ParcuCDPS displays a Rossman-fold common throughout the CDPS family. The active site includes the four conserved residues previously identified: S26, Y167, E171 and Y191 (Fig. ). Additionally, we hypothesised D58 was acting as a potential active site residue, within hydrogen bond distance from the catalytic serine (S26), which could be important for S26 to act as a nucleophile. Indeed, when the hydrogen bond between S26 and D58 is disrupted by mutating D58 to either an alanine or an asparagine, ParcuCDPS loses over 90% of activity, while preserving its structure (Fig. , Supplementary Figure ). A Ser/Asp dyad was observed in Phospholipase A2, and future work could be directed towards better understanding the catalytic mechanism of ParcuCDPS. Moreover, the side chain of the predicted catalytically important Y167 points away from the active site, removing the conventional hydrogen bonding network seen in other CDPSs. Y167 plays the same role as Y178 in AlbC which is hypothesised to stabilise the aminoacyl moiety formed from the binding of the first substrate to P1. Further comparison with other CDPSs from the XYP family reveals RMSD values ranging from 2.7-2.9 Å. This structural comparison highlighted a divergence in secondary structure. Fig. depicts the core fold in transparent colour and the divergent regions shown in solid colour. ParcuCDPS structure diverges from the common fold in helix α3 and beta-strand β3, where these regions change direction. In the common CDPS fold helix α3 and α4 exist as a single continuous helix whereas, in ParcuCDPS a glycine residue (G84) provides a significant bend and change of direction, splitting the helix into two. The direction of β3 is another important deviation seen in ParcuCDPS' structure, as it changes direction (compared to common CDPS fold) at I56 to divide the active site pocket. This directional change of β3 facilitates the placement of D58 into H-bonding distance of the active site S26. These two features differentiate ParcuCDPS from the previously solved CDPS structures which are fairly conserved with respect to each other.