Conspectus
N-Acetylgalactosamine
(GalNAc)-type O-glycosylation
is an essential posttranslational modification (PTM) that plays fundamental
roles in biology. Malfunction of this PTM is exemplified by the presence
of truncated O-glycans in cancer. For instance, the
glycoprotein MUC1 is overexpressed in many tumor tissues and tends
to carry simple oligosaccharides that allow for the presentation of
different tumor-associated antigens, such as the Tn or sTn antigens
(GalNAc-α-1-O-Thr/Ser and Neu5Acα2-6GalNAcα1-O-Ser/Thr,
respectively). In other cases, such as tumoral calcinosis associated
with O-glycosylation of the fibroblast growth factor 23, O-glycans are absent or less abundant. Significant progress has been
made in determining the three-dimensional structures of biomolecules
that recognize GalNAc, such as antibodies, lectins, mucinases, GalNAc-transferases,
and other glycosyltransferases. Analysis of the complexes between
these entities and GalNAc-containing glycopeptides, in most cases
derived from crystallographic or NMR analysis, provides an understanding
of the key structural elements that control molecular recognition
of these glycopeptides. Here, we describe and compare the binding
sites of these proteins in detail, focusing on how the GalNAc moieties
interact selectively with them. We also summarize the differences
and similarities in GalNAc recognition. In general, the recognition
of GalNAc-containing glycopeptides is determined by hydrogen bonds
between hydroxyl groups and the N-acetyl group of
GalNAc with proteins, as well as CH-π contacts in which the
hydrophobic α-face of the sugar and the methyl group of NHAc
can be involved. The latter interaction usually provides the basis
for selectivity. It is worth noting that binding of these glycopeptides
depends primarily on recognition of the sugar moiety, with some exceptions
such as a few anti-MUC1 antibodies that primarily recognize the peptide
backbone and use the sugar to facilitate shape complementarity or
to establish a limited number of interactions with the protein. Focusing
specifically on the GalNAc moiety, we can observe that there is some
degeneracy of interactions within the same protein families, likely
due to substrate flexibility. However, when all studied proteins are
considered together, despite the commonalities within each protein
family, no pattern can be discerned between the different families,
apart from the presence of common residues such as Tyr, His, or Asp,
which are responsible for hydrogen bonds. The lack of a pattern can
be anticipated, given the diverse functions of mucinases, glycosyltransferases,
antibodies, and lectins. Finally, it is important to point out that
the conformational differences observed in solution in glycopeptides
bearing GalNAc-α-1-O-Ser or GalNAc-α-1-O-Thr also can
be found in the bound state. This unique characteristic is exploited,
for instance, by the enzyme C1GalT1 to broadly glycosylate both acceptor
substrates. The findings summarized in this review may contribute
to the rational structure-guided ...