Mitochondrial contact site and cristae organizing system (MICOS) machinery supports heme biosynthesis by enabling optimal performance of ferrochelatase

Abstract

Heme is an essential cofactor required for a plethora of cellular processes in eukaryotes. In metazoans the heme biosynthetic pathway is typically partitioned between the cytosol and mitochondria, with the first and final steps taking place in the mitochondrion. The pathway has been extensively studied and its biosynthetic enzymes structurally characterized to varying extents. Nevertheless, understanding of the regulation of heme synthesis and factors that influence this process in metazoans remains incomplete. Therefore, we investigated the molecular organization as well as the physical and genetic interactions of the terminal pathway enzyme, ferrochelatase (Hem15), in the yeast Saccharomyces cerevisiae . Biochemical and genetic analyses revealed dynamic association of Hem15 with Mic60, a core component of the mitochondrial contact site and cristae organizing system (MICOS). Loss of MICOS negatively impacts Hem15 activity, affects the size of the Hem15 high-mass complex, and results in accumulation of reactive and potentially toxic tetrapyrrole precursors that may cause oxidative damage. Restoring intermembrane connectivity in MICOS-deficient cells mitigates these cytotoxic effects. These data provide new insights into how heme biosynthetic machinery is organized and regulated, linking mitochondrial architecture-organizing factors to heme homeostasis.

Ferrochelatase: Mapping the Intersection of Iron and Porphyrin Metabolism in the Mitochondria

“…These structures are either of wild-type or variant forms of the enzyme some with substrate, inhibitor or product bound. In addition to the structures from these two species there are also structures from Saccharomyces cerevisiae (PpfC) ( Karlberg et al, 2002 ; Dietz et al, 2021 ), Listeria monocytogenes (CpfC) ( Hofbauer et al, 2020 ), and Bacillus anthracis (CpfC) (unpublished PDB ID 2C8J). No structures are available for any plant ferrochelatases.…”

“…It is of note that the closed and release conformations have not been observed in any ferrochelatase structures from other organisms which may be due to crystallization conditions and inhibitors utilized in these studies. To date only the open and partially closed conformations, between that of the open and closed conformations, have also been observed in the structure of the S. cerevisiae enzyme ( Karlberg et al, 2002 ; Dietz et al, 2021 ).…”

“…In mouse and S. cerevisiae cells, ferrochelatase has been shown to interact with components of the mitochondrial membrane contact site and cristae organizing system (MICOS). Of the eight known proteins that make up this complex, five were found to physically interact with S. cerevisiae ferrochelatase via co-IP ( Dietz et al, 2021 ). Interestingly, mammalian homologs of MICOS have also been found to interact with human ferrochelatase ( Piel et al, 2016 ).…”

“…Protoporphyrin IX levels decreased while other porphyrin intermediates increased. Restabilizing this connection between the OMM and IMM via a synthetic tether rescued the phenotype ( Dietz et al, 2021 ). In sum, the integrity of this mitochondrial interface is crucial for efficient ferrochelatase activity and heme biosynthesis.…”

“…The mitochondrial ferrochelatase metabolon has been well studied ( Medlock et al, 2015 ; Piel et al, 2019 ; Dietz et al, 2021 ), however, neither the stoichiometry of this complex, nor the differences in the metabolon in different cell types and developmental stages has been fully determined. It is necessary to understand how the expression levels of participating proteins could affect the integrity of the complex and consequently the metabolism of heme.…”