Methanobactin: Metal binding properties, physiological function and biosynthesis
Methanobactins (mbs) are low molecular mass (< 1300 Da) modified peptides
secreted by many methanotrophs or methane oxidizing bacteria to sequester copper from the
environment. To date, methanobactin has been structurally characterized from six
methanotrophs and can be divided into two groups. Group I methanobactins are represented
by methanobactins from Methylosinus trichosporium OB3b and Methylosinus sp. LW4. This
group is characterized by the presence of two oxazolone rings with adjacent thioamide
groups. Two nitrogens from the oxazolone rings and two sulfurs from the thioamide groups
come together to form a copper coordination site in distorted tertrahedral geometry. This
group of methanobactin also has two cysteines in the mature protein that form a stable
disulfide bond. The second group is composed of methanobactins from four different
Methylocystis species. This group of methanobactins has a hairpin like structure following
copper binding with sulfate group attached to serine or threonine and is structurally more
dynamic than group I methanobactins. This group is also characterized by the presence of a
C-terminal oxazolone ring with an associated thioamide and either an N-terminal
imidazolone or pyrazinedione group and an associated thioamide.
The first part of this dissertation is focused on the study of methanobactin binding to various non-copper metals such as mercury and gold and the role of that binding of noncopper
metals has on the physiology of methanotrophs. The second part of this dissertation is
focused on understanding the post-translational modifications required for methanobactin.
Until recently, the biosynthesis was assumed to be via a non-ribosomal peptide synthase or
polyketide synthase. However, during the course of my dissertation, we determined that
methanobactin is indeed produced ribosomally and post-translationally modified. I show in
this dissertation that TonB-dependent transporter gene in the methanobactin gene cluster is
involved in the uptake of copper-bound methanobactin. In addition, we demonstrate that
mbnN is involved in the deamination of the N-terminal oxazolone ring, a post-translational
modification required in the formation of the N-terminal oxazolone ring in the methanobactin from Methylosinus trichosporium OB3b.
The results and methods used in this research would further help determine the role of
other genes involved in biosynthesis of methanobactin and bioengineer methanobactin for
various human and animal health purposes. In light of recent evidence of methanobactin
being effective chelator of excess copper in Wilson’s disease in rat models, understanding the
biosynthesis of methanobactin has become more important.