comprise of continuous repeat of a consensus octapeptide (Ala-Gly-Thr-Gly-SerThr-Leu-Thr) and function as templates for the formation of small ice crystal seeds
termed “ice nuclei.” This facilitates ice formation at high subzero temperature, while
“ice minus” bacteria do not possess Ina proteins and lower the ice nucleation
temperature. Turner et al. (1991) have classified ice nucleating protein into three
chemically distinct classes depending on A, B, and C structure. The class C structure
was composed of aggregates of ice-nucleating protein (INP). The class B structure
was a glycoprotein with sugar residue including glucose, mannose, etc., attached to
the protein core and the class A structure was a lipoglycoprotein that was covalently
anchored to the cell surface via a mannose-PI (phosphatidylinositol) that is similar to
the anchoring of many proteins to cell membranes of eukaryotic cells (Kozloff et al.
1991).
6.13.3 Antifreeze Proteins
In contrast, another strategy used by organism to survive freezing temperature is the
production of anti-nucleating proteins or antifreeze proteins (AFPs). These are
structurally diverse group of proteins that have the ability to modify ice crystal
structure. Unlike the widely used automotive antifreeze (ethylene glycol), AFPs do
not lower freezing point in proportion to concentration. Rather, they work in a
non-colligative manner. This allows them to act as antifreeze at concentrations
300–500 times lower than the other dissolved solutes. This minimizes their effect
on osmotic pressure (Fletcher et al. 2001). The unusual capabilities of AFPs are
attributed to their binding ability at specific ice crystal surfaces (Jorov et al. 2004).
AFPs create a difference between the melting point and freezing point known as
thermal hysteresis (TH), which is a measure of the antifreeze activity. The addition
of AFPs at the interface between solid ice and liquid water inhibits the thermodynamically favored growth of the ice crystal. Ice growth is kinetically inhibited by the
AFPs covering the water-accessible surfaces of ice (Jorov et al. 2004). This mechanism is also known as freeze avoidance. Besides causing TH, the AFPs act by
another mechanism called freeze tolerance. In most of the freezing conditions,
formation of ice takes place as a multicrystalline mass. Growth of large ice crystals
occurs at the expense of smaller crystals, a phenomenon termed ice recrystallization.
AFPs limit the growth of ice crystals at subzero temperatures by being adsorbed on
the ice surface (Fig. 6.2). This mechanism is known as recrystallization inhibition
(RI) (Duman 2001).
Several bacteria have been reported to exhibit antifreeze activity. These include
Pseudomonas putida GR12-2, which was originally isolated from the high arctic
Canadian soil, Rhodococcus erythropolis, isolated from the midguts of beetle larvae,
Micrococcus cryophilus, isolated from chilled sausages, a Moraxella sp. isolated
from Antarctic soils, an additional 11 γ- and α-proteobacteria isolated from Antarctic
lakes and Antarctic strain Flavobacterium xanthum (Muryoi et al. 2004; Kawahara
et al. 2007).
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termed “ice nuclei.” This facilitates ice formation at high subzero temperature, while
“ice minus” bacteria do not possess Ina proteins and lower the ice nucleation
temperature. Turner et al. (1991) have classified ice nucleating protein into three
chemically distinct classes depending on A, B, and C structure. The class C structure
was composed of aggregates of ice-nucleating protein (INP). The class B structure
was a glycoprotein with sugar residue including glucose, mannose, etc., attached to
the protein core and the class A structure was a lipoglycoprotein that was covalently
anchored to the cell surface via a mannose-PI (phosphatidylinositol) that is similar to
the anchoring of many proteins to cell membranes of eukaryotic cells (Kozloff et al.
1991).
6.13.3 Antifreeze Proteins
In contrast, another strategy used by organism to survive freezing temperature is the
production of anti-nucleating proteins or antifreeze proteins (AFPs). These are
structurally diverse group of proteins that have the ability to modify ice crystal
structure. Unlike the widely used automotive antifreeze (ethylene glycol), AFPs do
not lower freezing point in proportion to concentration. Rather, they work in a
non-colligative manner. This allows them to act as antifreeze at concentrations
300–500 times lower than the other dissolved solutes. This minimizes their effect
on osmotic pressure (Fletcher et al. 2001). The unusual capabilities of AFPs are
attributed to their binding ability at specific ice crystal surfaces (Jorov et al. 2004).
AFPs create a difference between the melting point and freezing point known as
thermal hysteresis (TH), which is a measure of the antifreeze activity. The addition
of AFPs at the interface between solid ice and liquid water inhibits the thermodynamically favored growth of the ice crystal. Ice growth is kinetically inhibited by the
AFPs covering the water-accessible surfaces of ice (Jorov et al. 2004). This mechanism is also known as freeze avoidance. Besides causing TH, the AFPs act by
another mechanism called freeze tolerance. In most of the freezing conditions,
formation of ice takes place as a multicrystalline mass. Growth of large ice crystals
occurs at the expense of smaller crystals, a phenomenon termed ice recrystallization.
AFPs limit the growth of ice crystals at subzero temperatures by being adsorbed on
the ice surface (Fig. 6.2). This mechanism is known as recrystallization inhibition
(RI) (Duman 2001).
Several bacteria have been reported to exhibit antifreeze activity. These include
Pseudomonas putida GR12-2, which was originally isolated from the high arctic
Canadian soil, Rhodococcus erythropolis, isolated from the midguts of beetle larvae,
Micrococcus cryophilus, isolated from chilled sausages, a Moraxella sp. isolated
from Antarctic soils, an additional 11 γ- and α-proteobacteria isolated from Antarctic
lakes and Antarctic strain Flavobacterium xanthum (Muryoi et al. 2004; Kawahara
et al. 2007).
206
P. K. Mishra et al.