Characterization of Recombinant Barley D-Enzyme and Its Ability to Influence Wort Sugar Profiles

Recombinant barley disproportionating enzyme 1 (rDPE1) is a functional D-enzyme capable of disproportionating maltotriose (G₃) into glucose (G₁) and maltopentaose (G₅) in the reaction, G₃ + G₃ ←→ G₁ + G₅. Maltotriose accumulates in wort during mashing and cannot be further broken down by diastatic power enzymes. This reaction mechanism could theoretically reduce maltotriose levels in wort replacing them with more easily fermentable sugars (glucose) or substrates (maltopentaose) to be broken down by diastatic power enzymes like β-amylase. Barley rDPE1 was most active between pH 6.5 and 6.8 but was functional between pH 4.5 and 8.0. Activity levels were relatively stable between 37 °C and 50 °C, peaking at 45 °C, with half the activity remaining after incubation at 55 °C. The thermostability and pH activity profiles of rDPE1 allow for at least partial survival under mashing conditions. rDPE1 released glucose from maltotriose, maltotetraose, and maltopentaose, but could not use maltose, maltohexaose, or maltoheptaose as substrates. Moreover, barley rDPE1 synthesized maltotetraose, maltohexaose, and maltoheptaose when given maltotriose as a substrate. An assortment of maltooligosaccharides were formed by rDPE1 when given the substrates maltotriose, maltotetraose, maltopentaose, maltohexaose, and maltoheptaose. The most novel reaction observed was the accumulation of maltose when rDPE1 used maltotetraose as a substrate. Barley rDPE1 (63 µg) was added to a 60-min 65 °C isothermal mash (1 mL) at 5, 30, and at 60 min (i.e., post-mash) and significantly lowered glucose, fructose, and maltotriose levels and increased maltotetraose, maltopentaose, and non-fermentable sugar levels.