Chalkophores

Abstract

Copper-binding metallophores, or chalkophores, play a role in microbial copper homeostasis that is analogous to that of siderophores in iron homeostasis. The best-studied chalkophores are members of the methanobactin (Mbn) family-ribosomally produced, posttranslationally modified natural products first identified as copper chelators responsible for copper uptake in methane-oxidizing bacteria. To date, Mbns have been characterized exclusively in those species, but there is genomic evidence for their production in a much wider range of bacteria. This review addresses the current state of knowledge regarding the function, biosynthesis, transport, and regulation of Mbns. While the roles of several proteins in these processes are supported by substantial genetic and biochemical evidence, key aspects of Mbn manufacture, handling, and regulation remain unclear. In addition, other natural products that have been proposed to mediate copper uptake as well as metallophores that have biologically relevant roles involving copper binding, but not copper uptake, are discussed.

Keywords: bioinorganic chemistry; chalkophore; copper homeostasis; metallophore; methanobactin; natural product; siderophore.

Figure 1

Structures of methanobactins (Mbns) characterized to date. (a) The structure of copper-chelated Mbn (CuMbn) (top) from Methylosinus (Ms.) trichosporium OB3b. The C-terminal methionine, in gray, is sometimes absent from the final compound. The crystal structure (bottom) clearly shows the distorted tetrahedral geometry of the copper binding site. (b) The structure of CuMbn from Methylocystis (Mc.) sp. SB2. Based on the precursor peptide sequences for this and other Methylocystis Mbns, additional residues are present at the C terminus of these compounds during biosynthesis but have not yet been experimentally observed. (c) The structure of Mbn from Mc. hirsuta CSC1. Determined via X-ray crystallography, this and the remaining Methylocystis Mbns have six-membered rings (assigned as a pyrazinediol) as the first heterocycle. Residues in gray are sometimes absent. (d) The structure of Ms. sp. LW4 CuMbn, which, like Ms. trichosoporium OB3b CuMbn, contains two oxazolone rings and neighboring thioamide groups. (e) The structure of Mbn from Mc. sp. M. Residues in gray are sometimes absent. (f) The structure of CuMbn from Mc. rosea SV97. Residues in gray are sometimes absent. (g) Potential identities for the “N-terminal” heterocycle in Mbns from Methylocystis species. Figure adapted with permission from References and .

Figure 2

Current model for processes related to (a) copper homeostasis and methanobactin (Mbn) transport, regulation, and (b) biosynthesis in Methylosinus trichosporium OB3b. Abbreviations: ABC, ATP-binding cassette transporter; CuMbn, copper-chelated Mbn; ICM, intracytoplasmic membrane; IM, inner membrane; OM, outer membrane; pMMO, particulate methane monooxygenase; sMMO, soluble methane monooxygenase. Figure adapted with permission from References and .

Figure 3

Methanobactin (Mbn) operons. (a) Operon schematics for Mbn operon groups. (b) Phylogenetic tree of MbnB protein sequences, illustrating the divisions used to define the Mbn operon groups. (c) Sequence logos for MbnA sequences from different groups. Figure adapted with permission from References and .

Figure 4

Structures of putative chalkophores. (a) Coproporphyrin III is a porphyrin. Although not traditionally considered siderophores, porphyrin (heme) uptake is a source of iron for many species. (b) Yersiniabactin (Ybt) metal binding is partly mediated by heterocycles, which are not among the traditional catecholate, hydroxymate, and phenolate siderophore ligands. (c) SF2768 has unusual diisonitrile groups that are likely involved in copper binding.

Figure 5

Structures of select copper-binding siderophores. (a) Nicotianamine and (b) staphylopine share structural characteristics and biosynthesis genes, despite differing origins in eukaryotic and bacterial species. (c) Pyochelin (Pch) has a metal-chelating hydroxyphenol-thiazolyl-thiazolidinyl backbone similar to that of Yersiniabactin (Ybt). (d) Pyoverdine I (Pvd) is a member of the large family of primarily peptidic pyoverdine siderophores. (e) Pyridine-2,6-dithiocarboxylic acid (pdtc) is a small metal-binding secondary metabolite. (f) Schizokinen was originally identified in gram-positive Bacillus and cyanobacterial Anabaena species.