Characterization of the active site in the thiocyanate-forming protein from Thlaspi arvense (TaTFP) using EPR spectroscopy

Abstract

Glucosinolates are plant thioglucosides, which act as chemical defenses. Upon tissue damage, their myrosinase-catalyzed hydrolysis yields aglucones that rearrange to toxic isothiocyanates. Specifier proteins such as thiocyanate-forming protein from Thlaspi arvense (TaTFP) are non-heme iron proteins, which capture the aglucone to form alternative products, e.g. nitriles or thiocyanates. To resolve the electronic state of the bound iron cofactor in TaTFP, we applied continuous wave electron paramagnetic resonance (CW EPR) spectroscopy at X-and Q-band frequencies (∼9.4 and ∼34 GHz). We found characteristic features of high spin and low spin states of a d ⁵ electronic configuration and local rhombic symmetry during catalysis. We monitored the oxidation states of bound iron during conversion of allylglucosinolate by myrosinase and TaTFP in presence and absence of supplemented Fe²⁺. Without added Fe²⁺, most high spin features of bound Fe³⁺ were preserved, while different g'-values of the low spin part indicated slight rearrangements in the coordination sphere and/or structural geometry. We also examined involvement of the redox pair Fe³⁺/Fe² in samples with supplemented Fe²⁺. The absence of any EPR signal related to Fe³⁺ or Fe²⁺ using an iron-binding deficient TaTFP variant allowed us to conclude that recorded EPR signals originated from the bound iron cofactor.

Keywords: allylglucosinolate; catalysis; electron paramagnetic resonance; non-heme iron proteins; specifier proteins.