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.