Two distinct catalytic pathways for GH43 xylanolytic enzymes unveiled by X-ray and QM/MM simulations

Xylanolytic enzymes from glycoside hydrolase family 43 (GH43) are involved in the breakdown of hemicellulose, the second most abundant carbohydrate in plants. Here, we kinetically and mechanistically describe the non-reducing-end xylose-releasing exo-oligoxylanase activity and report the crystal structure of a native GH43 Michaelis complex with its substrate prior to hydrolysis. Two distinct calcium-stabilized conformations of the active site xylosyl unit are found, suggesting two alternative catalytic routes. These results are confirmed by QM/MM simulations that unveil the complete hydrolysis mechanism and identify two possible reaction pathways, involving different transition state conformations for the cleavage of xylooligosaccharides. Such catalytic conformational promiscuity in glycosidases is related to the open architecture of the active site and thus might be extended to other exo-acting enzymes. These findings expand the current general model of catalytic mechanism of glycosidases, a main reaction in nature, and impact on our understanding about their interaction with substrates and inhibitors

Kontrollierte milde und moderate hypothermie

Controlled hypothermia is used as a therapeutic intervention to provide neuroprotection and (more recently) cardioprotection. The growing insight into the underlying pathophysiology of apoptosis and destructive processes at the cellular level, and the mechanisms underlying the protective effects of hypothermia, have led to improved application and to a widening of the range of potential indications. In many centres hypothermia has now become part of the standard therapy for post-anoxic coma in certain patients, but for other indications its use still remains controversial. The negative findings of some studies may be partly explained by inadequate protocols for the application of hypothermia and insufficient attention to the prevention of potential side effects. This review deals with some of the concepts underlying hypothermia-associated neuroprotection and cardioprotection, and discusses some potential clinical indications as well as reasons why some clinical trials may have produced conflicting results. Practical aspects such as methods to induce hypothermia, as well as the side effects of cooling are also discussed