Conjugation of Amine-Functionalized Polyesters With Dimethylcasein Using Microbial Transglutaminase

A self-cascade system based on Ag nanoparticle/single-walled carbon nanotube nanocomposites as an enzyme mimic for ultrasensitive detection of L-cysteine

L-Cysteine, widely used in medicine and the food industry, is of great essentiality to organisms and the food quality. Given that current detection approaches require exacting lab conditions and tedious sample treatment, there is a pressing demand for developing a method that possesses advantages of user friendliness, prominent performance, and cost-effectiveness. Herein, a self-cascade system was developed for the fluorescence detection of L-cysteine based on the ingenious performance of Ag nanoparticle/single-walled carbon nanotube nanocomposites (AgNP/SWCNTs) and DNA-templated Ag nanoclusters (DNA-AgNCs). The fluorescence of DNA-AgNCs could be quenched on account of the adsorption of DNA-AgNCs on AgNP/SWCNTs by π-π stacking. With the cooperation of Fe²⁺, AgNP/SWCNTs with oxidase and peroxidase-like activities could catalyze the oxidation of L-cysteine to produce cystine and hydrogen peroxide (H₂O₂) and then break the O-O bond of H₂O₂ to generate a hydroxyl radical (·OH), which could cleave the DNA strand into different sequence fragments which subsequently peeled off from the AgNP/SWCNTs, resulting in a "turn-on" fluorescence response. In this paper, AgNP/SWCNTs with multi-enzyme activities was synthesized enabling the reaction to proceed in just one step. The successful preliminary applications for the L-cysteine detection in pharmaceutical, juice beverage, and serum samples indicated that the developed method exhibited great potential in medical diagnosis, food monitoring, and the biochemical field, which also broadened the horizon for follow-up research.

Microbial Transglutaminase-Mediated Formation of Erythropoietin-Polyester Conjugates

Erythropoietin (EPO) is a glycoprotein hormone that has been used to treat anemia in patients with chronic kidney disease and in cancer patients who are receiving chemotherapy. Here, we investigated the accessibility of the glutamine (Gln, Q) residues of recombinant human erythropoietin (rHuEPO) towards a thermoresistant variant microbial transglutaminase (mTGase), TG¹⁶ with the aim of developing novel rHuEPO conjugates that may potentially enhance its biological efficacy. As a model bioconjugation, we studied the reactivity of rHuEPO towards TG¹⁶ with a low molar mass amine group containing substrate, monodansyl cadaverine (MDC). The reactions were carried out at a T_m of 54.3 °C, the transition temperature of rHuEPO. Characterization by SDS-PAGE and mass spectrometry confirmed the conjugates formation. Then, we examined the conjugation of rHuEPO with a biodegradable and biocompatible polyester, poly(D-sorbitol adipate) (PDSA). To achieve this, PDSA was enzymatically synthesized using lipase B from Candida antartica (CAL-B), chemically modified with side chains having free primary amine (NH₂) groups that can be acyl acceptor substrate of TG¹⁶, thoroughly characterized by ¹H NMR spectroscopy, and then applied for the TG¹⁶-mediated conjugation reaction with rHuEPO. rHuEPO conjugates generated by this approach were identified by SDS-PAGE proving that the amine-grafted PDSA is accepted as a substrate for TG¹⁶. The successful conjugation was further verified by the detection of high molar mass fluorescent bands after labelling of amine-grafted PDSA with rhodamine B-isothiocyanate. Overall, this enzymatic procedure is considered as an effective approach to prepare biodegradable rHuEPO-polymer conjugates even in the presence of N- and O-glycans.