Towards a Molecular Understanding of Cold-activity of
Enzymes from Psychrophiles
Nicholas J. RUSSBLL*
Microbiology Laboratories, Department of Biological Sciences, Wye College University of London, Wye, Ashford, Kent TN25 5AH, UK
Considering the facts that the greater proportion of the planet on which we live is cold, rarely rising above 5℃, and most of the biota are adapted to grow at temperatures well below what is often considered as "normal" (i.e. 37℃), it is surprising that we know relatively less about the molecular basis of cold-activity of enzymes from psychrophilic, compared with mesophilic or thermophilic organisms
(1). However, recently, there has been a sharp increase in research effort and productivity in this field, with the realization that such cold-active enzymes offer novel opportunities for biotechnological exploitation (2).
Thus, following cloning of the genes for several cold-active enzymes (1-3), there has been the landmark f1rst crystallization of a cold-active protein (4). On the basis of homology models of proteins derived from the gene Sequences it has become apparent that those structural modifications which confer cold-activity are not merely the opposite of those which give thermostability: for example, in triose phosphate isomerase the thermophilic enzyme has modifications to the body of α-helices which stabilize the helix dipole, whereas the psychrophilic enzyme has altered helix-capping residues (5). Another feature which has become apparent from the homology models is that, amongst the various cold-active enzymes there are diferent structural features used to achieve the same mechanistic goal of maintaining structural flexibility and catalytic activity at low temperatures (1,3)-reminiscent of the different strategies used by thermostable proteins from thermophiles. The psychrophilic adaptations include: more polar and less hydrophobic residues; additonal glycine residues and low arginine/lysine ratio; fewer hydrogens bonds, aromatic interactions and ion pairs; lack of salt bridges; additional surface loop(s) with decreased proline content; and reduced hydrophobic interactions between subunits. No cold-active enzyme displays all these features: each has a suite of changes (relative to mesophilic/thermophilic counterparts) which confer the necessary conformational f1exibility to the active site, but at the expense of activity at higher temperatures. Therefore, cold_active enzymes are generally more thermolabile, although there are examples of enzymes from psychrophiles with remarkably wide thermal ranges of activity.
With the crystallization of the psychrophilic (c-amylase (4) and more recently citrate synthase (6), there will now become available direct structural data from X-ray
