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dc.contributor.authorBoynton, Tye O'Hara
dc.description.abstractHeme is an intrinsic and indispensable molecule in most forms of life, performing a myriad of functions such as gas transport, electron transport, and cellular signaling. In eukaryotes, heme is synthesized by a tightly regulated pathway of eight enzymes that is both well understood and fully characterized. In prokaryotes, however, there is a notable absence of identifiable biosynthetic enzymes in the terminal portion of the pathway. Recognized forms of protoporphyrinogen oxidase (PPO, HemY), the penultimate step in heme biosynthesis, are lacking in 50% of heme synthesizing genomes to date. An identifiable form of coproporphyrinogen oxidase (CPO), the antepenultimate step, is likewise missing in a subset of bacteria. Herein we describe the presence and characterization of three variant enzymes to fill these steps. The first, HemG, was shown to be a flavodoxin-like enzyme that has evolved PPO acitivity via a long chain insert loop specific to this function. Through the use of quinones, HemG functions during both aerobic and anaerobic respiration and is found in the enterics. Using comparative genomics, we have further identified another variant PPO, HemJ, which was proven to contain catalytic function through characterization and complementation of a hemJ Acinetobacter baylyi ADP1 mutant. Finally, we have proven the existence of novel CPO present in the Firmicutes that requires a cobalt cofactor and utilizes oxygen. This is distinct from the two currently known forms, HemF and HemN, which use only oxygen or a radical-SAM mechanism respectively. In addition to these variant enzymes, we show evidence of novel behavior for HemY in the Firmicutes and Actinobacteria, which requires the presence of the following pathway enzyme HemH and a newly identified enzyme HemQ to function correctly. This data indicates a novel mechanism of heme formation, despite overall HemY sequence homology with eukaryotes. It is intriguing that, while highly conserved elsewhere, these steps exhibit this degree of diversity in a pathway where the final intermediates are highly toxic. With our findings, enzymes with protoporphyrinogen oxidase activity can now be asserted in all heme-synthesizing bacteria. With regards to CPO activity, the Firmicutes
dc.subjectProkaryotic Heme Biosynthesis
dc.subjectProtoporphyrinogen oxidase
dc.subjectCoproporphyrinogen oxidase
dc.titleTerminal steps of heme biosynthesis in prokaryotes
dc.title.alternativeidentification and characterization of novel enzymes
dc.description.advisorHarry Dailey
dc.description.committeeHarry Dailey
dc.description.committeeWilliam B. Whitman
dc.description.committeeJan Mrazek
dc.description.committeeWilliam Lanzilotta

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