生物化学

Heme is an iron-containing porphyrin which acts as a prosthetic group in several enzymes involved in disparate functions, such as respiration and H₂O₂-scavenging. Escherichia coli is able to produce heme endogenously since it contains all the enzymes involved in the nine-step biosynthesis pathway, which in absence of stress and in iron-replete media proceeds unabated. However, we recently showed that two steps are affected by H₂O₂ stress (Mancini and Imlay, 2015). To compensate, two enzymes, namely the ferrochelatase (HemH) and an isozyme of coproporphyrinogen III oxidase (HemF), are activated by the H₂O₂-responsive regulator OxyR. Genetic mutations that block either adaptation cause the intracellular accumulation of protoporphyrin IX and coproporphyrinogen III, the substrates of HemH and HemF, respectively. We here describe a method used to extract and quantify protoporphyrin IX and coproporphyrin III, the product of the spontaneous oxidation of coproporphyrinogen III.