Chemistry Reference
In-Depth Information
19
Animal and Human Lectins
Hans - Joachim Gabius
Previous chapters (Chapters 6 - 11 ) have illustrated the enormous structural diver-
sity of glycans. An elaborate synthetic machinery including enzymes for introduc-
ing distinct substitutions and remodeling underlies the establishment of the
exceptionally high coding capacity of glycans. The question immediately arises as
to how this information is turned into biological responses. An intermolecular
contact within the decoding process is especially favored at branch ends of mature
chains. They are spatially readily accessible (for a primer on the interaction of
glycans with proteins, please see Chapter 13). The limited fl exibility around gly-
cosidic linkages in glycans accounts for the presentation of only few key- like con-
formers suited for contact to a lectin (for conformational aspects of glycans, please
see Chapters 2 and 13 ; for defi nition of the term ' lectin ' , Chapter 15.3 ).
Given all these favorable characteristics of glycans at branch ends to enable a
role as biochemical signals (structural variability, spatial accessibility and limited
fl exibility), the level of diversity reached on the sugar side should be matched in
numbers by lectins. What held true for bacteria and viruses (please see Chapter
17) and plants (please see Chapter 18), in which numerous lectins had been
detected, was questioned for higher animals as late as 1973 [1] (one year before
the fi rst mammalian lectin was isolated; please see Chapter 15 for details). The
following decades witnessed a remarkable surge in our knowledge about lectins
in higher animals, both in number of families and family members [2]. With
lectins now fi nally being accepted as 'a common cell component' [1], lectin-car-
bohydrate interactions are expected to entail a wide range of functions, a key topic
of this chapter. The chapter fi rst provides an overview on protein folds with lectin
activity and next on lectin functions. Triggering such responses depends on the
binding of distinct glycoconjugates, that is the bioactive lectin ligands, as will be
exemplifi ed. Not only their presence but also spatial parameters will be identifi ed
to be crucial for bioactivity. As a consequence, six levels, at which the affi nity of a
glycan for a lectin can be adjusted, will be presented at the end of this chapter.
This synopsis can serve as a guideline to interpret structural changes in glycosyl-
ation as regulatory events.
