Methanobactin and the Link between Copper and Bacterial Methane Oxidation

Abstract

SUMMARYMethanobactins (mbs) are low-molecular-mass (<1,200 Da) copper-binding peptides, or chalkophores, produced by many methane-oxidizing bacteria (methanotrophs). These molecules exhibit similarities to certain iron-binding siderophores but are expressed and secreted in response to copper limitation. Structurally, mbs are characterized by a pair of heterocyclic rings with associated thioamide groups that form the copper coordination site. One of the rings is always an oxazolone and the second ring an oxazolone, an imidazolone, or a pyrazinedione moiety. The mb molecule originates from a peptide precursor that undergoes a series of posttranslational modifications, including (i) ring formation, (ii) cleavage of a leader peptide sequence, and (iii) in some cases, addition of a sulfate group. Functionally, mbs represent the extracellular component of a copper acquisition system. Consistent with this role in copper acquisition, mbs have a high affinity for copper ions. Following binding, mbs rapidly reduce Cu2+to Cu1+. In addition to binding copper, mbs will bind most transition metals and near-transition metals and protect the host methanotroph as well as other bacteria from toxic metals. Several other physiological functions have been assigned to mbs, based primarily on their redox and metal-binding properties. In this review, we examine the current state of knowledge of this novel type of metal-binding peptide. We also explore its potential applications, how mbs may alter the bioavailability of multiple metals, and the many roles mbs may play in the physiology of methanotrophs.

Bacterial copper storage proteins

“…However, the observation that Csp3 is present alone in 22 methanotrophs (Csp1 is rarely present on its own) could indicate the function of this protein is independent of Csp1 and Mbn (see section about the functions of Csps). Much attention has been paid to the role of Mbn in Cu acquisition and utilization by methanotrophs (17,(50)(51)(52)(54)(55)(56)(57)(58)(59)(60). However, the currently identified Mbn operon is only found in just over 10% of sequenced methanotroph genomes, suggesting other systems are important for these processes.…”

“…Copper availability appears to be largely constrained by the use of high affinity sites in proteins (12)(13)(14), although 'pools' of Cu bound by other molecules are important (4,5,11,(15)(16)(17)(18).…”

“…The mbnA gene, which codes for leader (cleaved) and core (modified) peptides has been identified in an operon along with proteins either shown or suggested to be involved in modification reactions, apo-Mbn export, and re-incorporation of Cu(I)-Mbn (17,51,(55)(56)(57)(58)(59). Related Mbn operons are present in some non-methanotrophic bacteria (17,51,59). Work in our lab has found that Mbns bind Cu(I) with affinities in the 10 21 M -1 range, and have Cu(II) affinities that are ~ 6-10 orders of magnitude weaker (52,54,60).…”

“…Previous Minireviews in the Thematic The ability to utilize large amounts of Cu results in methanotrophs having highly interesting Cu-handling systems. This includes methanobactin (Mbn) (17,(50)(51)(52), which has been considered comparable to certain Fe-binding siderophores (53), and thus termed a chalkophore (50). Mbn is a modified peptide (Fig.…”

“…1, A and B) that is part of a highly specialized Cu-uptake system, secreted to sequester Cu under limiting conditions (17,50). The mbnA gene, which codes for leader (cleaved) and core (modified) peptides has been identified in an operon along with proteins either shown or suggested to be involved in modification reactions, apo-Mbn export, and re-incorporation of Cu(I)-Mbn (17,51,(55)(56)(57)(58)(59). Related Mbn operons are present in some non-methanotrophic bacteria (17,51,59).…”