[phenixbb] PEG dependent structural changes in crystal structure

[shivendra singh]

Dear All,
I have been working on a protein which initially got crystallised in
condition having PEG1500 as precipitant. The space group was P21 and got
solved with reasonable Rfree. Analysis of its structure showed large
deviation and very distinct active site architecture along with
disorderedness in one of its long loop (no density) in comparison with the
expected result, based on related homologous structures. The structure does
not seem to be active with one of its active site residue moved apart from
other catalytic amino acids. Also the substrate entry tunnel looks
distorted. The purified enzyme used for crystallisation showed optimum
activity in vitro. This led us to screen it again for some other
crystallisation condition and got another crystal hit in condition having
PEG3350 as precipitant. Rest of the components of crystallisation cocktail
were same. The data belonged to P212121 space group. The regions which were
disordered or distorted in earlier case were observed to be ordered and in
their expected orientation and position. The enzyme is not reported to be
in different structural or functional states as observed.
I am wondering how the protein from the same batch showed two distinct
structural organizations in conditions with varying PEGs.
What may cause it to follow such transition.
Whether it has some significant functional aspect or just a result of
improper crystal packing.

Thanks.

Shiv
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