Chemistry Reference
In-Depth Information
Table 16.1
The strategic roles of Ca
2+
in lectin activity.
Function
Type of lectin
Structural role in organizing lectin site (no
direct contact to ligand)
Leguminous lectins homologous to ConA, possibly lectin
chaperones in quality control (calnexin), animal lectin-type
cargo receptor (ERGIC-53) but not yeast orthologs,
Anguilla
anguilla
agglutinin
Structural role and direct contact to anionic
group(s) of ligand or neutralization of anionic
charges
Pentraxins, laminin G -like module (?), annexin A2, cation-
dependent mannose - 6 - phosphate receptor
Direct contact to neutral group(s) of ligand
with/without structural role
C - type lectins,
Pseudomonas aeruginosa
lectin I (two coordina-
tion bonds),
Pseudomonas aeruginosa
lectin II (four coordina-
tion bonds)
16.4
Conclusions
This chapter documents how calcium ions fulfi ll their mission in lectins. The dif-
ferent strategies are summarized in Table 16.1. This survey inspires the notion
that lectin activity might be regulatable by local/transient changes in Ca
2+
concen-
tration or the lectin's affi nity to Ca
2+
. In this respect, the detection of a pH-sensitive
switch for Ca
2+
and sugar binding in the endocytic asialoglycoprotein receptor of
rat liver (please see Chapters 15 and 19 for its function) when exposed to the
endosomal pH of 5.4 teaches us a salient lesson on regulation of binding activity
by the microenvironment [25]. Beyond lectins, Ca
2+
is operative in other sugar-
binding proteins, such as in enzymes degrading glycosaminoglycans and pectate
or glycosyltransferases. Therefore, when talking about sugar-binding activities in
lectins and also sugar-processing enzymes, the remarkable talents of Ca
2+
deserve
to be appreciated.
Summary Box
Lectins single out distinct sugar epitopes from the glycomic complexity for
binding. This process is often solely determined by functional groups of amino
acids and water molecules. In a series of cases, Ca
2+
is involved. It is a versatile
means toward the following ends: organizing the binding site without contact
to the ligand, neutralizing the negative charge in mannose-6-phosphate as well
as interacting with charged and even neutral ligands.
