Four layers of organization from sequence to assembly
Proteins are polymers of amino acids, and their structure unfolds across four levels. Primary structure is the amino acid sequence, determined by the DNA gene encoding the protein. Each amino acid differs in its side chain (R group), giving it unique chemical properties. The order of amino acids encodes all the information needed to build the final structure.
Secondary structure emerges from hydrogen bonding between backbone atoms (not side chains). Regular patterns include alpha helices (like a spring) and beta sheets (strands lying side by side). These structures are stabilized by hydrogen bonds and are found in most proteins. Some regions are loops or turns, lacking regular structure.
Tertiary and quaternary assembly
Tertiary structure is the overall 3D shape of the entire protein, determined by how the side chains interact. Hydrophobic residues cluster in the interior, away from water. Disulfide bonds covalently link distant cysteines. Ionic interactions, hydrogen bonds, and van der Waals forces stabilize the fold. The shape determines the protein's function: an enzyme's active site, an antibody's binding groove, a receptor's ligand-binding pocket all emerge from tertiary structure.
Quaternary structure describes how multiple protein subunits assemble into a complex. Hemoglobin is four subunits held together by non-covalent interactions; a change in one subunit's tertiary structure can allosterically affect the others. This hierarchical architecture, from sequence to assembly, allows for remarkable specificity and regulation. Misfolded proteins, where tertiary or quaternary structure goes awry, cause disease (Alzheimer's, Parkinson's, cystic fibrosis).