NMR studies on mechanism of isomerisation of fructose 6-phosphate to glucose 6-phosphate catalysed by phosphoglucose isomerase from Thermococcus kodakarensis

Paradigm shift in xylose isomerase usage: a novel scenario with distinct applications

Isomerases are enzymes that induce physical changes in a molecule without affecting the original molecular formula. Among this class of enzymes, xylose isomerases (XIs) are the most studied to date, partly due to their extensive application in industrial processes to produce high-fructose corn sirups. In recent years, the need for sustainable initiatives has triggered efforts to improve the biobased economy through the use of renewable raw materials. In this context, D-xylose usage is crucial as it is the second-most abundant sugar in nature. The application of XIs in biotransforming xylose, enabling downstream metabolism in several microorganisms, is a smart strategy for ensuring a low-carbon footprint and producing several value-added biochemicals with broad industrial applications such as in the food, cosmetics, pharmaceutical, and polymer industries. Considering recent advancements that have expanded the range of applications of XIs, this review provides a comprehensive and concise overview of XIs, from their primary sources to the biochemical and structural features that influence their mechanisms of action. This comprehensive review may help address the challenges involved in XI applications in different industries and facilitate the exploitation of xylose bioprocesses.

TM1385 from Thermotoga maritima functions as a phosphoglucose isomerase via cis-enediol-based mechanism with active site redundancy

Phosphoglucose isomerases (PGIs) belong to a class of enzymes that catalyze the reversible isomerization of glucose-6-phosphate to fructose-6-phosphate. PGIs are crucial in glycolysis and gluconeogenesis pathways and proposed as serving additional extracellular functions in eukaryotic organisms. The phosphoglucose isomerase function of TM1385, a previously uncharacterized protein from Thermotoga maritima, was hypothesized based on structural similarity to established PGI crystal structures and computational docking. Kinetic and colorimetric assays combined with ¹H nuclear magnetic resonance (NMR) spectroscopy experimentally confirm that TM1385 is a phosphoglucose isomerase (TmPGI). Evidence of solvent exchange in ¹H NMR spectra supports that TmPGI isomerization proceeds through a cis-enediol-based mechanism. To determine which amino acid residues are critical for TmPGI catalysis, putative active site residues were mutated with alanine and screened for activity. Results support that E281 is most important for TmPGI formation of the cis-enediol intermediate, and the presence of either H310 or K422 may be required for catalysis, similar to previous observations from homologous PGIs. However, only TmPGI E281A/Q415A and H310A/K422A double mutations abolished activity, suggesting that there are redundant catalytic residues, and Q415 may participate in sugar phosphate isomerization upon E281 mutation. Combined, we propose that TmPGI E281 participates directly in the cis-enediol intermediate step, and either H310 or K422 may facilitate sugar ring opening and closure.

Phosphotungstic acid heterogenized by assembly with pyridines for efficient catalytic conversion of fructose to methyl levulinate

Solid acid-catalyzed sugar degradation has been considered to be an efficient approach to synthesize alkyl levulinates (which can be used as fuel additives and surfactants). However, those catalytic processes typically involve harsh reaction conditions and high cost for catalyst preparation. We prepared a series of phosphotungstic acid organic hybrids by a simple solvothermal method, and used them as heterogeneous catalysts for the selective degradation of fructose to methyl levulinate (ML) in methanol with high efficiency under mild reaction conditions. The catalysts were characterized systematically, and the effects of different substituents in pyridine, reaction temperature/time, catalyst dose, and fructose concentration studied. The 3-FPYPW hybrid prepared from 3-fluoropyridine and phosphotungstic acid exhibited superior catalytic activity for the synthesis of ML (82.5%) from fructose (97.8%). A possible reaction pathway was proposed. In addition, the catalyst could be separated from the reaction mixture readily, and reused without remarkable loss of reactivity.

A high yield of methyl levulinate (82.5%) was achieved from fructose via a one-pot multi-step conversion process using acidic 3-FPYPW as a heterogeneous catalyst.