Siderocalin Combats Mycobacterial Infections - Iron Acquisition by the Genus Mycobacterium - page 57

This process depends on carboxymycobactin; without it, mycobacteria-bound myco-
bactin does not accumulate iron from extracellular ferric transferrin [ 31 ].
In addition to accepting iron from ferric carboxymycobactin, mycobactins also
remove iron directly from the host macrophages in a unique mechanism. The lipo-
philic siderophores are water soluble enough to diffuse throughout a macrophage
while also being membrane permeable. The macrophage internal iron stores are all
vulnerable to sequestration by mycobactins. Once iron is bound, the ferric myco-
bactins accumulate in lipid droplets of the macrophages to be trafficked to phago-
somes where they may deliver the iron to inhabiting mycobacteria [ 39 ].

4.3 Scn Binds Carboxymycobactins

Iron piracy by mycobacteria does not go unchallenged by the human immune sys-
tem. Scn, being part of the immunoresponse to bacterial infections, attempts to
ward off mycobacterial infections by intercepting mycobacterial siderophores.
The Scn ligand binding domain, or calyx, is shallow, broad, and lined with
polar and positively charged residues (Arg81, Lys125, Lys134) [ 4 , 40 ]. It is also
quite rigid, with three binding pockets inside the calyx that impose a steric limita-
tion on which ligands are Scn-compatible. Furthermore, Scn resists any confor-
mational change when exposed to changes in pH, ionic strength or upon ligand
binding [ 4 ]. The isolation of ferric enterobactin in the protein calyx unveiled the
most direct clue behind the function of Scn. Ferric enterobactin is a metal complex
carrying a -3 charge due to its three catecholate units. The catecholate moieties
are also aromatic and planar, allowing each to fit complementarily into each of
the three binding pockets of Scn, optimized by hybrid electrostatic and cation-pi
interactions that occur between the positive residues and catecholates as well as
the overall negative charge of the complex [ 14 , 41 ]. The affinity of Scn for ferric
enterobactin is so great that the dissociation constant ( K D ) for the protein-ligand
complex, 0.41 nM [ 4 ], rivals the affinity of ferric enterobactin for its innate recep-
tor FepA ( K D = 0.27 nM) [ 42 ] (Table 4.1 ).

Table 4.1 Dissociation
constants ( K D ) of various
ligands with siderocalin a

Ferric (siderophore b ) K D (nM)

Fe(Ent) 0.41 (1)

Fe(CMB), n = 3 >9,000

Fe(CMB), n = 4 2,360 (9)

Fe(CMB), n = 5 1,100 (30)

Fe(CMB), n = 6 654 (20)

Fe(CMB), n = 7 128 (1)

Fe(CMB), n = 8 280

a From Ref. [ 10 ]
b Ent Enterobactin, CMB Carboxymycobactin