Chemistry Reference
In-Depth Information
16
Ca
2+
: Mastermind and Active Player for Lectin Activity
(Including a Gallery of Lectin Folds)
Hans - Joachim Gabius
A set of monosaccharides is the biochemical equivalent of an alphabet for code
word generation by oligomerization (please see Chapter 1), and the resulting mes-
sages are decoded by lectins (please see Chapters 13 and 15 for basic aspects, and
Chapters 17 - 19 and 24 - 30 for lectin functions). To accomplish this feat functional
groups of amino acids and water molecules as well as stacking between aromatic
and sugar rings cooperate (please see Chapter 13 for details). What's more, Ca
2+
can be recruited for assignments in sugar binding. This ion exerts its activity in
remarkably different ways. This chapter systematically illustrates crystallographic
evidence for the various natural strategies of teaming up Ca
2+
with amino acid
residues and water molecules to let the respective protein gain optimal lectin activ-
ity. At the same time, the illustrations provide a view on folding of Ca
2+
- dependent
bacterial, plant and vertebrate lectins. We start with the function of Ca
2+
to struc-
turally organize the lectin site without direct contact to the sugar, calling on
the best-known lectin. The classical protocol for its crystallization is given in the
Info Box .
16.1
Ca
2+
: Organizing the Active Site
The agglutinin concanavalin A ( ConA ) from
Canavalia ensiformis
has a prominent
position in the history of lectinology (please see Chapter 15 for details). Its fold is
established by an antiparallel
- sandwich evocative of a jelly -roll (a Danish pastry)
(Figure 16.1a). ConA is the structural role model for many leguminous lectins
(please see Chapter 18 for functions and applications of plant lectins). It requires
the presence of a calcium ion and a further metal ion, naturally Mn
2+
, at distinct
preformed sites for lectin activity [1]. Invariably, four amino acids and two water
molecules are engaged for contact, and the driving force for the orchestrated
structural rearrangements with loop stabilization and a
trans - /cis -
isomerization of
a peptide bond is provided upon ion accommodation [2, 3]. As shown in Figure
16.1a, occupancy of the two sites for metal ions, for example, directs the amide
β
