Arabidopsis thaliana flavoprotein AtHAL3a catalyzes the decarboxylation of4 '-phosphopantothenoylcysteine to 4 '-phosphopantetheine, a key step in coenzyme A biosynthesis

Arabidopsis thaliana flavoprotein AtHAL3a is related to plant growth and sa lt and osmotic tolerance. AtHAL3a shows sequence homology to the bacterial flavoproteins EpiD and Dfp. EpiD, Dfp, and AtHAL3a are members of the homo- oligomeric flavin-containing Cys decarboxylase (HFCD) protein family. We de monstrate that AtHAL3a catalyzes the decarboxylation of (R)-4 ' -phospho-N- pantothenoylcysteine to 4 ' -phosphopantetheine. This key step in coenzyme A biosynthesis is catalyzed in bacteria by the Dfp proteins. Exchange of Hi s-90 of AtHAL3a for Asn led to complete inactivation of the enzyme. Dfp and AtHAL3a are characterized by a shortened substrate binding clamp compared with EpiD. Exchange of the cysteine residue of the conserved ACGD motif of this binding clamp resulted in loss of (R)-4 ' -phospho-N-pantothenoylcyste ine decarboxylase activity. Based on the crystal structures of EpiD H67N wi th bound substrate peptide and of AtHAL3a, we present a model for the bindi ng of (R)-4 ' -phospho-N-pantothenoylcysteine to AtHAL3a.