Chemistry Reference
In-Depth Information
Figure 16.1
Continued
16.2
Ca
2+
: Contacting Charged Ligands
The human pentraxin serum amyloid P component (SAP) contains two Ca
2+
about 4 Å apart and bridged by amino acid side-chains (Figure 16.1d). As methyl
4,6 -
O -
(1 - carboxyethylidene) -
DG ) can displace SAP
from amyloid fi bers, this substance was used as ligand. Its carboxyl group is in
contact to both Ca
2+
(distances of 2.39 and 2.45 Å, respectively) [9, 10]. Beyond
that, Gln148 and Asn59 share assignments in binding Ca
2+
and galactose (here
the O6 atom) [10]. Thus, the calcium ions ensure the correct positioning of
side-chain amides and sugar oxygen atoms. We have herewith witnessed an
example of a direct contact between Ca
2+
and a negatively charged group of the
ligand. It opens a new aspect for Ca
2+
functionality beyond its eminent structural
role.
Moving from a serum protein to basement membranes, a pertinent protein
domain mediating binding to heparin and
β
-
D
- galactopyranoside ( MO
β
-dystroglycan is found in laminin,
agrin and perlecan (please see Chapter 11.5). With a root mean square deviation
of 2.7 Å relative to SAP, this laminin G-like module is also classifi ed as a
α
- sand-
wich protein (Figure 16.1e) [11, 12]. Binding-site positioning has special features:
whereas the two Ca
2+
are at the center of one face of the
β
β
- sandwich in SAP (Figure
