The terminal heme synthetic enzyme, Coproheme Decarboxylase, coordinates heme synthesis and uptake in response to iron in Mycobacteria

Abstract

Heme is both an essential cofactor and an abundant source of nutritional iron for the human pathogen Mycobacterium tuberculosis (Mtb). While heme is required for Mtb survival and virulence, it is also potentially cytotoxic. Since Mtb has the ability to both make and uptake heme, the de novo synthesis of heme and its acquisition from the host must be balanced in order to mitigate heme toxicity. However, the mechanisms employed by Mtb to regulate heme uptake, synthesis, and bioavailability are poorly understood. By integrating ratiometric heme sensors with mycobacterial genetics, cell biology, and biochemistry, we determined that the terminal heme biosynthetic enzyme, coproheme decarboxylase (ChdC), plays a role in regulating both heme bioavailability and uptake in Mtb. Moreover, we found that Mtb has a preference for scavenging reduced ferrous heme and exhibits a cell surface heme reductase activity that is regulated by ChdC. In Mtb, ChdC expression is down-regulated when iron is limiting, which in-turn increases both heme import and bioavailability. Such a mechanism may serve to protect cells from heme toxicity while trying to meet the nutritional demand for iron. Our results demonstrate that heme synthesis and uptake are tightly integrated in mycobacteria and represent the first example of a heme synthetic enzyme playing a role in controlling heme uptake. Significance StatementHeme is an essential but potentially cytotoxic cofactor and iron source for the pathogen, Mycobacterium tuberculosis (Mtb). To understand how Mtb coordinates heme uptake and synthesis to mitigate heme toxicity, we integrated heme sensors with mycobacterial genetics and biochemical approaches to probe the interplay between heme synthesis and scavenging. We discovered that the terminal heme synthetic enzyme, coproheme decarboxylase (ChdC), negatively regulates heme uptake and utilization in response to iron availability through a mechanism involving control of a ferric heme reductase. During iron limitation, ChdC is downregulated, thereby enhancing exogenous heme reduction, uptake and utilization while simultaneously suppressing heme synthesis, which allows Mtb to avoid heme toxicity. Our results highlight the close coordination between heme synthesis and uptake in mycobacteria. ClassificationBiological sciences : Biochemistry