ENZYMES FROM COLD-ADAPTED MICROORGANISMS - THE CLASS-C BETA-LACTAMASEFROM THE ANTARCTIC PSYCHROPHILE PSYCHROBACTER-IMMOBILIS A5

A heat-labile beta-lactamase has been purified from culture supernatan ts of Psychrobacter immobilis A5 grown at 4 degrees C and the correspo nding chromosomal ampC gene has been cloned and sequenced. All structu ral and kinetic properties clearly relate this enzyme to class C beta- lactamases. The kinetic parameters of P. immobilis beta-lactamase for the hydrolysis of some p-lactam antibiotics are in the same range as t he values recorded for the highly specialized cephalosporinases from p athogenic mesophilic bacteria. By contrast, the enzyme displays a low apparent optimum temperature of activity and a reduced thermal stabili ty. Structural factors responsible for the latter property were analys ed from the three-dimensional structure built by homology modelling. T he deletion of proline residues in loops, the low number of arginine-m ediated H-bonds and aromatic-aromatic interactions, the lower global h ydrophobicity and the improved solvent interactions through additional surface acidic residues appear to be the main determinants of the enz yme flexibility.