What is crystallography and why is so useful for proteins?

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regarding one experimental approach to understanding GPCR activation, I wanted to discuss how crystallography was used to deliver new insight on allosteric regulation of GPCRs by endogenous ions, cholesterol, lipids, and polypeptides. One specific example Katritch et al. discusses A2AAR receptor which is inactive and described as a tight water-filled channel that connects extracellular and intracellular parts to the receptor. Through crystallography, it was discovered that there was a 200-A3 allosteric pocket in the channel that was initially believed to fit a Na+ ion, but was undetected in previous crystallographic structures. A number of GPCRs: Trp2466.48, Tyr2887.53, Asn241.50, Asp522.50, Ser913.39, Asn2807.45, Asn2847.49 bounds the channel and all function as a hydrogen bonding network. Through crystallography, the A2AAR receptor was revealed to have the Na+ ion coordinated by Asp522.50 and Ser913.39. It was then final discovered through the same method that when the receptor was activated, the Na+ pocket falls apart and the ion leaves until reactivation.

Responding to the second part of the question, I believe there was such an exponential growth in crystal structures of GPCRs because of breakthrough discoveries in protein engineering and crystallography. When I check the sources that Katritch et al. uses to claim this, I see that the researchers overcame crystallography problems by engineering fusion proteins to overcome structural flexibility issues.

Question: What is crystallography and why is so useful for proteins?

Katritch, V., Cherezov, V., & Stevens, R. C. (2013). Structure-function of the G proteincoupled receptor superfamily. Annual review of pharmacology and toxicology, 53, 531-556.

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