Which method is commonly used to determine protein three-dimensional structure at atomic resolution?

• A. Gel electrophoresis
• B. PCR
• C. X-ray crystallography
• D. Mass spectrometry

Answer: (C)

To determine a protein’s atomic arrangement, you need a method that reveals positions of individual atoms with precision. X-ray crystallography does exactly this: you grow crystals of the protein, shine X-rays on them, and measure how the rays diffract. From that diffraction pattern you compute an electron density map and fit the atomic model into it, giving a detailed three-dimensional structure with typically sub-angstrom to around 1–2 Å resolution. This level of detail is why it’s the go-to technique for understanding active sites, ligand binding, and overall folding.

Gel electrophoresis separates molecules by size or charge and doesn’t provide 3D coordinates. PCR amplifies DNA, not protein structure. Mass spectrometry measures mass and can infer composition or sequence and, in some specialized forms, limited structural clues, but it doesn’t deliver a full atomic-resolution model of a protein’s shape.