Bivalent cations and amino‐acid composition contribute to the thermostability of Bacillus licheniformis xylose isomerase

Comparative analysis of genome sequence data from mesophilic and hyperthermophilic micro‐organisms has revealed a strong bias against specific thermolabile amino‐acid residues (i.e. N and Q) in hyperthermophilic proteins. The N + Q content of class II xylose isomerases (XIs) from mesophiles, moderate thermophiles, and hyperthermophiles was examined. It was found to correlate inversely with the growth temperature of the source organism in all cases examined, except for the previously uncharacterized XI from Bacillus licheniformis DSM13 (BLXI), which had an N + Q content comparable to that of homologs from much more thermophilic sources. To determine whether BLXI behaves as a thermostable enzyme, it was expressed in Escherichia coli, and the thermostability and activity properties of the recombinant enzyme were studied. Indeed, it was optimally active at 70–72 °C, which is significantly higher than the optimal growth temperature (37 °C) of B. licheniformis. The kinetic properties of BLXI, determined at 60 °C with glucose and xylose as substrates, were comparable to those of other class II XIs. The stability of BLXI was dependent on the metallic cation present in its two metal‐binding sites. The enzyme thermostability increased in the order apoenzyme < Mg2+–enzyme < Co2+–enzyme ≈ Mn2+–enzyme, with melting temperatures of 50.3 °C, 53.3 °C, 73.4 °C, and 73.6 °C. BLXI inactivation was first‐order in all conditions examined. The energy of activation for irreversible inactivation was also strongly influenced by the metal present, ranging from 342 kJ·mol−1 (apoenzyme) to 604 kJ·mol−1 (Mg2+–enzyme) to 1166 kJ·mol−1 (Co2+–enzyme). These results suggest that the first irreversible event in BLXI unfolding is the release of one or both of its metals from the active site. Although N + Q content was an indicator of thermostability for class II XIs, this pattern may not hold for other sets of homologous enzymes. In fact, the extremely thermostable α‐amylase from B. licheniformis was found to have an average N + Q content compared with homologous enzymes from a variety of mesophilic and thermophilic sources. Thus, it would appear that protein thermostability is a function of more complex molecular determinants than amino‐acid content alone.