions suffice to maintain protein stability.
  • Extracellular Space: Essential for stabilizing excreted or cell-surface proteins against harsh extracellular environments (e.g., changes in pH and ambient temperature).

Principles of Ligand Binding and Enzyme Catalysis

• Ligand Binding:

  • Ligand: Any substance bound specifically by a protein.
  • Requires high specificity and physical complementarity through noncovalent interactions.
  • Binding Site: A precise surface cavity formed by folded amino acid residues.
  • Governs protein localization, activity, and interaction networks (e.g., antibodies binding viral/bacterial antigens or white blood cell receptors).

• Enzymatic Transformation:

  • Substrate: Specific ligand bound by an enzyme.
  • Active Site: Specialized catalytic pocket binding the substrate.
  • Transition State: The high-energy, distorted physical conformation adopted by the substrate within the active site, lowers the overall activation energy of the chemical reaction.
  • Metabolic Pathways: Interconnected chains of enzymatic reactions where the product of one enzyme becomes the substrate for the next ($A \xrightarrow{\text{Enzyme 1}} B \xrightarrow{\text{Enzyme 2}} C \dots$).
  • Many clinical pharmaceuticals act as competitive or non-competitive enzyme inhibitors.

Nonprotein Helpers: Cofactors and Coenzymes

• Definitions:
  • Cofactors: Inorganic ions or small nonprotein molecules required for enzyme activity.
  • Iron ($Fe^{2+}$): Held in heme groups within hemoglobin, myoglobin, and cytochromes to bind oxygen or electrons.
  • Zinc ($Zn^{2+}$): Essential cofactor in carboxypeptidase to hydrolyze peptide bonds.
  • Coenzymes: Small organic nonprotein molecules aiding enzyme function, frequently derived from dietary vitamins.
  • Biotin: Assists enzymes in transferring carboxyl groups.
  • Retinal: Light-absorbing coenzyme bound to rhodopsin in eye photoreceptor cells.

Mechanisms of Protein Regulation

• Cellular Control Mechanisms:
  • Regulation of gene expression (transcription/translation rates) and protein degradation rates controls total protein abundance.
  • Subcellular compartmentalization concentrates or isolates enzymes.
• Direct Activity Modulation:
  • Regulatory Sites: Distinct binding sites separate from the active site used to modulate enzyme activity.
  • Feedback Inhibition: A downstream end-product of a metabolic pathway binds an regulatory site on an early-acting enzyme in the same pathway, inhibiting its activity when product concentrations accumulate (negative regulation).
  • Positive Regulation: An intermediate or product from one metabolic branch binds and stimulates an enzyme operating in a parallel or competing pathway.

• Allosteric Transitions:
  • Allosteric proteins switch reversibly between distinct active and inactive structural conformations based on ligand binding at regulatory sites.
  • Example: ADP binding to an enzyme positively shifts its equilibrium from an inactive conformation ($10\%$ active without ADP) to a fully active conformation ($100\%$ active with ADP).

Reversible Covalent Modifications

• Protein Phosphorylation:
  • Over $1/3$ of all proteins in a mammalian cell are phosphorylated at any given time.
  • Protein Kinase: Catalyzes the transfer of a terminal phosphate group from ATP to a hydroxyl group on a serine, threonine, or tyrosine side chain.
  • Protein Phosphatase: Removes the phosphate group via hydrolysis.
  • Conformational Impact: The attached phosphate group introduces two negative charges ($-PO_4^{2-}$), driving major conformational changes through electrostatic repulsions or attractions with nearby charged side chains.
  • Docking Sites: Phosphorylation creates binding recognition sites for signaling complexes (e.g., autophosphorylated Receptor Tyrosine Kinases).

• Other Regulatory Modifications:
  • Acetylation: Addition of acetyl groups to lysine residues (e.g., histone modification regulating chromatin structure).
  • Palmitoylation / Fatty Acid Attachment: Covalent attachment of fatty acids to cysteine side chains to anchor cytosolic proteins into plasma membranes.
  • Ubiquitination: Attachment of ubiquitin peptides to target proteins for proteasomal degradation.
  • Combinatorial Regulation: Multiple modifications occur on single proteins; for example, the tumor suppressor p53 is modified at more than 20 distinct sites in response to DNA damage.

Nucleotide-Driven Molecular Switches and Motors

• GTP-Binding Proteins (GTPases):
  • Act as molecular switches that are active when bound to GTP and inactive when bound to GDP.
  • Hydrolysis of bound GTP to GDP + $P_i$ shifts the protein into an inactive conformation.
  • Reactivation requires slow GDP dissociation followed by rapid binding of a new GTP molecule, driven by cellular signals.

• Motor Proteins:
  • Generate directional mechanical forces responsible for muscle contraction, spindle movement during mitosis, and organelle transport along filaments (e.g., Myosin walking along actin filaments).
  • Unidirectional Movement: Movement is driven unidirectionally because one step of the cycle is rendered completely irreversible by the hydrolysis of bound ATP to ADP + $P_i$

• Protein Machines:
  • Highly coordinated multi-protein assemblies operating as linked complexes.
  • Coupled ATP or GTP hydrolysis drives synchronized, sequential conformational shifts across all subunits to perform complex tasks (e.g., DNA replication, cell division, and protein synthesis).

Higher-Order Cellular Organization

• Scaffold Proteins:
  • Large, flexible proteins containing specific binding sites for multiple interacting proteins.
  • Bring enzyme networks into close physical proximity to dramatically accelerate reaction rates and confine metabolic processes to specific sub-cellular areas.
  • Long scaffold RNA strands can perform similar organizing roles.

• Biomolecular Condensates:
  • Dynamic, membrane-less intracellular compartments formed by phase separation driven by multiple weak interactions among protein and RNA scaffolds.
  • Concentrates specific "client" molecules within a localized liquid-like droplet separated from the surrounding cytosol.
  • Example - Nucleolus: A nuclear condensate where rRNA scaffolds assemble ribosomal proteins into intact ribosome subunits.

Analytical Methods for Protein Isolation and Structure Determination

• Purification and Separation:
  • Cell Homogenization: Cells or tissues are broken open to release cytosol and organelles (producing a cell homogenate/extract).
  • Fractionation: Initial separation of cell extracts into sub-fractions.
  • Chromatography: Separates protein mixtures based on physical properties.
  • Affinity Chromatography: Matrix beads are covalently coupled to a specific ligand/protein X; target binding proteins adhere to the column while non-binding proteins pass through. Target proteins are eluted using high salt concentrations or pH changes.
  • Electrophoresis (SDS-PAGE): Polypeptides migrate through a polyacrylamide gel matrix under an electric field, separating based on molecular weight and net charge.

• Identification by Mass Spectrometry:
  • A specific protein spot is excised from an electrophoresis gel.
  • Cleaved into smaller peptide fragments using the site-specific protease trypsin.
  • Peptides are ionized (blasted with a laser into a gaseous charged state) and accelerated toward a detector in a mass spectrometer.
  • Time-of-flight measurements yield precise mass-to-charge ($m/z$) spectrum ratios.
  • Experimental peptide masses are matched against theoretical masses calculated from genomic sequence databases to identify the protein and clone its corresponding gene.

• 3D Structure Determination Techniques:
  • X-ray Crystallography: Measures diffraction patterns of X-rays passing through crystallized proteins.
  • Nuclear Magnetic Resonance (NMR) Spectroscopy: Analyzes magnetic properties of atomic nuclei in solution.
  • Cryo-Electron Microscopy (cryoEM): Rapidly freezes protein samples in vitreous ice and images them using electron beams to reconstruct 3D atomic structures.
  • AI and Structure Prediction: Machine learning tools (e.g., AlphaFold) analyze evolutionary sequence patterns and family domain structures to predict folded conformations for over half of all known proteins.

Genetic Engineering Applications

• Recombinant Protein Production: Bacteria, yeast, or mammalian expression systems are engineered to mass-produce therapeutic human proteins (e.g., insulin, human growth hormone, and fertility drugs).
• Synthetic Biology and Environmental Engineering: Design of novel enzymes capable of metabolizing toxic wastes or synthesizing pharmaceuticals.
• Targeted Drug Delivery Systems: Design of artificial synthetic proteins containing molecular cages/latches that open selectively upon binding key target compounds, enabling site-specific drug release, gene activation, or targeted cell death.