Structure of hUPP1. (A) The global fold of hUPP1 is conserved with its bacterial homologues with only slight modifications. The enzyme's N-terminus possesses additional architecture (purple) and the third helix in the E. coli protein (α3) is replaced by a sheet-turn-sheet structure (blue). All secondary structure is annotated so as to maintain consistency with prior microbial structures. (B) Shown is the dimeric biological unit for hUPP1. It is noteworthy that both of the aforementioned structural modifications (purple & blue) result in substantial increases in the buried surface area within the dimer interface of the enzyme. (C) Structure-based sequence alignment between the two human enzyme isoforms and structurally analyzed bacterial homologues reveals strict conservation of all of the residues lining the enzyme active site (orange) with the sole exception of a phenylalanine to tyrosine variation at the N-terminus of a residue that contacts primarily inhibitory molecules, such as BAU. There are also substantial differences in the loop lining the back of the ligand-binding pocket that similarly interacts only with inhibitor chemical groups and not natural substrates (boxed). Modifications underlying loss of dimer trimerization in hUPP1 are highlighted in brown. Interestingly, the glutamate implicated in bacterial enzymes in K+ coordination at the dimer interface (yellow) is absent from both human proteins. Residues of hUPP1 that are disordered in both structures are hatched.