STRUCTURAL ADAPTATIONS OF THE COLD-ACTIVE CITRATE SYNTHASE FROM AN ANTARCTIC BACTERIUM

Background: The structural basis of adaptation of enzymes to low tempe rature is poorly understood, Dimeric citrate synthase has been used as a model enzyme to study the structural basis of thermostability, the structure of the enzyme from organisms living in habitats at 55 degree s C and 100 degrees C having previously been determined, Here the stud y is extended to include a citrate synthase from an Antarctic bacteriu m, allowing us to explore the structural basis of cold activity and th ermostability across the whole temperature range over which life is kn own to exist. Results: We report here the first crystal structure of a cold-active enzyme, citrate synthase, isolated from an Antarctic bact erium, at a resolution of 2.09 Angstrom. In comparison with the same e nzyme from a hyperthermophilic host, the cold-active enzyme has a much more accessible active site, an unusual electrostatic potential distr ibution and an increased relative flexibility of the small domain comp ared to the large domain, Several other features of the cold-active en zyme were also identified: reduced subunit interface interactions with no intersubunit ion-pair networks; loops of increased length carrying more charge and fewer proline residues; an increase in solvent-expose d hydrophobic residues; and an increase in intramolecular ion pairs. C onclusions: Enzymes from organisms living at the temperature extremes of life need to avoid hot or cold denaturation yet maintain sufficient structural integrity to allow catalytic efficiency, For hyperthermoph iles, thermal denaturation of the citrate synthase dimer appears to be resisted by complex networks of ion pairs at the dimer interface, a f eature common to other hyperthermophilic proteins. For the cold-active citrate synthase, cold denaturation appears to be resisted by an incr ease in intramolecular ion pairs compared to the hyperthermophilic enz yme, Catalytic efficiency of the cold-active enzyme appears to be achi eved by a more accessible active site and by an increase in the relati ve flexibility of the small domain compared to the large domain.