Aromatic stacking interactions govern catalysis in aryl-alcohol oxidase

This is the peer reviewed version of the following article: [Ferreira, P., Hernández-Ortega, A., Lucas, F., Carro, J., Herguedas, B., Borrelli, K. W., Guallar, V., Martínez, A. T. and Medina, M. (2015), Aromatic stacking interactions govern catalysis in aryl-alcohol oxidase. FEBS J, 282: 3091–3106. doi:10.1111/febs.13221], which has been published in final form at [10.1111/febs.13221]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving." http://olabout.wiley.com/WileyCDA/Section/id-820227.html The version posted may not be updated or replaced with the final published version (the Version of Record).
Aryl-alcohol oxidase (AAO, EC 1.1.3.7) generates H2O2 for lignin degradation at the expense of benzylic and other π system-containing primary alcohols, which are oxidized to the corresponding aldehydes. Ligand diffusion studies on Pleurotus eryngii AAO showed a T-shaped stacking interaction between the Tyr92 side chain and the alcohol substrate at the catalytically competent position for concerted hydride and proton transfers. Bi-substrate kinetics analysis revealed that reactions with 3-chloro- or 3-fluorobenzyl alcohols (halogen substituents) proceed via a ping–pong mechanism. However, mono- and dimethoxylated substituents (in 4-methoxybenzyl and 3,4-dimethoxybenzyl alcohols) altered the mechanism and a ternary complex was formed. Electron-withdrawing substituents resulted in lower quantum mechanics stacking energies between aldehyde and the tyrosine side chain, contributing to product release, in agreement with the ping–pong mechanism observed in 3-chloro- and 3-fluorobenzyl alcohol kinetics analysis. In contrast, the higher stacking energies when electron donor substituents are present result in reaction of O2 with the flavin through a ternary complex, in agreement with the kinetics of methoxylated alcohols. The contribution of Tyr92 to the AAO reaction mechanism was investigated by calculation of stacking interaction energies and site-directed mutagenesis. Replacement of Tyr92 by phenylalanine does not alter the AAO kinetic constants (on 4-methoxybenzyl alcohol), most probably because the stacking interaction is still possible. However, introduction of a tryptophan residue at this position strongly reduced the affinity for the substrate (i.e. the pre-steady state Kd and steady-state Km increase by 150-fold and 75-fold, respectively), and therefore the steady-state catalytic efficiency, suggesting that proper stacking is impossible with this bulky residue. The above results confirm the role of Tyr92 in substrate binding, thus governing the kinetic mechanism in A
This work was supported by the BIO2013-42978-P (to MM), BIO2011-26694 (to ATM), “Juan de la Cierva” (to FL) and CTQ2010-18123 (to VG) Grants of the Spanish Ministry of Economy and Competitiveness (MINECO) and by the INDOX (KBBE-2013-7-613549, to ATM) and PELE (ERC-2009-Adg 25027, to VG) European projects.
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