Re: how big a protein can one still crystallize ?

Dear Nicolas,

You're in luck. I am a protein crystallographer by trade and your question is near and dear to me.

First of all, it is important to draw a line between proteins and polymers. They are two considerably different beasts and it is not fair or proper to lump them together, especially with respect to crystallization.

While polymers do crystallize (sort of) - their crystals are not really three dimensional - typically they tend to form mostly two-dimensional lattices with some amount of order in third dimension. Additionally polymers are rarely present as single species (i.e. a specific molecular weight) which makes it even more difficult to characterize them as crystalline or amorphous. So let's forget about polymers and focus on proteins!

There is no way to predict whether a specific protein crystallizes or not. One of the reasons why proteins are able to crystallize at all is that unlike polymers proteins can possess a specific and consistent three dimensional structure, defined by their amino acid sequence. (Unfortunately, some proteins have large segments of poorly ordered sequence which is not good for crystallization). Purified proteins in solution should be homogenous - both chemically and structurally, in order to crystallize. Oligomeric proteins crystallize just as well as monomeric ones do -- as long as these oligomers are specific (i.e. only tetramers or only dimers in solution). Size does not have much to do with the success or failure of any specific protein to crystallize, although it seems that larger proteins tend to be more recalcitrant.

Some of the largest proteins and protein assemblies that were crystallized include whole viral capsids and the ribosome (although ribosome contains more RNA than protein). On the other hand some very small proteins have not been crystallized so far. The reason for the latter may be that these proteins lack internal order - parts of them could be 'wobbly' which results in non-homogenous population of slightly different protein molecules in solution, which is a situation that is bad news for crystallization.

This is a very large and complex field of science, and even after more than ten years working with protein crystals I cannot claim to understand them completely. So please feel free to ask more questions.

Good luck and happy New Year!

Artem Evdokimov