Metabolic engineering of Escherichia coli for secretory production of free haem

Haem has widespread applications in healthcare and food supplement industries. Escherichia coli has previously been engineered to produce a small amount of haem intracellularly through the C4 pathway, requiring extraction for applications. Here we report secretory production of free haem by engineered E. coli strains, using the C5 pathway and the optimized downstream pathway for haem biosynthesis. Furthermore, knocking out ldhA, pta and also yfeX—encoding a putative haem-degrading enzyme—results in 7.88 mg l−1 of total haem with 1.26 mg l−1 of extracellular haem in flask cultivation. Fed-batch fermentations of the engineered strain overexpressing a haem exporter CcmABC from glucose only and glucose supplemented with l-glutamate secrete 73.4 and 151.4 mg l−1 of haem, respectively, which are 63.5% of 115.5 mg l−1 and 63.3% of 239.2 mg l−1 of total haem produced. The engineered E. coli strain reported here will be useful for microbial production of free haem. Microbial production of haem for applications in healthcare and food supplement industry requires high-performing strains. Here, Lee and co-workers report secretory production of free haem by metabolically engineered Escherichia coli strains to produce up to 239 mg l−1 total haem.