Radical enzymes for the biosynthesis of tetrapyrroles

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During the biosynthesis of tetrapyrroles like hemes and chlorophylls coproporpyhrinogen III is decarboxylated by two structurally not related enzymes. The oxygen-sensitive coproporpyhrinogen III oxidase HemN uses a [4Fe-4S]-cluster and S-adenosyl-L-methionine to form a substrate radical required for catalysis. The oxygen-dependent HemF uses histidine-coordinated manganese for the same activity. For HemN the natural electron donor(s) and acceptor(s) will be identifed. The redox behavior of the Fe-S-cluster and the location(s) of the substrate radical will be determined spectroscopically. The nature of enzymatic S-adenosyl-L-methionine binding and utilization will be clarified. For HemF missing manganese ligands will be identified. Changes in oxidation state and metal coordination during the reaction of manganese with oxygen and substrate will be analyzed. Potential enzyme and substrate radicals involved in catalysis will be demonstrated. Substituents of the tetrapyrrolic substrate important for the recognition by both enzymes will be evaluated using synthetic analogs. Finally, to understand structure-function relationships the atomic structure of both enzymes must be elucidated using anaerobic crystallisation with substrate analogs and X-ray analysis.