Some like it cold: biocatalysis at low temperatures

Abstract

In the last few years, increased attention has been focused on a class of organisms called psychrophiles. These organisms, hosts of permanently cold habitats, often display metabolic fluxes more or less comparable to those exhibited by mesophilic organisms at moderate temperatures. Psychrophiles have evolved by producing, among other peculiarities, "cold-adapted" enzymes which have the properties to cope with the reduction of chemical reaction rates induced by low temperatures. Thermal compensation in these enzymes is reached, in most cases, through a high catalytic efficiency associated, however, with a low thermal stability. Thanks to recent advances provided by X-ray crystallography, structure modelling, protein engineering and biophysical studies, the adaptation strategies are beginning to be understood. The emerging picture suggests that psychrophilic enzymes are characterized by an improved flexibility of the structural components involved in the catalytic cycle, whereas other protein regions, if not implicated in catalysis, may be even more rigid than their mesophilic counterparts. Due to their attractive properties, i.e., a high specific activity and a low thermal stability, these enzymes constitute a tremendous potential for fundamental research and biotechnological applications.

Structure of Phenylalanine Hydroxylase from Colwellia psychrerythraea 34H, a Monomeric Cold Active Enzyme with Local Flexibility around the Active Site and High Overall Stability

“…Many enzymes from psychrophilic organisms show elevated K m levels for substrates and cofactors compared with mesophilic homologs, indicative of reduced affinity (49,50). Moreover, the K m values often increase further with increasing temperature (53). We observed a slight increase in both the K m (BH 4 ) and [S 0.5 ](L-Phe) of CpPAH when the assay temperature was raised from 10 to 25°C (Table 3).…”

“…The increasing availability of crystal structures from other psychrophilic organisms, notably bacteria, has further revealed that cold-adapted enzymes often show high flexibility and thermolability compared with their mesophilic and thermophilic counterparts (50,53,79). Most cold-adapted enzymes show a global rather uniform distribution of their flexibility throughout the three-dimensional structure (50).…”

“…Overall conformational flexibility in cold-adapted enzymes appears to be obtained by the following: (i) a lower number of prolines and arginines; (ii) more glycines; (iii) fewer hydrogen bonds and fewer ion pair interactions; (iv) higher numbers of serine and methionine; and (v) reduced hydrophobicity (particularly for internal residues) expressed as lower (Ile ϩ Leu)/ (Ile ϩ Leu ϩ Val) ratio; or (vi) density of charged surface residues (50,53,79). As can be seen in Table 5 (Figs.…”

Adaptation of model proteins from cold to hot environments involves continuous and small adjustments of average parameters related to amino acid composition

“…Several reviews summarized the properties of proteins isolated from thermophiles (Jaenicke and Zavodsky, 1990;Jaenicke, 1991;Adams, 1993;Vieille et al, 1996;Jaenicke and Böhm, 1998;Niehaus et al, 1999;Vieille and Zeikus, 2001;Sterner and Liebl, 2001) and psychrophiles Gerday et al 1997;Sanders et al, 2003;Georlette et al 2004;Siddiqui and Cavicchioli, 2006). Comprehensive studies on crystal structures of thermophilic proteins did not reveal unusual conformations specific to the source type (Petukhov et A c c e p t e d m a n u s c r i p t 5 al., 1997; Facchiano et al, 1998;Karshikoff and Ladenstein, 1998;Szilagyi and Zavodszky, 2000;Kumar et al, 2000).…”

“…Temperature adaptation of proteins is mostly relevant for the catalytic properties of the enzymes, as they must adapt the rate of the catalyzed reaction to the growth temperature of the organism. Indeed, thermophilicity studies revealed that psychrophiles synthesize cold-adapted enzymes endowed with a specific activity at low temperatures, significantly higher compared to that possessed by the mesophilic counterparts (Georlette et al, 2004;Siddiqui and Cavicchioli, 2006). On the other hand, the specific activity of hyperthermophilic enzymes reaches its optimum only at high temperatures, close to the optimum growth conditions of the source (Vieille et al, 1996;Vieille and Zeikus, 2001).…”