Non-enzymatic formation of isoprene and 2-methyl-3-buten-2-ol (2-MBO) by manganese

Abstract

It has been suggested that isoprene synthesis by isoprene synthase (IspS) proceeds via a substrate-assisted mechanism. The authors observed a non-enzymatic isoprene formation by Mn²⁺, which represents the basis of IspS enzyme reaction. Because IspS and many other terpene synthases require Mn²⁺ metal ions as cofactor, this study characterized the formation reaction for the first time. Metal ions including Mn²⁺ non-enzymatically produced both isoprene and 2-methyl-3-buten-2-ol (2-MBO) from dimethylallyl pyrophosphate (DMADP). Isoprene formation was most enhanced by Fe²⁺ and, to a lesser extent, by Mn²⁺ or Cu²⁺. Ni²⁺, Co²⁺, Mg²⁺, and Ba²⁺ exhibited a low activity to generate both isoprene and 2-MBO. The proportion of isoprene and 2-MBO varied with the Mn²⁺ concentration: isoprene predominated over 2-MBO at a higher Mn²⁺ concentration. Similarly, isoprene formation by Mn²⁺ increased exponentially as temperature increased with predominance of isoprene over 2-MBO at higher temperature. Both isoprene and 2-MBO formation was enhanced by acidic and neutral pH compared to alkaline conditions. Molecular dynamic simulation of DMADP suggested that the formation reaction is initiated by deprotonation of hydrogen on allyl terminal carbon by phosphate oxygen and generates carbocation and allyl anion intermediates. This is followed by quenching to produce isoprene or by hydroxyl addition to form 2-MBO. Thus, this study provided an insight into reaction mechanism of isoprene and 2-MBO biosynthesis and highlighted some parts of isoprene emission from terrestrial plants, which could be formed by non-enzymatic mechanism.