Molecular adaptations of enzymes from psychrophilic organisms

The dominating adaptative character of enzymes from cold-evolving organisms 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 changes during catalysis at low temperatures. Molecular modelling of the 3-D structure of cold-adapted enzymes reveals that only subtle modifications of their conformation can be related to the structural flexibility. The observed structural features include: 1) the reduction of the number 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 alpha-helices; 2) a lower hydrophobicity of the hydrophobic clusters forming the core of the protein; 3) deletion or substitution of proline residues in loops or turns connecting secondary structures; 4) improved solvent interactions with a hydrophilic surface via additional charged side chains; 5) the occurence of glycine clusters close to functional domains; and 6) a looser coordination of Ca2+ ions. No general rule from the molecular changes observed; rather, each enzyme adopts its own strategy by using one or a combination of these altered interactions. Enzymes from thermophiles reinforce the same type of interactions indicating that there is a continuity in the strategy of protein adaptation to temperature. (C) 1997 Elsevier Science Inc.