Structural adaptation of enzymes to low temperatures

A systematic comparative analysis of 21 psychrophilic enzymes belonging to different structural families from prokaryotic and eukaryotic organisms is reported. The sequences of these enzymes were multiply aligned to 427 homol ogous proteins from mesophiles and thermophiles. The net flux of amino acid exchanges from meso/thermophilic to psychrophilic enzymes was measured, To assign the observed preferred exchanges to different structural environmen ts, such as secondary structure, solvent accessibility and subunit interfac es, homology modeling was utilized to predict the secondary structure and a ccessibility of amino acid residues for the psychrophilic enzymes for which no experimental three-dimensional structure is available. Our results show a clear tendency for the charged residues Arg and Glu to be replaced at ex posed sites on alpha -helices by Lys and Ala, respectively, in the directio n from 'hot' to 'cold' enzymes. Val is replaced by Ala at buried regions in alpha -helices, Compositional analysis of psychrophilic enzymes shows a si gnificant increase in Ala and Asn and a decrease in Arg at exposed sites, B uried sites in beta -strands tend to be depleted of Val. Possible implicati ons of the observed structural variations for protein stability and enginee ring are discussed.