The 1.7 angstrom crystal structure of the apo form of the soluble quinoprotein glucose dehydrogenase from Acinetobacter calcoaceticus reveals a novelinternal conserved sequence repeat

The crystal structure of a dimeric apo form of the soluble quinoprotein glu cose dehydrogenase (s-GDH) from Acinetobacter calcoaceticus has been solved by multiple isomorphous replacement followed by density modification, and was subsequently refined at 1.72 Angstrom resolution to a final crystallogr aphic R-factor of 16.5 % and free R-factor of 0.8 %. The s-GDH monomer has a beta-propeller fold consisting of six four-stranded anti-parallel beta-sh eets aligned around a pseudo 6-fold symmetry axis. The enzyme binds three c alcium ions per monomer, two of which are located in the dimer interface. T he third is bound in the putative active site, where it may bind and functi onalize the pyrroloquinoline quinone (PQQ) cofactor. A data base search une xpectedly showed that four uncharacterized protein sequences are homologous to s-GDH with many residues in the putative active site absolutely conserv ed. This indicates that these homologs may have a similar structure and tha t they may catalyze similar PQQ-dependent reactions. A structure-based sequence alignment of the six four-stranded beta-sheets i n s-GDH's beta-propeller fold shows an internally conserved sequence repeat that gives rise to two distinct conserved structural motifs. The first str uctural motif is found at the corner of the short beta-turn between the inn er two beta-strands of the beta-sheets, where an Asp side-chain points back into the beta-sheet to form a hydrogen-bond with the OH/NH of a Tyr/Trp si de-chain in the same beta-sheet. The second motif involves an Arg/Lys side- chain in the C beta-strand of one beta-sheet, which forms a bidentate salt- bridge with an Asp/Glu in the CD loop of the next beta-sheet. These intra a nd inter-beta-sheet hydrogen-bonds are likely to contribute to the stabilit y of the s-GDH beta-propeller fold. (C) 1999 Academic Press.