Evolution of enzymatic activities in the orotidine 5'-monophosphate decarboxylase suprafamily: structural basis for catalytic promiscuity in wild-type and designed mutants of 3-keto-L-gulonate 6-phosphate decarboxylase.

3-Keto-L-gulonate 6-phosphate decarboxylase (KGPDC) and D-arabino-hex-3-ulose 6-phosphate synthase (HPS), members of the orotidine 5'-monophosphate decarboxylase (OMPDC) suprafamily, catalyze reactions that involve the formation of Mg(2+)-ion stabilized 1,2-enediolate intermediates. The active sites of KGPDC and HPS share several conserved residues, including the presumed ligands for the Mg(2+) and a catalytic histidine residue that has been implicated in protonation of the intermediate in the KGPDC-catalyzed reaction. As reported in the previous manuscript, both enzymes are naturally promiscuous, with KGPDC from Escherichia coli catalyzing a low level of the HPS reaction and the HPS from Methylomonas aminofaciens catalyzing a significant level of the KGPDC reaction. Interestingly, the promiscuous HPS reaction catalyzed by KGPDC can be significantly enhanced by replacing no more than four active site residues from KGPDC reaction with residues from HPS. In this manuscript, we report structural studies of wild-type and mutant KDGPC's that provide a structural explanation for both the natural promiscuity for the HPS reaction and the enhanced HPS activity and diminished KGPDC activity catalyzed by active site mutants.

Site-directed sulfhydryl labeling of the oxaloacetate decarboxylase Na+ pump of Klebsiella pneumoniae: helix VIII comprises a portion of the sodium ion channel.

Helix VIII of the beta-subunit of the oxaloacetate decarboxylase of Klebsiella pneumoniae contains the functionally important residues betaN373, betaG377, betaS382, and betaR389. Using a functional oxaloacetate decarboxylase mutant devoid of Cys residues in the beta-subunit, each amino acid residue in helix VIII was replaced individually with Cys. Structural and dynamic features of this region were studied by using site-directed sulfhydryl modification of 20 single-Cys replacement mutants with methanethiosulfonate (MTS) reagents in the absence or presence of Na(+) ions. The pattern of accessibility of the MTS reagents from the periplasmic side of helix VIII shows a periodicity which suggests that this region is alpha-helical. In particular, a water-accessible face comprising betaN373, betaG377, betaS382, betaM386, and betaV390 may be part of a Na(+) channel. Cys residues introduced in the cytoplasmically oriented part of helix VIII were accessible to three different water-soluble MTS compounds and therefore believed to be exposed to water on this side of the membrane. Most residues located in the upper part of helix VIII (residues betaN373-betaV381C) were protected by Na(+) ions for inactivation by the MTS reagents. The distinct results on accessibility toward the different MTS reagents obtained in the presence or absence of Na(+) ions may suggest a conformational change upon binding of Na(+) in this region. The betaR389C mutant had a reduced activity and a pH optimum at pH 9, which could be restored to a wild-type pH optimum of 6.5 and to a 400% gain in activity upon chemical modification with 2-aminoethyl methanethiosulfonate.

Synthesis, structure and activity of artificial, rationally designed catalytic polypeptides.

Biological macromolecules with catalytic activity can be created artificially using two approaches. The first exploits a system that selects a few catalytically active biomolecules from a large pool of randomly generated (and largely inactive) molecules. Catalytic antibodies and many catalytic RNA molecules are obtained in this way. The second involves rational design of a biomolecule that folds in solution to present to the substrate an array of catalytic functional groups. Here we report the synthesis of rationally designed polypeptides that catalyse the decarboxylation of oxaloacetate via an imine intermediate. We determine the secondary structures of the polypeptides by two-dimensional NMR spectroscopy. We are able to trap and identify intermediates in the catalytic cycle, and to explore the kinetics in detail. The formation of the imine by our artificial oxaloacetate decarboxylases is three to four orders of magnitude faster than can be achieved with simple amine catalysts: this performance rivals that of typical catalytic antibodies.