Abstract
Catalytic breakdown of polysaccharides can be achieved more efficiently by means of the enzymes lytic polysaccharide monooxygenases (LPMOs). However, the LPMO mechanism has remained controversial, preventing full exploitation of their potential. One of the controversies has centered around an active site tyrosine, present in most LPMO classes. Recent investigations have for the first time obtained direct (spectroscopic) evidence for the possibility of chemical modification of this tyrosine. However, the spectroscopic features obtained in the different investigations are remarkably different, with absorption maximum at 420 and 490 nm, respectively. In this paper we use density functional theory (DFT) in a QM/MM formulation to reconcile these (apparently) conflicting results. By modeling the spectroscopy as well as the underlying reaction mechanism we can show how formation of two isomers (both involving deprotonation of tyrosine) explains the difference in the observed spectroscopic features. Both isomers have a [TyrO–Cu–OH]+ moiety with the OH in either the cis- or trans-position to a deprotonated tyrosine. Although the cis-[TyrO–Cu–OH]+ moiety is well positioned for oxidation of the substrate, preliminary calculations with the substrate reveal that the reactivity is at best moderate, making a protective role of tyrosine more likely.
Highlights
Carbohydrate polymers such as starch, cellulose, and chitin comprise a large, renewable resource, both as alternatives to fossil fuel and as a carbon source for commercial chemicals.[1]
Using an approach based on density functional theory (DFT), combined with molecular mechanics (QM/MM), we here propose that the spectroscopic differences have origin in different species and we suggest a mechanism for their formation: our mechanism follows Path II in Fig. 1: since previous theoretical studies including the substrate[23,24,25] (Path I) have shown that oxyl species can be formed from the reductant and H2O2, Path II departs from [Tyr–OH–CuO]+ (1), abstracting H from tyrosine
The three intermediates 1–3 in Fig. 1 can attain both singlet and triplet spin states. Since this was a point of disagreement between the experiments suggesting deprotonation of tyrosine,[38,39] we initially probe the energy-difference between the singlet and triplet spin states
Summary
Carbohydrate polymers such as starch, cellulose, and chitin comprise a large, renewable resource, both as alternatives to fossil fuel and as a carbon source for commercial chemicals.[1]. The enzymes that are called lytic polysaccharide monooxygenases (LPMOs) have been discovered to boost[1,2,3,4,5,6] polysaccharide degradation and have been developed into a key ingredient for efficient polysaccharide decomposition. This decomposition was believed to be solely hydrolytic until studies on the chitinolytic bacterium Serratia marcescens[3] showed that LPMOs employ oxidative chemistry: the LPMOs catalyze oxidation of the otherwise unreactive glycosidic C–H bonds on either the C1 or C4 (or both) carbons in cellulose and. The rst discovered members[2,3] of the LPMO family are denoted as “Auxiliary Activity (AA)” enzymes AA9 and AA10 and many additional members (from AA11 and AA13 to AA16) have been categorized since the original discovery.[8,9,10,11,12,13,14] The different LPMOs have somewhat different amino-acid sequences (even within the same families), and target a wide range of different polysaccharide substrates.[15,16,17,18] Despite this variation, a common feature is an active site with a copper ion, coordinated by two histidine residues which are known as the histidine brace[8] (see Fig. 1)
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