A wide range of molecular determinants confer conformation flexibility of
enzymes. These determinants involved in protein stability are either reduced in the
number or modified to increase flexibility and to reduce rigidity in protein of coldadapted microorganisms. These determinants include changes in the frequency of
particular molecular bonds (fewer ion pairs, arginine-mediated hydrogen bond, and
aromatic interactions) and amino acid side chains (more polar and less hydrophobic
residues, a decrease in proline residues in loops, a reduction in arginine residues, or
low arginine/lysine ratio), increased interaction with solvent (water and associated
ions), reduced hydrophobic interactions between subunit and lose anchoring of
N and C termini (Feller and Gerday 1997; Russell 2000; Sheridan et al. 2000;
Marx et al. 2004; Margesin et al. 2005a). Obviously, no cold-active enzymes display
all these features; the strategy can be differerent from enzyme to enzyme (Margesin
et al. 2007). Characterization of β-galactosidase obtained from psychrotolerant strain
of Arthrobacter from Antarctic dry-valley soil showed temperature optima near
18
C and it remained 50% active at 0
C. It was also found 2.1 and 5 times more
active than β-galactosidase from E. coli at 20 and 10
C, respectively. Comparison
between β-galactosidase of this Arthrobacter strain with another β-galactosidase
(temperature optima around 40
C) from psychrotolerant Arthrobacter
psychrolactophilus revealed that except the decrease in proline residues, most of
the criteria for structural features believed so far to confer cold stability and coldactive nature of the enzyme were not satisfied. Again, most of the trends suggested
for cold-active enzymes were not found, when the amino acid composition of the
cold-active β-galactosidase was compared to E. coli β-galactosidase. The thermolability of the enzymes was explained by the fact that it was a tetramer, which
dissociated at 25
C into the inactive monomers (Coker et al. 2003). Therefore, the
above study indicates that cold stability of enzyme differed from bacterial strain to
strain.
Structural feature contributing to gross thermostability structure of glycerol
hydrolases are distribution of hydrogen bonds, ion pairs, and amino acid composition (Panasik et al. 2000). These structural features do not always help in
generalizing the structural basis of adaptation of enzyme activities at different
extremes of temperature. Thermostability or cold-active nature of enzymes could
be explained by synergistic and co-operative intramolecular interaction with compatible solutes, viz. sugars and amino acid (Wintrode et al. 2000; Zartler et al. 2001).
Recently, Sundareswaran et al. (2010) reported that CSM2, a cold-sensitive mutant
of psychrophilic Pseudomonas syringae, grows like wild-type cells when cultured at
22 and 28
C. But the growth is retarded at 4
C. In CSM2, AAT (aspartate aminotransferase) is identified as the mutated gene. The expression of AAT in Pseudomonas syringae was transiently enhanced when cells were shifted from 22 to 4
C
indicating that AAT is cold-inducible. Complementation of the mutated AAT
transformed CSM2 from a cold-sensitive phenotype to a cold-resistant phenotype
like the wild-type cells. This finding indicates the importance of aspartate aminotransferase enzyme in the growth of psychrophilic bacteria at low temperatures.
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P. K. Mishra et al.
enzymes. These determinants involved in protein stability are either reduced in the
number or modified to increase flexibility and to reduce rigidity in protein of coldadapted microorganisms. These determinants include changes in the frequency of
particular molecular bonds (fewer ion pairs, arginine-mediated hydrogen bond, and
aromatic interactions) and amino acid side chains (more polar and less hydrophobic
residues, a decrease in proline residues in loops, a reduction in arginine residues, or
low arginine/lysine ratio), increased interaction with solvent (water and associated
ions), reduced hydrophobic interactions between subunit and lose anchoring of
N and C termini (Feller and Gerday 1997; Russell 2000; Sheridan et al. 2000;
Marx et al. 2004; Margesin et al. 2005a). Obviously, no cold-active enzymes display
all these features; the strategy can be differerent from enzyme to enzyme (Margesin
et al. 2007). Characterization of β-galactosidase obtained from psychrotolerant strain
of Arthrobacter from Antarctic dry-valley soil showed temperature optima near
18
C and it remained 50% active at 0
C. It was also found 2.1 and 5 times more
active than β-galactosidase from E. coli at 20 and 10
C, respectively. Comparison
between β-galactosidase of this Arthrobacter strain with another β-galactosidase
(temperature optima around 40
C) from psychrotolerant Arthrobacter
psychrolactophilus revealed that except the decrease in proline residues, most of
the criteria for structural features believed so far to confer cold stability and coldactive nature of the enzyme were not satisfied. Again, most of the trends suggested
for cold-active enzymes were not found, when the amino acid composition of the
cold-active β-galactosidase was compared to E. coli β-galactosidase. The thermolability of the enzymes was explained by the fact that it was a tetramer, which
dissociated at 25
C into the inactive monomers (Coker et al. 2003). Therefore, the
above study indicates that cold stability of enzyme differed from bacterial strain to
strain.
Structural feature contributing to gross thermostability structure of glycerol
hydrolases are distribution of hydrogen bonds, ion pairs, and amino acid composition (Panasik et al. 2000). These structural features do not always help in
generalizing the structural basis of adaptation of enzyme activities at different
extremes of temperature. Thermostability or cold-active nature of enzymes could
be explained by synergistic and co-operative intramolecular interaction with compatible solutes, viz. sugars and amino acid (Wintrode et al. 2000; Zartler et al. 2001).
Recently, Sundareswaran et al. (2010) reported that CSM2, a cold-sensitive mutant
of psychrophilic Pseudomonas syringae, grows like wild-type cells when cultured at
22 and 28
C. But the growth is retarded at 4
C. In CSM2, AAT (aspartate aminotransferase) is identified as the mutated gene. The expression of AAT in Pseudomonas syringae was transiently enhanced when cells were shifted from 22 to 4
C
indicating that AAT is cold-inducible. Complementation of the mutated AAT
transformed CSM2 from a cold-sensitive phenotype to a cold-resistant phenotype
like the wild-type cells. This finding indicates the importance of aspartate aminotransferase enzyme in the growth of psychrophilic bacteria at low temperatures.
196
P. K. Mishra et al.