Protein Structure Notes (Transcript-Based) (copy)

Peptide Backbone and Primary Structure

• The peptide backbone is the part of the protein that participates in forming secondary structure; the side chains do not form the secondary structure directly. Side chains are involved later in tertiary structure.
• Primary structure is the linear sequence of amino acids with the peptide backbone at the top and side chains hanging down.
• Primary structure is read from the N-terminus (the end with the amino group) to the C-terminus (the end with the free carboxyl group).
• This directional reading is equivalent to reading from 5' to 3' in nucleic acids: $5' \rightarrow 3'$.
• Overall message: protein structure is central because structure dictates function, which in turn influences cellular behavior and biological outcomes.

Secondary Structure: Formation and Characteristics

• Secondary structure is formed by hydrogen bonds within the peptide backbone.
• The backbone curls and folds, forming recurring motifs; hydrogen bonds drive these patterns.
• Hydrogen bonds require partial charges on atoms: partial negative on carbonyl oxygens and partial positive on amide hydrogens.
• The backbone bonds and geometry lead to repetitive structures, like a snake-like curl.
• The peptide bond is shown in blue in diagrams; as the backbone bends, hydrogen bonds form between successive units.
• Explanation of partial charges: the carbonyl carbon–oxygen double bond creates a partial negative on O, while the adjacent amide nitrogen pulls electrons, creating a partial positive on the attached hydrogen; these enable H-bonds with nearby backbone units.
• The concept of “bonds as possible between different parts of the peptide backbone” gives rise to characteristic secondary structures such as alpha helices and beta pleated sheets.
• The term "pleated" refers to folds or corrugations in the sheet-like structure; think of pleated fabric or trousers being pleated – rigidity is not the key here, but a folded geometry.
• The two main secondary structures are alpha helices and beta sheets (beta pleated sheets).
• Side chains do not form the backbone H-bonds themselves, but they influence the bendiness and stability of the backbone indirectly.
• Amino acid sequence determines secondary structure in a complex way; this relationship is studied more in depth in bioinformatics, where properties of individual amino acids are analyzed.
• Visual representations of a beta pleated sheet show strands running back and forth; this is a typical depiction in standard protein diagrams.
• Example protein introduced for labs: tyrosinase, an enzyme involved in pigmentation (melanin synthesis).
  • Tyrosinase ends with “-ase,” indicating an enzyme.
  • It is involved in skin and hair pigmentation; a deficiency or variation in melanin production affects tanning and sun sensitivity.
  • Potatoes also contain melanin; this enzyme converts tyrosine partway toward melanin.
• Implication: some proteins are relatively flexible in their structure (e.g., certain segments can be more dynamic), while other regions are more tightly structured.

Three-Dimensional Structure and the Role of Side Chains (Tertiary Structure)

• After secondary structures form, the overall 3D shape is driven by interactions between side chains (R-groups).
• Types of interactions include polar interactions and hydrogen bonds; the presence and properties of side chains govern these interactions.
• A key example of a covalent interaction in tertiary structure is the disulfide linkage:
  • A disulfide linkage is a covalent bond between two sulfur atoms:
    \mathrm{R{-}S{-}S{-}R'}
    
  • It forms between two cysteine residues (cysteine contains a thiol group, –SH).
  • Cysteine is crucial because disulfide bonds require sulfur; nonpolar side chains generally do not form such bonds.
  • Disulfide bonds typically form between two cysteine residues, not within the same cysteine molecule.
  • A lighthearted aside notes that cysteine’s importance has even been humorously associated with Michelangelo’s use of cysteine in art; the point is to emphasize the structural importance of cysteine.
• Given a polypeptide that has folded into its functional form, the result may be a single functional unit (a single protein) with one major part, or a machine with multiple parts that can detach or move relative to each other.

Quaternary Structure: Multiple Polypeptide Chains

• Quaternary structure describes how multiple polypeptide chains associate to form a functional protein complex.
• Chains can be identical or different; interactions between chains are often maintained by hydrogen bonds or ionic interactions.
• Example: hemoglobin
  • Hemoglobin consists of two alpha chains and two beta chains: $\text{Hb} = (\alpha<em>2)(\beta</em>2)$
  • This arrangement allows cooperative binding of oxygen in the blood.
• Another example: collagen
  • Collagen is found in skin, hair, and nails, as well as connective tissues like tendons, ligaments, and cartilage.
• Genetic encoding note (hemoglobin): at least two genes encode different parts of hemoglobin; one set encodes alpha chains, another set encodes beta chains (e.g., alpha-globins).
• The distribution of acidic, basic, polar, and cysteine-containing side chains across subunits determines how subunits interact and assemble.

Transport Between Nucleus and Cytoplasm: An Illustrative Protein-Interaction Example

• A transport pathway into the nucleus involves a target protein marked by a carbohydrate tag during production.
• The tag is referenced as a carbohydrate (CHO) modification; the highlighted protein in orange interacts with a blue partner (beta subunit) that binds to pores in the nuclear membrane.
• In this model, alpha and beta subunits coordinate to shuttle proteins through nuclear pores into the nucleus.
• This example illustrates how protein-protein interactions and post-translational tags (carbohydrate tags) contribute to cellular localization and transport.

Special Case: Anthrax Lethal Factor and Protein Kinases

• Lethal factor is a proteolytic enzyme that targets kinases, which are enzymes that control cell growth and division.
• Mechanism: lethal factor binds to a protein kinase and hydrolyzes it (breaks it down).
  • Hydrolysis is the splitting of a large molecule into smaller pieces.
• Consequence: by dismantling kinases, the cell’s signaling and growth/division controls are disrupted, contributing to cell death in the infected cell.
• The key point is that the lethal factor possesses a shape that perfectly fits its target kinase, enabling this catalytic action.

Summary of Key Concepts and Terms

• Terminology and directionality:
  • N-terminus (amino end) to C-terminus (carboxyl end): $\text{N-terminus} \rightarrow \text{C-terminus}$
  • N to C directional reading parallels 5' to 3' orientation in nucleic acids: $5' \rightarrow 3'$
• Primary structure: amino acid sequence and the peptide backbone; side chains extend from the backbone.
• Secondary structure: alpha helices and beta sheets formed by backbone hydrogen bonding; side chains influence but do not form these bonds directly.
• Hydrogen bonding: backbone atoms form recurring patterns; partial charges drive these interactions.
• Beta pleated sheet: a folded/pleated arrangement of backbone segments; the term pleated refers to the sheet-like, folded arrangement.
• Tertiary structure: 3D folding due to interactions between side chains; includes hydrogen bonding, ionic interactions, hydrophobic effects, and covalent disulfide linkages (between cysteine residues).
• Disulfide linkage: a covalent S–S bond between two cysteines; requires sulfur-containing side chains.
• Cysteine significance: critical for disulfide bonding; used as an iconic example of covalent stabilization in proteins.
• Quaternary structure: assembly of multiple polypeptide chains into a functional protein complex (e.g., hemoglobin: $\text{Hb} = (\alpha<em>2)(\beta</em>2)$).
• Collagen: a structural protein found in skin, hair, nails, tendons, ligaments, and cartilage; highlights the role of protein architecture in connective tissues.
• Genetic encoding of protein subunits: different genes encode different subunits (e.g., alpha and beta chains of hemoglobin).
• Protein transport and tagging: carbohydrate tags can guide proteins to specific cellular locations (e.g., nuclear transport involving alpha and beta interactions and nuclear pores).
• Anthrax lethal factor and kinases: a real-world example of protein-protein interactions affecting cell signaling and viability; hydrolysis of kinases by lethal factor demonstrates how structure-specific interactions translate to function and pathology.

Connections to Core Principles and Real-World Relevance

• Structure determines function: the notes emphasize how backbone geometry, hydrogen bonding, side-chain interactions, and multimeric assembly govern how proteins work in cells.
• Dynamic versus static regions: some protein segments are flexible, enabling movement and conformational changes essential for function; other regions are tightly folded to provide stability.
• Disease and biotechnology relevance: understanding disulfide bonds, tertiary/quaternary structures, and transport pathways underpins drug design, enzyme engineering, and understanding disease mechanisms (e.g., toxins like anthrax lethal factor).
• Model organisms and enzymes: tyrosinase illustrates how structure relates to enzymatic activity in pigmentation; potato melanin production provides an illustrative parallel to mammalian melanin pathways.

Quick Reference: Key Equations and Notation

• Directionality of peptide chain: $\text{N-terminus} \rightarrow \text{C-terminus}$
• Nucleic acid-like directionality: $5' \rightarrow 3'$
• Disulfide linkage (cysteine): $\mathrm{Cys{-}S{-}S{-}Cys}$
• Hemoglobin quaternary assembly: $\text{Hb} = (\alpha<em>2)(\beta</em>2)$
• Hydrogen bonding motif (backbone): $\text{O}^{\delta-} \cdots \text{H}^{\delta+} \cdots \text{N}^{\delta-}$ (representing carbonyl O with amide H interactions)
• Carbohydrate tagging in transport: abbreviated as CHO in diagrams (carbohydrate tag involved in localization)
• Lethal factor mechanism: binds to kinases and hydrolyzes them (hydrolysis = breakdown into smaller pieces)