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    • br Ultrasonic pretreatment of substrates Substrate pretreatm

    2019-07-30


    Ultrasonic pretreatment of substrates Substrate pretreatment is widely used in the biofuel, textile and food industries since the substrates are always difficult to degrade. During some enzymatic hydrolysis reactions, the protective layer of the substrate impedes the reaction. Ultrasonic treatment could destroy the aggregation of the substrate and remove the indigestible cuticle, making the substrate more vulnerable to attack by enzymes [65]. Mechanical effects can unfold the substrates\' structures to change the conformation for an easier combination with enzymes [66]. In addition, ultrasound could degrade substrates and decrease the degree of polymerization directly. During depolymerization, homolytic and/or heterolytic cleavage of a leukotriene receptor agonist may occur, and the breakage of a CC bond in the macromolecule is the most common mechanism [67]. Ultrasound-induced degradation always occurs preferentially near the middle of the chain. Furthermore, the decrease in the particle size can enhance the mass transfer and accelerate the reaction [68]. Therefore, the efficiency and rate of enzymatic reactions as well as the product yield are significantly stimulated.
    Ultrasound assisted enzymatic reactions The enzymatic reaction is always the critical step during many processes, but it is relatively cost- and rate-limited [75]. Conventional enzymatic methods require a significantly long time, and the degree of the hydrolysis always has limitations. Using other techniques to assist enzymatic reactions has attracted much interest. Ultrasound has been known to be used for the intensification of several physical, chemical and biological processes, including enzymatic reactions [76]. Studies on ultrasound assisted enzymatic reactions always aim at accelerating the reaction rate. Ultrasound assisted enzymatic reactions are credible for both solid-liquid-phase systems and liquid-liquid-phase systems (as shown in Table 3).
    Conclusions
    Acknowledgements This work was financially supported by the National Key Research and Development Program of China (grant 2016YFD0400301) and the Key Research and Development Program of Zhejiang Province (grant 2017C02015).
    Introduction A common complication of the kinetics of the NAD(P)+-dependent aldehyde dehydrogenase (ALDH) enzymes is substrate inhibition by the aldehyde, but, in spite of its high occurrence, it has been neglected in most of the reported kinetic studies. Most likely, inhibition by the aldehyde substrate is a general feature of these enzymes as a consequence of both their ordered steady-state kinetic mechanism and structural features. A search in the literature showed that inhibition of ALDHs by high concentrations of the aldehyde substrate has been reported for decades [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]], but a complete kinetic characterization of this behavior was seldom performed. A few studies addressed the mechanism involved in substrate inhibition, which was proposed to be caused by (i) formation of the non-productive ternary complex E-NAD(P)H-aldehyde from which NAD(P)H can be released [3,4,7,12]; (ii) binding of the aldehyde to the free enzyme in competition with the coenzyme [10,14,26]; (iii) non-productive binding [24] or binding of two aldehyde molecules [19] in the aldehyde-binding site; and (iv) binding of the inhibitor substrate molecule to an inhibitory allosteric site [6]. Regardless of the mechanism and possible physiological relevance, if any, of substrate inhibition, and of its value as a tool to establish a kinetic mechanism—three aspects which are out of the scope of this work—we reasoned that if this phenomenon is not taken into account in kinetic studies of ALDHs, and of enzymes in general, important errors in the determination of the kinetic parameters and even in the determination of the kinetic mechanism will ensue. Here, we explored the kind and extent of these errors using theoretical simulations of the substrate saturation kinetics with different possible mechanisms of substrate inhibition in monosubstrate and Bi Bi ordered steady-state reactions. In addition, we exemplify these errors with experimentally determined initial velocity data obtained studying the betaine aldehyde dehydrogenase from Spinacia oleracea (SoBADH), an enzyme belonging to the family 10 of the ALDH superfamily.