Xylanase from the psychrophilic yeast Cryptococcus adeliae

Abstract

A xylanase belonging to family 10 is produced by Cryptococcus adeliae, an Antarctic yeast that exhibits optimal growth at low temperature. The mature glycosylated xylanase secreted by C. adeliae is composed of 338 amino acid residues and 26 +/- 3 osidic residues, and shares 84% identity with its mesophilic counterpart from C. albidus. The xylanase from C. adeliae is less thermostable than its mesophilic homologue when the residual activities are compared, and this difference was confirmed by differential scanning calorimetry experiments. In the range 0 degrees-20 degrees C, the cold-adapted xylanase displays a lower activation energy and a higher catalytic efficiency. All these observations suggest a less compact, more flexible molecular structure. Analysis of computerized molecular models built up for both psychrophilic and mesophilic xylanases indicates that the adaptation to cold consists of discrete changes in the tridimensional structure: of 53 substitutions, 22 are presumably involved in the adaptation process. These changes lead mainly to a less compact hydrophobic packing, to the loss of one salt bridge, and to a destabilization of the macrodipoles of the helices.

Molecular basis of cold adaptation

“…The effect of temperature on chemical reactions is basically described by the Arrhenius equation: k = Ae ϪEa/RT , where k is the rate constant, A is the pre-exponential factor that depends on the reaction, enzyme with enhanced catalytic efficiency k cat /K m . For extracellular enzymes that work at saturating substrate concentrations, adaptation consists mainly of increasing k cat Narinx et al 1997;Petrescu et al 2000). Alternatively, for secreted marine enzymes or intracellular enzymes that could face low substrate concentrations, a decrease in K m (Michaelis-Menten constant) providing a higher substrate affinity could be useful (Kim et al 1999;Bentahir et al 2000;Georlette et al 2000;Hoyoux et al 2001).…”

Molecular adaptations to cold in psychrophilic enzymes

“…Psychrophiles were first referred to as cold-adapted bacteria [12]. However, if a psychrophile is defined as an organism living permanently at temperatures close to the freezing point of water in thermal equilibrium with the medium, such a definition includes de facto a large range of species: yeast [13][14][15][16], algae [17][18][19], marine invertebrates and insects or polar fish, which are the largest psychrophiles [20,21]. These examples underline that psychrophiles are numerous, taxonomically diverse and have a widespread distribution.…”

Flavobacterium frigidarium sp. nov., an aerobic, psychrophilic, xylanolytic and laminarinolytic bacterium from Antarctica.

“…In comparison to thermophiles, relatively few psychrophilic xylanolytic organisms have been described. The psychrophilic xylanolytic organisms that have been described include the anaerobic prokaryote Clostridium vincentii (Mounfort et al, 1997) ; the aerobic prokaryote Hymenobacter roseosalivarius (Hirsch et al, 1998) and the eukaryotes Cryptococcus TAE85 (Gerday et al, 1997) and Cryptococcus adeliae (Petrescu et al, 2000). The phenotypic and genotypic features of isolate A2i T suggest that it is a member of the genus Flavobacterium.…”