Crystal structure of the plant PPC decarboxylase AtHAL3a complexed with an ene-thiol reaction intermediate |
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Authors: | Steinbacher Stefan Hernández-Acosta Pilar Bieseler Bastian Blaesse Michael Huber Robert Culiáñez-Macià Francisco Antonio Kupke Thomas |
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Institution: | Max-Planck-Institut für Biochemie, Abteilung für Strukturforschung, Am Klopferspitz 18a, D-82152 Martinsried, Germany. steinbac@biochem.mpg.de |
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Abstract: | The Arabidopsis thaliana protein AtHAL3a decarboxylates 4'-phosphopantothenoylcysteine to 4'-phosphopantetheine, a step in coenzyme A biosynthesis. Surprisingly, this decarboxylation reaction is carried out as an FMN-dependent redox reaction. In the first half-reaction, the side-chain of the cysteine residue of 4'-phosphopantothenoylcysteine is oxidised and the thioaldehyde intermediate decarboxylates spontaneously to the 4'-phosphopantothenoyl-aminoethenethiol intermediate. In the second half-reaction this compound is reduced to 4'-phosphopantetheine and the FMNH(2) cofactor is re-oxidised. The active site mutant C175S is unable to perform this reductive half-reaction. Here, we present the crystal structure of the AtHAL3a mutant C175S in complex with the reaction intermediate pantothenoyl-aminoethenethiol and FMNH(2). The geometry of binding suggests that reduction of the C(alpha)=C(beta) double bond of the intermediate can be performed by direct hydride-transfer from N5 of FMNH(2) to C(beta) of the aminoethenethiol-moiety supported by a protonation of C(alpha) by Cys175. The binding mode of the substrate is very similar to that previously observed for a pentapeptide to the homologous enzyme EpiD that introduces the aminoethenethiol-moiety as final reaction product at the C terminus of peptidyl-cysteine residues. This finding further supports our view that these homologous enzymes form a protein family of homo-oligomeric flavin-containing cysteine decarboxylases, which we have termed HFCD family. |
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Keywords: | coenzyme A biosynthesis flavoprotein HFCD proteins PPC decarboxylase reaction intermediate |
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