The mechanism of a one-substrate transketolase reaction

Transketolase catalyzes the transfer of a glycolaldehyde residue from ketose (the donor substrate) to aldose (the acceptor substrate). In the absence of aldose, transketolase catalyzes a one-substrate reaction that involves only ketose. The mechanism of this reaction is unknown. Here, we show that hydroxypyruvate serves as a substrate for the one-substrate reaction and, as well as with the xylulose-5-phosphate, the reaction product is erythrulose rather than glycolaldehyde. The amount of erythrulose released into the medium is equimolar to a double amount of the transformed substrate. This could only be the case if the glycol aldehyde formed by conversion of the first ketose molecule (the product of the first half reaction) remains bound to the enzyme, waiting for condensation with the second molecule of glycol aldehyde. Using mass spectrometry of catalytic intermediates and their subsequent fragmentation, we show here that interaction of the holotransketolase with hydroxypyruvate results in the equiprobable binding of the active glycolaldehyde to the thiazole ring of thiamine diphosphate and to the amino group of its aminopyrimidine ring. We also show that these two loci can accommodate simultaneously two glycolaldehyde molecules. It explains well their condensation without release into the medium, which we have shown earlier.

PEG-DOX, novel pH responsive prodrug nanoparticles

pH responsive prodrug nanoparticles (PEG-DOX) were prepared by attaching free Doxorubicin ( DOX) onto the amphipathic polyethylene glycol-aldehyde (PEG-CHO). The hydrophobic core of PEG-CHO enabled free DOX to be attached, while the hydrophilic outer layer of the carrier enabled the water solubility of the entire structure. This nanocarrier enabled a greater carrying capacity than free DOX, making its circulation time longer. The prodrug remained stable within neutral pH, ensuring its prolonged circulation time, but disassembled rapidly when reaching in the acidic environment of tumor tissues to release the free DOX. The newly designed nanocarriers have the potential to be applied clinically as a future DOX formulation in cancer chemotherapy.