Xylanases are hydrolytic enzymes which randomly cleave the beta 1,4 backbone of the complex plant cell wall polysaccharide xylan. Diverse forms of these enzymes exist, displaying varying folds, mechanisms of action, substrate specificities, hydrolytic activities (yields, rates and products) and physicochemical characteristics. Research has mainly focused on only two of the xylanase containing glycoside hydrolase families, namely families 10 and 11, yet enzymes with xylanase activity belonging to families 5, 7, 8 and 43 have also been identified and studied, albeit to a lesser extent. Driven by industrial demands for enzymes that can operate under process conditions, a number of extremophilic xylanases have been isolated, in particular those from thermophiles, alkaliphiles and acidiphiles, while little attention has been paid to cold-adapted xylanases. Here, the diverse physicochemical and functional characteristics, as well as the folds and mechanisms of action of all six xylanase containing families will be discussed. The adaptation strategies of the extremophilic xylanases isolated to date and the potential industrial applications of these enzymes will also be presented.

More than three-quarters of the Earth's surface is occupied by cold ecosystems, including the ocean depths, and polar and alpine regions. These permanently cold environments have been successfully colonized by a class of extremophilic microorganisms that are known as psychrophiles (which literally means cold-loving). The ability to thrive at temperatures that are close to, or below, the freezing point of water requires a vast array of adaptations to maintain the metabolic rates and sustained growth compatible with life in these severe environmental conditions.

The ability of psychrophiles to survive and proliferate at low temperatures implies that they have overcome key barriers inherent to permanently cold environments. These challenges include: reduced enzyme activity; decreased membrane fluidity; altered transport of nutrients and waste products; decreased rates of transcription, translation and cell division; protein cold-denaturation; inappropriate protein folding; and intracellular ice formation. Cold-adapted organisms have successfully evolved features, genotypic and/or phenotypic, to surmount the negative effects of low temperatures and to enable growth in these extreme environments. In this review, we discuss the current knowledge of these adaptations as gained from extensive biochemical and biophysical studies and also from genomics and proteomics.

Psychrophilic, mesophilic, and thermophilic ␣-amylases have been studied as regards their conformational stability, heat inactivation, irreversible unfolding, activation parameters of the reaction, properties of the enzyme in complex with a transition state analog, and structural permeability. These data allowed us to propose an energy landscape for a family of extremophilic enzymes based on the folding funnel model, integrating the main differences in conformational energy, cooperativity of protein unfolding, and temperature dependence of the activity. In particular, the shape of the funnel bottom, which depicts the stability of the native state ensemble, also accounts for the thermodynamic parameters of activation that characterize these extremophilic enzymes, therefore providing a rational basis for stability-activity relationships in protein adaptation to extreme temperatures.Our planet harbors a huge number of harsh environments that are considered as "extreme" from an anthropocentric point of view, as far as temperature, pH, osmolarity, free water, or pressure are concerned. Nevertheless, these peculiar biotopes have been successfully colonized by numerous organisms, mainly extremophilic bacteria and archaea. As the curiosity of scientists stimulates the exploration of new environments, it seems that there is no "empty space" for life on Earth and, for instance, even the supercooled cloud droplets contain actively growing bacteria (1). Among the extremophilic microorganisms, those living at extreme temperatures have attracted much attention. Thermophiles have revealed the unsuspected upper temperature for life at about 113°C (2, 3). Their enzymes have also demonstrated a considerable biotechnological potential such as the various thermostable DNA polymerases used in PCR that have boosted many laboratory techniques. At the other end of the temperature scale, metabolically active psychrophilic bacteria have been detected in liquid brine veins of sea ice at Ϫ20°C (4). These cold-loving microorganisms face the thermodynamic challenge to maintain enzyme-catalyzed reactions and metabolic rates compatible with sustained growth near or below the freezing point of pure water (5, 6). Directed evolution experiments have highlighted that, in theory, cold activity of enzymes can be gained by several subtle adjustments of the protein structure (7). However, in natural cold environments, the consensus for the adaptive strategy is to take advantage of the lack of selective pressure for stable proteins for losing stability, therefore making the enzyme more mobile or flexible at temperatures that "freeze" molecular motions and reaction rates (8).The crystal structures of extremophilic enzymes unambiguously indicate a continuum in the molecular adaptations to temperature. There is indeed a clear increase in the number and strength of all known weak interactions and structural factors involved in protein stability from psychrophiles to mesophiles (living at intermediate temperatures close to 37°C) and to thermophiles (2, 9 -1...

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