= Structural biologist
A \b[structural biologist] studies the three-dimensional organization of biological <macromolecules> and how it relates to their function. <Structural biology> combines measurement with a <macromolecular structural model>: a representation supported by experimental observations, rather than a direct photograph of every atom. Common approaches include <X-ray crystallography>, <cryo-electron microscopy> and <NMR spectroscopy>.
In <X-ray crystallography>, a <crystal> produces a diffraction pattern from which researchers infer an <electron density map> and refine a molecular model. A concrete dataset is <calmodulin> structure1CLL in the <Protein Data Bank>. Its primary record reports <X-ray diffraction> at $1.70\,\mathrm{\mathring A}$ resolution and a refined model containing four <calcium ions>.<Calmodulin structure 1CLL in the Protein Data Bank>[Experimental Data Snapshot and Primary Citation abstract]{ref}
An original unit conversion gives the scale:
$$
1.70\,\mathrm{\mathring A}=0.170\,\mathrm{nm}.
$$
That resolution value is not a universal error bar of $0.170\,\mathrm{nm}$ on every modeled atom. Some regions can be well constrained while flexible or disordered regions are poorly resolved. In1CLL, the primary paper's abstract explicitly identifies poorly defined terminal residues.<Calmodulin structure 1CLL in the Protein Data Bank>[Primary Citation abstract, poorly defined residues and refinement]{ref}
A structural biologist can then test a mechanistic proposal with complementary evidence. A model may suggest a <calcium ion> binding environment; comparing binding or activity measurements can test its functional consequences. A static arrangement alone does not establish every reaction step or rate. The scientific work lies in connecting the model, its uncertainty and the behavior of the molecular system.
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