A Biophysical Analysis of the Ocr Protein Gel
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Date
2007Author
Higham, Richard G
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Abstract
Ocr is unusual among proteins in its ability to form a transparent gel at high ammonium
sulphate concentrations. This transition was investigated using a combination of spectroscopic,
microscopic and rheological techniques. It occurs sharply at a concentration
of 3.2M ammonium sulphate and is not observed with other types of salt. Rheological
measurements showed that rather than precipitating under such conditions, ocr forms a
weak viscoelastic gel. Far UV circular dichroism spectra reveal that ocr does not denature
in the gel phase, while near UV CD spectra suggest the formation of long, helical
structures. Well resolved fibrils were observed using atomic force microscopy. They
were over 1µm in length and varied between 2.6nm to 10.4nm in height, corresponding
to the thickness and length of the ocr dimer.
Ocr is a highly charged protein (-56e at pH 8) and is shaped like a banana. We argue
that it is stabilized in specifically aggregated structures at large salt concentrations
by these physical properties. Electrostatic repulsions between proteins are screened by
salts, allowing proteins to approach close enough to aggregate. The charge on ocr is
high enough to resist such precipitation. However, at 3.2M ammonium sulphate we
suggest that the salt molecules bridge neighbouring ocr dimers via hydrogen bonds,
connecting amino acid carboxyl groups with the ammonium groups of the salt. The
banana-shaped dimers stack on top of each other, forming long helical fibrils that intertwine
into a semi flexible network.