Cold-adapted and mesophilic brachyurins.

Two different types of brachyurins, termed I and II, have been described in the literature. Within type I there are two subtypes, Ia and Ib. The prototype for the type I brachyurins is Fiddler crab collagenase I. Its cold-adapted analogue from Antarctic krill, termed euphaulysin, shares many of its characteristics. Both enzymes are distinguished by their broad substrate specificity as well as the ability to cleave collagen. The precursor form of euphaulysin has been expressed in Pichia pastoris and processed to its fully active form using cod trypsin. A molecular model of euphaulysin, based on the known crystal structure of crab collagenase I, indicates that the core structure of these enzymes is almost identical. As a cold-adapted enzyme, euphaulysin has a higher catalytic efficiency than crab collagenase I. It is also more sensitive to thermal inactivation and autolysis. Furthermore, euphaulysin has an increased length of several surface loops compared to crab collagenase I. Extended surface loops have been suggested to play a role in the cold activity of some bacterial enzymes. Sensitivity to autolysis is an important factor which contributes to the thermal instability of euphaulysin. Substitution of a highly exposed residue in the 'autolysis loop' of euphaulysin resulted in an increased stability of the enzyme towards thermal inactivation without altering its catalytic efficiency.