|
Thermal unfolding
of Sso7d from the hyperthermophile Sulfolobus
Solfataricus.
Stefan knapp
Karolinska institutet CSB NOVUM 14157 Huddinge Sweden
Sso7d is a small basic
histone like protein which is highly expressed
in the Archaeon Sulfolobus solfataricus. The protein has a SH3
(src-homology region 3) domain like fold and its secondary
structure
consists almost entirely of beta sheets. The thermal unfolding of
Sso7d was studied by circular dichroism spectroscopy and
differential
scanning calorimetry. The unfolding transition of Sso7d can be
described by a reversible two state process. Maximum stability
was
observed in the region between pH 4.5 and 7.0 where Sso7d unfolds
with
a melting temperature between 370 and 371.9 K an unfolding
enthalpy
between 62.9 and 65.4 kcal/mol and a heat capacity change of 620
cal/(mol K). The thermodynamic reason for the high melting
temperature
is a shallow stability curve with a broad stability maximum,
corresponding to the very small heat capacity change which was
obtained. The calculated stability curve of Sso7d has, despite of
its
high melting temperature, an only moderate intrinsic stability,
which
reaches its maximum of about 7 kcal/mol at 282 K. Sso7d is
particularly poorly stabilized (1 kcal/mol) at the maximal growth
temperature of Sulfolobus. A comparison of Sso7d with mesophilic
SH3
domains from the eucaryotic kinases Tec, Btk and Itk revealed
that SH3
domains of these proteins have at their physiological
temperatures
similar stabilization free energies as Sso7d at the optimal
growth
temperature of Sulfolobus. All SH3 domains are only weakly
stabilized
in terms of free energy of unfolding. The reason for the high
melting
temperature of Sso7d is a broadening and lifting of its stability
curve.
|
