: The functional form of a molecule (e.g., a four-unit hemoglobin complex), which may differ from the Asymmetric Unit (the smallest portion used to solve the crystal structure).
The archive uses several formats to store 3D coordinates and experimental data: Guide to Understanding PDB Data: Biological Assemblies P.D.B.
: An "Entity" is a unique molecule (like a specific protein chain), while an "Instance" (or Chain ID) is a specific copy of that molecule within the structure (e.g., Chain A and Chain B). 2. Standard File Formats : The functional form of a molecule (e