Proteins are large, complex molecules that perform a vast array of functions in living organisms. Their function is intimately linked to their structure, which is organized into four distinct levels: primary, secondary, tertiary, and quaternary.
The Four Levels of Protein Structure
Primary Structure
The primary structure is the linear sequence of amino acids in a polypeptide chain. Each protein has a unique sequence determined by the corresponding gene. Even a single amino acid change can alter the protein’s function, as seen in sickle cell anemia where a single valine replaces a glutamic acid.
Secondary Structure
The secondary structure refers to local folded structures that form within the polypeptide chain due to hydrogen bonding between backbone atoms. The two most common types are the alpha-helix, a right-handed coil stabilized by hydrogen bonds between every fourth amino acid, and the beta-sheet, a flat, pleated structure formed by hydrogen bonds between adjacent polypeptide segments running parallel or antiparallel.
Tertiary Structure
The tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It is stabilized by several types of interactions between side chains: hydrophobic interactions (nonpolar side chains cluster in the protein’s interior), hydrogen bonds between polar side chains, ionic bonds between oppositely charged side chains, and disulfide bridges (covalent bonds between cysteine residues).
Quaternary Structure
The quaternary structure describes how multiple polypeptide chains assemble into a functional protein complex. Hemoglobin, for example, is composed of four polypeptide subunits — two alpha and two beta chains — that work together to transport oxygen.
Protein Folding
Proteins fold into their native three-dimensional structures spontaneously or with the help of molecular chaperones. Misfolded proteins can form aggregates and are associated with diseases such as Alzheimer’s and Parkinson’s.
Practical Protein Structure Determination and Visualization
Three main experimental methods determine protein structures at atomic resolution. X-ray crystallography requires high-purity protein (>95%) concentrated to 5–20 mg/mL. Screen crystallization conditions using sparse-matrix kits (e.g., Hampton Research) with 96-well sitting-drop vapor diffusion trials. Optimize hits by varying pH, precipitant concentration, and temperature. Mount a diffraction-quality crystal (0.1–0.5 mm) in a cryo-loop, flash-cool at 100 K in liquid nitrogen, and collect diffraction data at a synchrotron beamline. Process data with XDS or iMosflm, solve the phase problem by molecular replacement (Phaser) using a homologous structure, build the model in Coot, and refine it with phenix.refine. Target resolution: <3.0 Å; a good structure has Rwork/Rfree < 0.20/0.25. Nuclear magnetic resonance (NMR) spectroscopy works for small proteins (<30 kDa) at 0.5–1 mM concentration in 10% D2O. Record 2D ¹⁵N-HSQC, 3D triple-resonance spectra (HNCA, HNCO, CBCA(CO)NH), and NOESY for distance restraints. Assign backbone resonances manually or using automated software (e.g., CARA, NMRFAM-SPARKY). Calculate structures from NOE-derived distance restraints and dihedral angles using CYANA or Xplor-NIH. Cryo-electron microscopy (cryo-EM) is the method of choice for large complexes (>150 kDa). Apply 3 µL of sample (0.1–1 mg/mL) to a glow-discharged holey carbon grid, blot for 3–4 seconds, and plunge-freeze in liquid ethane. Collect movies on a Titan Krios at 300 kV with a K2 or K3 detector. Process with RELION or CryoSPARC through motion correction, CTF estimation, particle picking (2D classification), ab initio reconstruction, 3D classification, and high-resolution refinement. For structure visualization, use PyMOL or ChimeraX to load PDB files. Display the protein as cartoons (secondary structure coloring), surfaces (electrostatic potential), or sticks (active site residues). Measure distances, angles, and hydrogen bonds between atoms.
Real-World Application
The SARS-CoV-2 spike protein (150 kDa ectodomain) was solved by cryo-EM to 3.3 Å resolution within months of the pandemic onset. The structure revealed the receptor-binding domain (RBD) in its “up” conformation bound to ACE2, enabling structure-based vaccine design. PyMOL visualization of the RBD-ACE2 interface identified key contact residues (K417, N501, Y453), guiding the development of neutralizing antibodies.
