Chapter 4: Proteins: Three-Dimensional Structure and Function

Protein organization is understood through four hierarchical levels: primary structure refers to the linear sequence of amino acids connected by peptide bonds, secondary structure encompasses recurring patterns like alpha helices and beta sheets stabilized by backbone hydrogen bonding, tertiary structure describes the complete folding of a single polypeptide chain through noncovalent interactions among side chains, and quaternary structure involves the assembly of multiple polypeptide subunits into functional complexes. The peptide bond exhibits partial double-bond character that restricts rotation around the carbon-nitrogen linkage, forcing the peptide group into a planar geometry and typically adopting a trans configuration. Consequently, rotation occurs only around the phi and psi dihedral angles, the allowable combinations of which are mapped on Ramachandran plots. Alpha helices are right-handed coiled structures with 3.6 residues per turn, stabilized by hydrogen bonds between backbone atoms four residues apart, while beta sheets form when extended polypeptide strands align either parallel or antiparallel to one another. Tertiary and quaternary structures emerge from the hydrophobic effect, whereby nonpolar residues preferentially bury themselves in the protein core away from the aqueous environment, thereby increasing system entropy and driving spontaneous folding to the lowest free-energy native state. Molecular chaperones facilitate correct folding in the crowded cellular milieu by binding and stabilizing folding intermediates. Specific protein examples illustrate structure-function relationships: myoglobin demonstrates straightforward oxygen binding through a heme prosthetic group, hemoglobin exhibits cooperative binding and allosteric regulation through conformational transitions between tense and relaxed states, collagen provides mechanical strength through triple-helix architecture and cross-linking, and antibodies recognize foreign antigens through variable binding domains derived from immunoglobulin fold motifs. X-ray crystallography and nuclear magnetic resonance spectroscopy serve as the principal experimental techniques for elucidating protein three-dimensional structures at atomic resolution.