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
From: CCP4 bulletin board [mailto:[email protected]] On Behalf
Of R. M. Garavito
Sent: Wednesday, April 08, 2015 10:04 AM
To: [email protected]
Subject: Re: [ccp4bb] Crystallisation of a minority fraction monomers
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.
Regards,
Michael
****************************************************************
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
FAX: (517) 353-9334 Email: [email protected]
****************************************************************
On Apr 8, 2015, at 9:28 AM, Roger Rowlett <[email protected]> wrote:
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: [email protected]
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.
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