Biomedical Engineering Reference
In-Depth Information
Figure 7.4
Proposed site of substrate entry for PfA-M17 hexamer. The molecular
surface of PfA-M17 is colored by chain (A-F). The active-site zinc and
carbonate of chain B are visible (magenta spheres). Chains C and D are
occluded in this view. The position of the loop (with the molecular surface
omitted) in chain B that sits at the entrance to the catalytic cavity is shown
as a yellow coil. (Reproduced with permission from McGowan et al.
42
).
partially disordered in the individual subunits of the hexamer in the X-ray
crystal structure, sits at the entrance to the catalytic cavity. The complete loop
was visible in electron density in one subunit chain in the structure where it
occludes the entrance to the enzyme interior (Figure 7.4). The flexibility of
this loop may regulate the substrate entry and product exit from the buried
aminopeptidase active site.
7.2.2.2 Active-Site Metal Ions
The active site of PfA-M17 contains two metal-binding sites, a readily
exchangeable site (site 1) and a tight binding site (site 2).
43
While PfA-M17
retains residual catalytic activity when the metal ion from site 1 is removed, the
removal of metal ions from both sites results in an inactive apo-enzyme that
cannot be re-activated by the addition of divalent metal cations.
43
However, the
divalent metal cation binding at site 1 of PfA-M17 can be functionally
exchanged for other metal cations, the enzyme displaying a preference in the
order Zn
21
4Mn
21
4Co
21
4Mg
21
.
43
The type of metal cation in the active site
of these enzymes can therefore influence the catalytic eciency against peptide
substrates as well as the binding of inhibitors.
The 0.24 nm X-ray crystal structure of PfA-M17_Zn
21
Zn
21
revealed a di-
zinc center in the active site with strong electron density for both zinc positions
