Radical new paradigm for heme degradation in Escherichia coli O157:H7

Abstract

All of the heme-degrading enzymes that have been characterized to date require molecular oxygen as a cosubstrate. Escherichia coli O157:H7 has been shown to express heme uptake and transport proteins, as well as use heme as an iron source. This enteric pathogen colonizes the anaerobic space of the lower intestine in mammals, yet no mechanism for anaerobic heme degradation has been reported. Herein we provide evidence for an oxygen-independent heme-degradation pathway. Specifically, we demonstrate that ChuW is a radical S-adenosylmethionine methyltransferase that catalyzes a radical-mediated mechanism facilitating iron liberation and the production of the tetrapyrrole product we termed “anaerobilin.” We further demonstrate that anaerobilin can be used as a substrate by ChuY, an enzyme that is coexpressed with ChuW in vivo along with the heme uptake machinery. Our findings are discussed in terms of the competitive advantage this system provides for enteric bacteria, particularly those that inhabit an anaerobic niche in the intestines.

NosN, a Radical S-Adenosylmethionine Methylase, Catalyzes Both C1 Transfer and Formation of the Ester Linkage of the Side-Ring System during the Biosynthesis of Nosiheptide

“…This enzyme is proposed to use a similar radical addition mechanism during the anaerobic degradation of heme, and also produces SAH rather than 5′-tA as a coproduct. 15 …”

Prokaryotic Heme Biosynthesis: Multiple Pathways to a Common Essential Product

“…Thus, many proposed HemNs are, in fact, not bona fide CgdH enzymes. Some annotated HemN proteins are known to be HemW, a heme chaperone (reviewed in reference 35), and ChuW, an oxygenindependent heme-degrading enzyme (373). This makes the interpretation of reported putative heme regulation data even more challenging.…”

Radical Breakthroughs in Natural Product and Cofactor Biosynthesis

“…Further examples of the impactful characterizations of radical SAM enzymes are emerging from other groups. For example, studies on homologs of ThiC, the enzyme responsible for complex rearrangement reactions during thiamine biosynthesis, revealed the existence of radical SAM enzymes responsible for the biosynthesis of the lower ligand of cobalamine in anaerobic bacteria 38 , while work on a highly underexplored class C radical SAM methylase led to the discovery of a novel mechanism in anaerobic heme degradation in pathogenic E. coli 39 . Considering the abundance of radical SAM enzymes and their highly underexplored nature, many more breakthrough findings are likely to be made through functional and mechanistic characterization of radical SAM enzymes.…”