— Ch. 1 · Four Levels Of Structure —
Protein structure.
~4 min read · Ch. 1 of 5
Frederick Sanger discovered the sequence of amino acids in insulin, establishing that proteins have defining amino acid sequences. This discovery revealed that a chain under 30 amino acids is often identified as a peptide rather than a protein. The primary structure refers to this specific sequence held together by peptide bonds made during protein biosynthesis. Counting residues always starts at the N-terminal end where the amino group remains free from a peptide bond. A single polypeptide chain may contain one or several domains that fold independently of the rest of the molecule. Linus Pauling suggested two main types of secondary structure in 1951: the alpha-helix and the beta-strand. These structures are defined by patterns of hydrogen bonds between the main-chain peptide groups. Tertiary structure describes the three-dimensional arrangement created by a single protein molecule folding into a compact globular shape. Quaternary structure involves the aggregation of two or more individual polypeptide chains operating as a single functional unit called a multimer. Complexes containing two subunits are called dimers while those with four subunits are termed tetramers. Hemoglobin serves as an example of a heterotetramer composed of two alpha and two beta chains.
History Of Discovery Methods
Around 90% of the protein structures available in the Protein Data Bank have been determined by X-ray crystallography. This method allows scientists to measure the three-dimensional density distribution of electrons in the crystallized state. Roughly 7% of known protein structures were obtained using nuclear magnetic resonance techniques for smaller molecules. Cryo-electron microscopy determines structures for larger protein complexes where resolution is typically lower than other methods. The maximum resolution from cryo-EM continues to increase steadily over time. Circular dichroism provides general secondary structure composition data through spectroscopic analysis. Vibrational spectroscopy characterizes the conformation of peptides and proteins at various scales. Two-dimensional infrared spectroscopy investigates flexible peptides that cannot be studied with standard methods. Fast parallel proteolysis probes structured fractions without requiring purification steps. Molecular dynamic simulations allow detailed studies once experimental determination is complete. These techniques collectively form the foundation of structural biology as a scientific field.