Enzyme structure
The ten known PURases belong to two groups (Table S1 ): four
belong to the cutinases (LCC, TfCut-2, Tcur_1278 T, and Tcur0390)
and are similar to PETases. No crystal structures are available for the
PURases PueA and PueB from Pseudomonas chlororaphis , but
structures of homologues indicate that they belong to superfamily 11 of
the LED, which in addition to the core domain has a mobile N-terminal
lid, which might mediate binding to the substrate interface and
substrate access, and an additional C-terminal β-sandwich domain. The
four PETases and PueA and PueB from P. chlororaphis are GX types37. Recently, a modelling study on the PURases fromPseudomonas chlororaphis predicted putative substrate binding
sites for PUR-like substrates 44. However, a
rearrangement of the substrate was observed upon the molecular
simulation of the complex, which is an additional challenge for the
identification of the substrate binding site 45.
Interestingly, most of the substrate binding residues predicted for PueA44 are conserved (Table S7 ).
The PURase from Delftia acidovorans (Uniprot identifier Q9WX47)
does not belong to the GX type hydrolases, but has a sequence similarity
to carboxylesterases of superfamily 13 and to the family PF00135 in
Pfam, and thus belongs to the GGGX-type hydrolases 37.
Other carboxylesterases in the LED are members of superfamily 4, which
have a mobile lid between β-strand -4 and -3 of the core domain. Because
the PURase from Delftia acidovorans shared less than 50%
sequence similarity to the sequences in the LED and due to the lack of
experimental structure information, it was assigned to a new
superfamily.