Some counter-arguments to Michael :
There is an “outside force doing the work”: macromolecule crystallization except rare exceptions is driven by competition for water molecules between the macromolecule and the precipitant. The exceptions are crystallization against low salt buffer, in which case the process is driven by hydrophobic “forces”.
And “packable” may play a role. A molecule which is of such shape and surface charge distribution that there is no way to pack it in a regular lattice will never crystallize.
Regarding the dimer vs. monomer debate, crystallization acts as a purification step. It seems perfectly plausible that crystal growth would “select” the monomeric state if dimers cannot be included in the growing crystal lattice, regardless of whether one is more soluble than the other. It all comes down to the initial crystal seed favored by the crystallization conditions. On a separate note, protein which forms dimers in solution trend to be more soluble in dimeric state than as monomers because dimerization usually buries a significant hydrophobic patch of molecular surface. If crystallization was only “selecting for the least soluble” oligomeric state we would rarely crystallize proteins as dimers.
Crystallization is such a confusing process J
Thierry
I just wanted to disagree with Roger's word choice, but not his argument (this is a "flame"-free response). Forget about "packing" and "packable" as there is no outside force doing the work. The molecules are just falling into a local energy minimum where favorable intra- and intermolecular interactions predominate. It is difference in the behavior of the ensemble versus of a solubilized, dispersed species (be it monomer or dimer). It is a phase behavior issue. Concerning Sebastian's case, while it is uncommon, the idea that a monomer has a crystalline phase state while the dimer does not is perfectly reasonable, and the crystals of the monomer grow due to mass action. I am sure the number of verified examples of this are limited. However, there are many cases where dimeric and tetrameric enzymes can be shown to be fully saturated with one or another bound substrate in solution, but show one or more empty active sites in the crystal. I know of several cases where this occurs, showing that selection of the species with the best set of favorable intra- and intermolecular interactions occurs.
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R. Michael Garavito, Ph.D.
Professor of Biochemistry & Molecular Biology
603 Wilson Rd., Rm. 513
Michigan State University
East Lansing, MI 48824-1319
Office: (517) 355-9724 Lab: (517) 353-9125
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The problem with crystallization is that is selects for the least soluble, most packable species. Sometimes that works against what you would like to know. That could include oligomerization state as well as conformational state. For example, some of the allosteric carbonic anhydrases stubbornly crystallize only in the T-state, despite crystallization conditions that are known to preferentially stabilize the R-state, and for which the predominant R-state population can be confirmed by other methods.
Cheers,
_______________________________________
Roger S. Rowlett
Gordon & Dorothy Kline Professor
Department of Chemistry
Colgate University
13 Oak Drive
Hamilton, NY 13346
tel: (315)-228-7245
ofc: (315)-228-7395
fax: (315)-228-7935
email: [log in to unmask]
On 4/8/2015 9:07 AM, Sebastiaan Werten wrote:
Dear all,
we are currently working on a protein that is known to exist in a monomer-dimer equilibrium. At the high concentrations used for crystallisation assays, the dimer is predominant and the monomer practically undetectable.
Nevertheless, one of the crystal forms that we have obtained contains the monomeric species, not the dimer.
I was wondering if anyone is aware of similar (published) cases, and if the phenomenon as such has been discussed in detail anywhere?
I did literature searches but so far couldn't find anything.
Any pointers would be much appreciated!
Best wishes,
Sebastiaan Werten.