MOLECULAR ADAPTATIONS OF ENZYMES FROM PSYCHROPHILIC ORGANISMS

The dominating adaptative character of enzymes from cold-evolving orga nisms is their high turnover number (k(cat)) and catalytic efficiency (k(cat)/K-m), which compensate for the reduction of chemical reaction rates inherent to low temperatures. This optimization of the catalytic parameters can originate from the highly flexible structure of these proteins providing enhanced abilities to undergo conformational change s during catalysis at low temperatures. Molecular modelling of the 3-D structure of cold-adapted enzymes reveals that only subtle modificati ons of their conformation can be related to the structural flexibility . The observed structural features include: 1) the reduction of the nu mber of weak interactions involved in the folded state stability like salt bridges, weakly polar interactions between aromatic side chains, hydrogen bonding, arginine content and charge-dipole interactions in a lpha-helices; 2) a lower hydrophobicity of the hydrophobic clusters fo rming the core of the protein; 3) deletion or substitution of proline residues in loops or turns connecting secondary structures; 4) improve d solvent interactions with a hydrophilic surface via additional charg ed side chains; 5) the occurence of glycine clusters close to function al domains; and 6) a looser coordination of Ca2+ ions. No general rule from the molecular changes observed; rather, each enzyme adopts its o wn strategy by using one or a combination of these altered interaction s. Enzymes from thermophiles reinforce the same type of interactions i ndicating that there is a continuity in the strategy of protein adapta tion to temperature. (C) 1997 Elsevier Science Inc.