Molybdopterin Radical in Bacterial Aldehyde Dehydrogenases

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Abstract
The EPR spectra of three different molybdoprotein aldehyde dehydrogenases, one purified from Comamonas testosteroni and two purified from Amycolatopsis methanolica, showed in their oxidized state a novel type of signal. These three enzymes contain two different [2Fe-2S] centers, one flavin and one molybdopterin cytosine dinucleotide, as cofactors all of which are expected to be EPR silent in the oxidized state. The new EPR signal is isotropic with g = 2.004 both at X-band and Q-band frequencies, consists of six partially resolved lines, and shows Curie temperature behavior suggesting that the signal is due to an organic radical with S = 1/2. The EPR spectra of Comamonas testosteroni aldehyde dehydrogenase obtained after cultivation in media containing 15NH4Cl and/or after substitution of H2O for D2O show the presence of both nitrogen and proton hyperfine interactions. Simulations of the spectra of the four possible isotope combinations yield a single set of hyperfine coupling constants. The electron spin shows hyperfine interaction with a single I = 1 (0.9 mT) ascribed to a N nucleus, with a single I = 1/2 (1.5 mT) ascribed to one nonexchangeable H nucleus, and with two, exchangeable, identical I = 1/2 spins (0.6 mT) ascribed to two identical exchangeable protons. Taken together, the observations and simulations rule out amino acid residues or flavin as the origin of the radical. The values of the various hyperfine coupling constants are consistent with the properties expected for a molybdenum(VI)-trihydropterin radical in which the N5 atom is engaged in two hydrogen-bonding interactions with the protein. The majority of the electron (spin) density of the radical is located at and around the N5 atom and at the proton bound to the C6 atom of the pterin ring. The EPR spectrum of the molybdopterin radical broadens above 65 K and is no longer detectable above 168 K, indicating that it is not magnetically isolated. The line broadening is ascribed to cross-relaxation with a nearby, rapidly relaxing, oxidized [2Fe-2S] center involving its magnetic S = 1 excited state in this process. The amount of radical was apparently not changed by addition of aldehydes or oxidants, but it disappeared upon reduction by sodium dithionite. Therefore, whether the molybdenum(VI) trihydropterin radical as detected here is a functional intermediate in catalysis remains to be investigated further.

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