A History of Iron Metabolism in the Mycobacteria - Iron Acquisition by the Genus Mycobacterium - page 27

ferric-form, be extracted into chloroform [ 88 , 89 ]. It was also found that the two
exochelins had different mechanisms for iron uptake: exochelin from M. smeg-
matis was taken up by an active (energy-dependent) transport system but the one
from M. bovis was by facilitated diffusion and was not energy dependent [ 90 , 91 ].
Furthermore, the M. smegmatis exochelin could not be taken up by M. bovis but
that from M. bovis could be taken up by M. smegmatis . Thus, two distinct types of
extracellular siderophores were being produced by the mycobacteria. Other water-
soluble exochelins were recovered from other non-pathogens: M. neoaurum [ 92 ]
and M. vaccae [ 93 ]. The latter siderophore was of interest as this species of myco-
bacteria did not appear to have a mycobactin.
2.5.2 The Water-Soluble Exochelins
It was nearly 20 years after the initial discoveries of the exochelins before their
structures were resolved though it was established soon after their initial isola-
tion that exochelin MS from M. smegmatis was probably a pentapeptide with three
N - epsilon -hydroxyornithines providing the chelating centre. Various research groups
in the UK had been approached by the author for assistance in trying to work out the
structures and to see how the ornithine residues, together with a beta -alanine and an
allo -threonine, were assembled. But none had been able to complete the work until
the author approached the group headed by Dudley Williams in the Department of
Chemistry at the University of Cambridge to help solve the structure of the water-
soluble exochelins. The structure of the chloroform-soluble exochelins was solved
by collaboration with a research group at Glaxo Research Laboratories.
The structure of the exochelin from M. smegmatis , then named as exoche-
lin MS, was determined by a PhD student, Gary Sharman working under Dudley
Williams at Cambridge University. Exochelin MS was an ornithinyl siderophore
with three hydroxamate groups that provided the iron chelating center (Fig. 2.8 a)
[ 94 ]. Further, but unpublished, work of the author indicated that the exochelin
from M. vaccae was probably similar if not identical to this exochelin. The struc-
ture of the exochelin from M. neoarum , called exochelin MN, was different [ 95 ]
but was still based on a peptide backbone as seen with exochelin MS (Fig. 2.8 b). It
had an unusual 2-hydroxyhistidine residue as part of its iron chelating center.
Although the structure of exochelin MS was not elucidated until 1995, quite a
lot of its properties and function had been worked out beforehand. Its uptake, see
above, was by an active transport process requiring the input of energy (i.e., ATP
was involved at some point of the mechanism) [ 90 ]. It was produced in a growth-
related manner and could readily solubilize iron from not only inorganic forms of
insoluble iron, such as ferric hydroxide and ferric phosphate, but also from ferritin
the storage form of iron found in all animals [ 88 , 89 ]. The involvement of myco-
bactin in the uptake of iron into M. smegmatis was not immediately apparent when
small concentrations of ferric-exochelin were used; however, when higher con-
centrations were used, a second uptake process became evident. This was a slower