NMR, mass spectrometry and chemical evidence reveal a different chemical structure for methanobactin that contains oxazolone rings

Methanobactin (mb) is a small copper-binding peptide produced by methanotrophic bacteria and is intimately involved in both their copper metabolism and their role in the global carbon cycle. The structure for methanobactin comprises seven amino acids plus two chromophoric residues that appear unique to methanobactin. In a previously published structure, both chromophoric residues contain a thiocarbonyl attached to a hydroxyimidazolate ring. In addition, one is attached to a pyrrolidine ring, while the other is attached to an isopropyl ester. A published X-ray determined structure for methanobactin shows these two chromophoric groups forming an N₂S₂ binding site for a single Cu(I) ion with a distorted tetrahedral geometry. In this report we show that NMR, mass spectrometry, and chemical data reveal a chemical structure that is significantly different than the previously published one. Specifically, the ¹H and ¹³C NMR assignments are inconsistent with an N-terminal isopropyl ester and point instead to a 3-methylbutanoyl group. Our data also indicate that oxazolone rings instead of hydroxyimidazolate rings form the core of the two chromophoric residues. Because these rings are directly involved in the binding of Cu(I) and other metals by methanobactin and are likely involved in the many chemical activities displayed by methanobactin, their correct identity is central to developing an accurate and detailed understanding of methanobactin-s many chemical and biological roles. For example, the oxazolone rings make methanobactin structurally more similar to other bacterially produced bactins and siderophores and suggest pathways for its biosynthesis.