Biomedical Engineering Reference
In-Depth Information
the peptide backbone detected approximately 43% of the time. It was concluded
from the study that AFGP8 has numerous structurally distinct degenerate minima,
and this set of low-energy minima was proposed to be important for antifreeze
activity. In particular, interconversion among multiple minima of AFGP8 can act
as a thermal reservoir that will retard the growth rate of the ice crystal locally where
it accumulates on the ice surface. This study overall suggests that conformational
flexibility to access a variety of low-energy minima states is critical for antifreeze
activity even though the Pro residues have an important role in stabilizing the
conformations adopted by the AFGP.
The self-diffusion of AFGP8 as a function of temperature was monitored by
NMR spectroscopy (243-303 K) in an independent study to evaluate if AFGPs
undergo any global structural rearrangements in the supercooled regime that could
be the cause for their function [
65
]. These results suggested that AFGP is more
hydrated (ca. 30% increase in the bead radius) than other globular proteins, but
showed no evidence for any major structural reorganization in order to function.
Nishiumra et al. have also detected the presence of hydrated forms of syAFGP3 with
water molecules by using cold spray ionization time-of-flight mass spectrometry
[
140
]. The involvement of the Thr residue in the hydration of AFGPs is supported
by recent studies that have demonstrated that the Thr g-methyl group is an important
determinant of both the carbohydrate orientation and the degree of hydration [
147
].
Substituting d-GalNAcThr with d-GalNAcSer affects the strength of hydrogen bond
involving the N-acetyl group and results in a significant increase in conformational
degrees of freedom as well as the orientation of the neighboring water molecules.
Consequently, the evolution of AFGPs with Thr in their structure, rather than Ser,
may be attributed to the contribution of Thr to hydrophobicity and hydration effects,
as well as the restricted conformational space of Thr glycosides compared with Ser
glycosides [
142
].
The helical content of AFGP in the liquid, supercooled and frozen states of
the solution film have been compared by attenuated total reflection (ATR)-FTIR
spectroscopy [
148
]. The results showed that the AFGP molecules do not show
any large conformational changes during supercooling whereas they change their
conformation and become significantly helical upon freezing. It was proposed that
the increase in helicity was to allow the AFGPs to increase the number of adsorption
sites and maximize the interaction with ice crystals.
A number of studies on the interaction of AFGPs with surfaces have been
carried out to probe the contributions of hydrophobic and hydrophilic interactions
to adsorption. These studies are not of direct relevance to the biological mechanism
of action of AFGPs, as these surfaces are models for an ice/vacuum interaction.
However, such models can provide insights into surface properties such as nucle-
ation events and surface roughening, and have been useful in the delineation of the
features required for the assembly and development of functional materials. Sarno
et al. examined the adsorption of AFGP8 onto mica and highly ordered pyrolytic
graphite (HOPG), which contains both hydrophilic and hydrophobic binding sites
(step edges or planes) [
149
]. The results showed that AFGP8 binds preferentially to
the hydrophilic step edges of the surface. It was further shown that AFGP8 deposited
