Structure and function of enzymes involved in the biosynthesis and transport of heme

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Hemes are essential cofactors for energy-conserving, electron transport complexes and various enzymes and proteins. We are systematically investigating the structure-function relationship of enzymes and porphyrin transporters involved in the complex heme biosynthetic pathway. Recently, we have determined the first crystal structures of a natural mischarging glutamyl-tRNA synthetase, 5-aminolevulinic acid synthase (HemA) and the “Radical SAM enzyme” coproporphyrinogen III oxidase (HemN). The catalytic mechanisms of these and two further enzymes (HemB, HemY) were successfully investigated using biophysical methods. Along these lines we will next investigate the structure-function relationship of a novel oxygen independent protoporphyrinogen IX oxidase (HemG). Its unique FMN-based catalysis which includes a 6-electron transfer to membrane-localized electron transport chains will be studied. Our solved structure of HemA serves as basis for the stepwise elucidation of its catalytic mechanism using a combination of mutant enzymes and substrate analogs. In vivo and in vitro protein-protein and protein-metabolite interactions between various heme biosynthetic enzymes and their functional consequences for metabolic channeling will be studied. Finally, the molecular basis of tetrapyrrole transport via mitochondrial membranes will be studied using a structurefunction approach. The long term goal is the detailed understanding of the biochemistry and regulation of one of the central biosynthetic pathways in life at the atomic level.