Proteins retain their function in the crystal

Probably the most convincing evidence that crystalline structures can safely be used to draw conclusions about molecular function is the observation that many macromolecules are still functional in the crystalline state. For example, substrates added to suspensions of crystalline enzymes are converted to product, albeit at reduced rates, suggesting that the enzyme s catalytic and binding sites are intact. The lower rates of catalysis can be accounted for by the reduced accessibility of active sites within the crystal, in comparison to solution. 

It makes sense, therefore, after obtaining crystals of a protein and before embarking on the strenuous process of obtaining a structure, to determine whether the protein retains its function in the crystalline state. If the crystalline form is functional, the crystallographer can be confident that the model will show the molecule in its functional form. 

X-ray structures are compatible with other structural evidence 

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