The outer membrane palmitoyltransferase PagP possesses regioselectivity for the palmitoylation of the (R)-3-hydroxymyristate chain at position 2 on the proximal glucosamine unit of lipid A. The residues Arg45 and Arg49 in the L1 loop appear to poise their guanidinium groups so as to interact with the proximal and distal phosphate groups at positions 1 and 4’ of lipid A, respectively. Both single and double substitution of these arginine residues with serine has no effect on the folding, stability, phospholipase and palmitoyltransferase activities. Additionally, the arginine to serine substitutions display wild-type regioselectivity and specific activity in the palmitoylation of the biosynthetic precursor lipid IVA as indicated by collision induced fragmentation MS/MS. In vivo, lipid A analysis in a msbB/pagP deletion strain shows no difference in acylation pattern as compared to the wild-type. These results establish both in vitro and in vivo that the arginine to serine substitutions have no effect on lipid A regioselectivity. / Thesis / Bachelor of Science (BSc)

The rifamycins are a class of antibiotics which were once used almost exclusively to treat tuberculosis, but are currently receiving renewed interest. Resistance to rifamycins is most commonly attributed to mutations in the drug target, RNA polymerase. Yet environmental isolates are also able to enzymatically inactivate rifamycins in a number of ways. Recently, rifamycin resistance determinants from the environment were found to be closely associated with a so called rifamycin associated element (RAE). The region containing the RAE from an environmental strain was shown to induce gene expression in the presence of rifamycins, hinting at an inducible system for rifamycin resistance. In this work, we examine the RAE from a model organism for Streptomyces genetics, Streptomyces venezuelae. We confirm that the promoter region containing the RAE upstream of a rifamycin monooxygenase rox is inducible by rifamycins. The strains of S. venezuelae generated in this work can be used in future genetic studies on the RAE. As well, the rifamycin monooxygenase Rox was purified for the first time and characterized biochemically. The structure of Rox was obtained with and without the substrate rifampin. Steady state kinetics for the enzyme were determined with a number of substrates, and its ability to confer resistance to rifamycins was examined. Monooxygenated rifamycin SV compound was purified and structurally characterized by NMR analysis. We proposed an aromatic hydroxylase type mechanism for Rox, in which the enzyme hydroxylates the aromatic core of the rifamycin scaffold and causes a non-enzymatic C-N bond cleavage of the macrolactam ring. This is a new mechanism of rifamycin resistance, and sheds some light on the decomposition of rifamycins mediated by monooxygenation, which is still poorly understood. / Thesis / Master of Science (MSc) / Antibiotic resistance represents a major threat to global health. Infections that were once readily treatable are no longer so due to the rise in multidrug resistant bacteria. As our arsenal of effective antibiotics is depleted, new drugs are being discovered less and less frequently. This has caused the scientific community to get creative in coming up with treatments: trying combinations of antibiotics, using antibiotics which were once considered too toxic, and repurposing antibiotics for different bacteria. Rifamycins are a class of antibiotics most commonly used in the treatment of tuberculosis. However, they are becoming more widely used as a result of antibiotic resistance. There are a number of different ways bacteria can become resistant to the harmful effects of rifamycins: by modifying the target so the drug can no longer bind to it, actively pumping the drug out of the cell, or by changing the drug in some way so it is no longer effective. Bacteria in the environment use antibiotics as a form of chemical warfare to gain an advantage over their neighbours; therefore, they have had millions of years to evolve very effective methods of antibiotic resistance. By surveying what kinds of antibiotic resistance are in the environment, we can predict what we might see one day in a medical setting. In this thesis, I have studied a protein that bacteria make to inactivate rifamycins. The rifamycin monooxygenase Rox adds an oxygen to the rifamycin scaffold; this causes spontaneous cleavage of the rifamycin backbone and changes the conformation of the drug so it can no longer bind to its target. I have also investigated the regulation of this and other genes in the bacterial strain Streptomyces venezuelae. By understanding how this process works, we can potentially design inhibitors to stop this from happening, should this method of resistance ever become clinically prevalent.

No description available.

Radical SAM enzymes are ancient, essential enzymes. They perform radical chemical reactions in virtually all living organisms and are involved in producing antibiotics, generating greenhouse gases, human health, and likely many other essential roles that have yet to be established. A wide variety of reactions have been characterized from this group of enzymes, including hydrogen abstractions, the transferring of methylthio groups, complex cyclization and rearrangement reactions, and others. However, many radical SAM enzymes have yet to be identified or characterized. There have been great leaps forward in the amount of enzyme sequences that are available in public databases, but experiments to investigate what chemical reactions the enzymes perform take a great deal of time. In our work, we utilize Hidden Markov Models to identify possible radical SAM enzymes and predict their possible functions through BLAST alignments and homology modelling. We also explore their distribution across the tree of life and determine how it is correlated with organism oxygen tolerances, because the core iron-sulfur cluster is oxygen sensitive. Trends in the abundances of radical SAM enzymes depending on oxygen tolerances were more apparent in prokaryotes than in eukaryotes. Although eukaryotes tend to have fewer radical SAM enzymes than prokaryotes, we were able to analyze uncharacterized radical SAM enzymes from both an aerobic eukaryote (Entamoeba histolytica) and a eukaryote capable of oxygenic photosynthesis (Gossypium barbadense), and predict the reactions they catalyze. This work sets the stage for the functional characterization of these essential yet elusive enzymes in future laboratory experiments. / Master of Science in Life Sciences / Radical SAM enzymes are ancient, essential enzymes that perform chemical reactions in virtually all living organisms. We do know that they are involved in producing antibiotics, human health, and generating greenhouse gases. We also know that there are many radical SAM enzymes whose functions remain a mystery. There have been great leaps forward in the amount of enzyme sequences that are available in public databases, but experiments to investigate what chemical reactions enzymes perform take a great deal of time. The experiments are especially difficult for radical SAM enzymes because the oxygen we breathe can break the enzymes down in a laboratory. In our work, we utilize computational techniques to identify possible radical SAM enzymes and predict what reactions they might catalyze. Because these enzymes are vulnerable to oxygen in laboratory environments, we also explore whether organisms that breathe oxygen have fewer of these enzymes than organisms that perform anaerobic respiration instead. We found that does seem to be the case in microbes like bacteria and archaea, but the results were not as consistent for eukaryotes. We then chose radical SAM enzymes we had identified from both an aerobic eukaryote (Entamoeba histolytica) and a eukaryote capable of producing oxygen (Gossypium barbadense), and predicted the reactions they catalyze. This work sets the stage for the functional characterization of these essential yet elusive enzymes in future laboratory experiments.

Small molecules containing a 7-deazapurine moiety are ubiquitous in nature. They comprise a broad range of structurally diverse antibiotics produced by terrestrial and marine microorganisms that possess demonstrated antibiotic and antineoplastic activity. In addition, queuosine, a hypermodified nucleoside located in the wobble position of select tRNAs that is almost universally conserved throughout biology, contains a 7-deazapurine functional group. The since their initial identification over 50 years ago, the chemical transformations underlying the biosynthesis of 7-deazapurines have remained elusive. This work describes the identification of a cluster of co-localized genes in the Streptomyces rimosus chromosome that are responsible for the biosynthesis of the 7-deazapurine containing antibiotics toyocamycin and sangivamycin. Further, the in vitro conversion of GTP to the previously identified queuosine biosynthetic intermediate 7-cyano-7-deazaguanine (preQ₀) is demonstrated using purified, recombinant enzymes. Also included herein is a kinetic, spectroscopic, and mechanistic characterization of QueE, an enzyme that catalyzes the third step in the biosynthesis of 7-deazapurines using a radical-mediated rearrangement. A possible mechanism for the reaction catalyzed by QueD, the second step in the deazapurine biosynthetic pathway, is explored based on X-ray crystallographic data of site directed QueD mutants containing bound substrate. Finally, hitherto unrecognized gene clusters that are likely devoted to the biosynthesis of 7-deazapurines other than queuosine are described.

No description available.

No description available.

ADP-L-glycero-D-manno-heptose 6-epimerase (HldD) catalyzes the inversion of configuration at C-6" of the heptose moiety of ADP-D-glycero-D-manno-heptose and ADP-L-glycero-D-manno-heptose. H1dD operates in the biosynthesis of L-glycero-D-manno-heptose, a conserved sugar in the core region of lipopolysaccharide (LPS) of Gram-negative bacteria. This work supports a direct redox mechanism whereby H1dD uses its tightly bound NADP+ to oxidize the substrate at C-6", generating a ketone intermediate. Reduction from the opposite face generates the epimeric product. An analog of the ketone intermediate, ADP-ß-D-manno-hexodialdose 8, was shown to undergo dismutation giving equal amounts of ADP-mannose 9and ADP-mannuronate 10. Observation of transient NADPH during dismutation established participation of the tightly bound cofactor. Further studies address how HldD is able to access both faces of the ketone intermediate with correct alignment of NADPH, the ketone intermediate, and a catalytic acid/base residue. It is proposed that Escherichia coli K-12 HldD contains two catalytic acid/base residues, tyrosine 140 and lysine 178, each of which facilitates redox chemistry on opposite faces of the ketone intermediate. The ketone intermediate may access either base via rotation about the C-5"/C-6" bond. The observation that two single mutants, Y140F and K178M, have severely compromised epimerase activities, yet retain dismutase activity, supports this hypothesis. UDP-N-acetylglucosamine 5-inverting 4,6-dehydratase (PseB) is a unique sugar nucleotide dehydratase that inverts the C-5" stereocentre during conversion of UDP-N-acetylglucosamine to UDP-2-acetyl-2,6-dideoxy-ß-L-arabino-4-hexulose. PseB catalyses the first step in the biosynthesis of pseudaminic acid, which is found as a post-translational modification on the flagellin of Campylobacter jejuni and Helicobacter pylon. PseB uses its tightly bound NADP+ to oxidize UDP-G1cNAc at C-4", enabling dehydration. The a,ß unsaturated ketone intermediate thus generated is reduced by delivery of a hydride from NADPH to C-6", and a proton to C-5". Consistent with this mechanism, a solvent derived deuterium becomes incorporated into the C-5" position of product during catalysis in D20. Likewise, PseB catalyzes solvent isotope exchange into the H5" position of the product, and theelimination of HF from UDP-6-deoxy-6-fluoro-G1cNAc 23. Mutants of the putative catalytic residues aspartate 126, lysine 127 and tyrosine 135 have severely compromised dehydratase, solvent isotope exchange, and HF elimination activities.

No description available.

ADP-L-glycero-D-manno-heptose 6-epimerase (HldD) catalyzes the inversion of configuration at C-6" of the heptose moiety of ADP-D-glycero-D-manno-heptose and ADP-L-glycero-D-manno-heptose. H1dD operates in the biosynthesis of L-glycero-D-manno-heptose, a conserved sugar in the core region of lipopolysaccharide (LPS) of Gram-negative bacteria. This work supports a direct redox mechanism whereby H1dD uses its tightly bound NADP+ to oxidize the substrate at C-6", generating a ketone intermediate. Reduction from the opposite face generates the epimeric product. An analog of the ketone intermediate, ADP-ß-D-manno-hexodialdose 8, was shown to undergo dismutation giving equal amounts of ADP-mannose 9and ADP-mannuronate 10. Observation of transient NADPH during dismutation established participation of the tightly bound cofactor. Further studies address how HldD is able to access both faces of the ketone intermediate with correct alignment of NADPH, the ketone intermediate, and a catalytic acid/base residue. It is proposed that Escherichia coli K-12 HldD contains two catalytic acid/base residues, tyrosine 140 and lysine 178, each of which facilitates redox chemistry on opposite faces of the ketone intermediate. The ketone intermediate may access either base via rotation about the C-5"/C-6" bond. The observation that two single mutants, Y140F and K178M, have severely compromised epimerase activities, yet retain dismutase activity, supports this hypothesis. UDP-N-acetylglucosamine 5-inverting 4,6-dehydratase (PseB) is a unique sugar nucleotide dehydratase that inverts the C-5" stereocentre during conversion of UDP-N-acetylglucosamine to UDP-2-acetyl-2,6-dideoxy-ß-L-arabino-4-hexulose. PseB catalyses the first step in the biosynthesis of pseudaminic acid, which is found as a post-translational modification on the flagellin of Campylobacter jejuni and Helicobacter pylon. PseB uses its tightly bound NADP+ to oxidize UDP-G1cNAc at C-4", enabling dehydration. The a,ß unsaturated ketone intermediate thus generated is reduced by delivery of a hydride from NADPH to C-6", and a proton to C-5". Consistent with this mechanism, a solvent derived deuterium becomes incorporated into the C-5" position of product during catalysis in D20. Likewise, PseB catalyzes solvent isotope exchange into the H5" position of the product, and theelimination of HF from UDP-6-deoxy-6-fluoro-G1cNAc 23. Mutants of the putative catalytic residues aspartate 126, lysine 127 and tyrosine 135 have severely compromised dehydratase, solvent isotope exchange, and HF elimination activities.