Biomedical Engineering Reference
In-Depth Information
The role of electrostatic interactions and particularly salt bridges in the stabilization
of protein and its interactions with other molecules is widely investigated (Sheinerman
and Honig, 1999; Sindler et al., 1998; Xiao and Honig, 1999; Nohaile et al., 2001). As
an illustration, it was shown that a barnase and its intracellular inhibitor barstar
association rate constant of is increased to over by electrostatic
forces (Schreiber and Fersht, 1994). The importance of buried salt bridges in the stability
of protein was demonstrated by the example of the unfolding of barnase (Tissit et al.,
1996). Replacing the Asp residues in the bridges Arg-69-Asp-93 and Arg-83-Asp-75 led
to lowering the enzyme stability by up to 5.4 kcal/mol. Nevertheless theoretical
calculations and experiments indicate that hydrophobic interactions are more stabilizing
than salt bridges in protein folding (Sindler et al., 1998). The loss of stability is related to
a substantial reduction in the degeneracy of the lowest-energy state.
Other factors also affect protein stability and hyperstability (Vetriani et al, 1998;
Jaenicke, 1996, 1998, 2000; Daniel and Danson, 2001). These include the formation of a
network of surface ionic pairs, hydrogen bonding, local interactions, the stabilization of
polypeptides helices (the packing and docking of domains, association of subunits,
conjugation with prosthetic groups and carbohydrate moieties, etc).
4.3. Enzymes from extreme thermophilic bacteria.
4.3.1. OVERVIEW
In recent years, increasing attention has been focused on proteins derived from extreme
thermophylic bacteria (Daniel and Cowan, 2000; Vetriani et al., 1998; Jaenicke, 1996;
1998, 2000; Adams and Kelly, 2001; and references therein). The increasing use of these
proteins in biotechnology has given new impetus to studies focused on their structure
and stability. At the same time, thermostable proteins prove challenging as the ideal
candidates for investigating the relationships between the structure and intramolecular
dynamics of the enzyme on the one hand, and their function and stability on the other.
Proteins isolated from thermophylic and especially from hyperthermophylic micro-
organisms are unusually stable with respect to high temperatures, organic solvents and
detergents (Nucci et al. 1993; Britton et al., 1999; Daniel and Cowan, 2000; Jaenicke,
1998, 2000; D'Auria et al., 1999). A series of homologous proteins and enzymes with
widely different stabilities was shown to be similar in sequence, subunit composition,
and enzymatic activity, e.g. the nature of the catalytic group of the active site, the
chemical mechanism of the reaction and specificity. Other than exhibiting high stability,
these enzymes also exhibit very poor catalytic behavior at ambient temperature, although
they are dramatically activated at high temperatures above 50°C and can reach maximum
activity at 80-90 °C and even 115-120 °C under effect of pressure (200- 500 atm)
(D'Auria 1999; Sun and Clark, 2001).
A number of challenging problems regarding the physico-chemical molecular level
are posed to biochemists and biophysicists, i.e. (1) the specificity of the protein
intramolecular structure giving such high thermostability and resistance to outer
effectors; (2) the physical reasons for such poor catalytic activity at ambient temperature,
Search WWH ::
Custom Search