Question

One of the main assumptions in protein X-ray crystallography is, that proteins (for that matter all other biological macromolecules as well) in their crystalline state have folding topologies identical to those of the proteins in solutions or in their natural cellular environment. Several lines of evidence support this notion. Which one of the following is the most compelling to you? (Rate A trough F from most to less compelling and give your reasoning.

A) Except in some isolated cases, the structures of the same protein determined by X-ray crystallography and NMR methods are similar.

B) A crystalline protein can be essentially considered as in solution, because the solvent molecules, e.g. hydration water and solutes co-crystallize with the protein.

C) Proteins are generally resistant against unwanted side effects when subjected to the X-ray irradiation; therefore they maintain their overall physical state.

D) Enzymes usually retain their catalytic activity in the crystalline state.

E) Electron density maps can be obtained at high, atomic resolution and assigned with high accuracy.

Answer 1

B is most compelling since protein exist in its environmental medium and when x-ray crystallography is done the protein is made in crystalline form in such a way that its properties do not change.

E says that electron density maps can be obtained at high atomic resolution and assigned with high accuracy. this is because proteins aree macromolecules with high electron density on them. X-ray crystallography gives 3-D structure which is of high resolution.

D states that enzymes usually retain their catalytic activity in the crystalline state this is somewhat true. since the crystalline state has to be isolated in such a way that the functioning of the protein shall not change.

C is not compelling enough since proteins often tend to lose their physical properties when irrradiated with x-rays. there has to be a very carefull amount of irradiation.

A is not compelling enough since the structures determined by X-ray crsytallography and NMR are differently analysed. X-ray crsytallography does not give the positions of hydrogens whereas NMR gives. X-ray crstallography provides information about correlated thermal motion within or between crystal unit cells while NMR allows one to obtain structural information from sample in the solid phase.