Extreme catalysts from low-temperature environments

Abstract

Cold-loving or psychrophilic organisms are widely distributed in nature as a large part of the earth's surface is at temperatures around 0°C. To maintain metabolic rates and to prosper in cold environments, these extremophilic organisms have developed a vast array of adaptations. One main adaptive strategy developed in order to cope with the reduction of chemical reaction rates induced by low temperatures is the synthesis of cold-adapted or psychrophilic enzymes. These enzymes are characterized by a high catalytic activity at low temperatures associated with a low thermal stability. A study of protein adaptation strategies suggests that the high activity of psychrophilic enzymes could be achieved by the destabilization of the active site, allowing the catalytic center to be more flexible at low temperatures, whereas other protein regions may be destabilized or as rigid as their mesophilic counterparts. Due to these particular properties, psychrophilic enzymes offer a high potential not only for fundamental research but also for biotechnological applications.[Key words: psychrophile, extremophiles, cold adaptation, enzyme kinetics, flexibility strategy]Life under low-temperature conditions was identified as early as 1840 by Hooker, who observed that algae were associated with sea ice. In 1887, Forster was the first who reported that microorganisms isolated from fish could grow well at 0°C (1). The term "psychrophilic" was first used in 1902 by Schmidt-Nielsen to describe such cold-adapted organisms (2). Psychrophile is defined as an organism, prokaryotic or eukaryotic, living permanently at temperatures close to the freezing point of water in thermal equilibrium with the medium. Thus psychrophiles are numerous, including a large range of species of gram-positive and gram-negative bacteria, yeast, algae, marine invertebrates, insects and polar fish, and are widely distributed (3). Psychrophiles have developed mechanisms of adaptation to temperature including a huge range of structural and physiological adjustments in order to cope with the deleterious effect of low temperatures. Indeed, they display metabolic fluxes at low temperatures that are more or less comparable to those exhibited by closely related mesophiles living at moderate temperatures (4-6). This is explained by the capability of these psychrophilic organisms to produce "cold-adapted" enzymes which are able to cope with the reduction of chemical reaction rates induced by low temperatures. However, most cellular adaptations to low temperatures and the underlying molecular mechanisms are not fully understood and are still being investigated. Moreover, a study of proteins and enzymes from cold-adapted organisms is not only useful in the understanding of some general processes related to the protein structure and function but also in protein folding investigations. In addition, cold-active and heat-labile psychrophilic enzymes possess an interesting biotechnological potential.

Thermodynamic and Structural Adaptation Differences between the Mesophilic and Psychrophilic Lactate Dehydrogenases

“…In our case, this restriction is more profound for the more flexible protein, cgLDH, as expected for the cold-adapted enzyme. 31,32 …”

Serine Hydroxymethyltransferase from the Cold Adapted Microorganism Psychromonas ingrahamii: A Low Temperature Active Enzyme with Broad Substrate Specificity

“…However, the relationships between activity, flexibility and stability still remain controversial. Overall, it seems that each psychrophilic enzyme adopts its own adaptive strategy [27–30]. We should mention that in a recent in silico analysis we have carried out on the structural adaptation of psychrophilic SHMTs, it turned out that a significant increase of frequency of flexible residues was consistently observed [12].…”

The effect of temperature adaptation on the ubiquitin-proteasome pathway in notothenioid fishes

“…It is unlikely that the extremely cold temperatures of Antarctic waters denature proteins given the long evolutionary time frame (11-14 million years) that notothenioid fishes have had to adapt to subzero temperatures and undergo appropriate modifications to maintain stability under natural conditions. There are numerous studies that have investigated the modifications of cold-adapted proteins that allow them to function at subzero temperatures (Jaenicke, 1990;Hoyoux et al, 2004;Siddiqui and Cavicchioli, 2006). While cold denaturation of proteins has been demonstrated in vitro by sub-zero temperatures, these studies were not conducted in a physiological system that accurately represents the conditions experienced by a protein in a cell.…”