Protein Motifs & Domains

Protein Structure Terminology

  • Primary Protein Structure: This refers to the unique linear sequence of amino acids that make up a polypeptide chain. This sequence is fundamental as it dictates all subsequent levels of protein structure.

  • Secondary Protein Structure: This level of structure is formed by hydrogen bonding of the peptide backbone, causing the amino acids to fold into repeating patterns.

    • Alpha Helix (α\alpha-helix): A common secondary structure where the polypeptide chain coils into a spiral shape. Hydrogen bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of an amino acid four residues away along the backbone.
    • Beta Sheet (Pleated Sheet, β\beta-sheet): Another common secondary structure formed by hydrogen bonds between the protein backbone segments that lie side-by-side. These segments can be parallel or antiparallel, forming a characteristic pleated appearance.

  • Tertiary Protein Structure: This describes the three-dimensional folding pattern of a protein, primarily due to interactions between the side chains (R-groups) of the amino acids. These interactions include hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.

  • Quaternary Protein Structure: This level of structure applies to proteins consisting of more than one amino acid chain (i.e., multiple polypeptide subunits) interacting to form a functional complex.

    • Examples of Quaternary Structures:
      • Dimer: A protein composed of two subunits (e.g., ARNT PAS-B, PDB ID = 2HV1).
      • Trimer: A protein composed of three subunits (e.g., E. blattae acid phosphatase, PDB ID = 2EOI).
      • Tetramer: A protein composed of four subunits (e.g., B. anthracis dihydrodipicolinate synthase, PDB ID = 1XL9).
      • Pentamer: A protein composed of five subunits (e.g., Shiga-like toxin I B, PDB ID = 1CZG).

Protein Motifs & Domains

  • Motif (or Super Secondary Structure/Fold): This term refers to a recurring, recognizable combination of secondary structures (e.g., helix-loop-helix, β\beta-hairpin, or β\beta-α\alpha-β\beta unit). Motifs are generally small, stable arrangements of common secondary structures, often associated with a particular function or binding property.

  • Domain: A domain is a larger, independently folding unit within a polypeptide chain. Protein domains often have distinct functions and can evolve, function, and exist independently of the rest of the protein. Proteins can be composed of multiple domains, sometimes arranged in a