Crystal Structure of the Plant PPC Decarboxylase AtHAL3a Complexed with an Ene-thiol Reaction Intermediate

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 FMNH2 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 FMNH2. The geometry of binding suggests that reduction of the CαCβ double bond of the intermediate can be performed by direct hydride-transfer from N5 of FMNH2 to Cβ of the aminoethenethiol-moiety supported by a protonation of Cα 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.