The most important selective pressure of low temperatures is exerted towards chemical reaction rates, most of which exponentially drop with decreasing temperature. Enzymes are biological catalyst and involved in most of the chemical reaction in the cell, those are necessary for the cell survival. Adaptation of the cell to the low temperature calls for presence of intracellular enzymes, which are active at low temperature. These cold active enzymes are high catalytic efficiency (Kcat/Km) at low and moderate temperature (0-30ºC) at which homologous enzymes produced by microorganism from other thermal classes are poorly active or not active at all. In addition, these enzymes are generally thermolable; their activity is shifted towards low temperature. The commonly accepted hypothesis for this cold adaptation is the activity-stability-flexibility relationship, which suggests that psycrophilic enzymes increase the flexibility of their structure to cope freezing effect of cold habitats.
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 cold adapted micro-organisms. 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. Characterization of β-galactosidase obtained from psycrotolerant strain of Arthrobactor from Antarctic dry - valley soil showed temperature optima near 18 ºC and it remained 50% activity at 0ºC. This was 2.1 and 5 times more active then β-galactosidase from E. coli at 20ºC and 10ºC, respectively. Comparison between β-galactosidase of this Arthrobacter strain with another β-galactosidase from psycrotolerant Arthrobacter psycrolactophilus that temperature optima around 40ºC, revealed that except the decrease in proline residues, most of the criteria for structural features, believed so far to confer cold stability and cold active nature of the enzyme were not satisfied. Again most of the trends suggested for cold actives 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 25ºC into the inactive monomers.
Structural feature contributing to thermostability of glycerol hydrolases, revealed that distribution of hydrogen bonds, ion pairs and amino acid composition are gross structural basis of thermostability. These structural compositors do not always help in generalizing the structural basis of adaptation of enzymes activities at different extremities of temperature. Thermostability or cold active nature of enzymes could be explained by synergistic and co-operative intramolecular interaction (With compatable solutes), which is largely remain unknown.
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