Crystal Structure of the Human Cytosolic Sialidase Neu2: Evidence for the Dynamic Nature of Substrate Recognition.

Gangliosides play key roles in cell differentiation, cellcell
interactions, and transmembrane signaling. Sialidases
hydrolyze sialic acids to produce asialo compounds,
which is the first step of degradation processes
of glycoproteins and gangliosides. Sialidase involvement
has been implicated in some lysosomal storage
disorders such as sialidosis and galactosialidosis. Neu2
is a recently identified human cytosolic sialidase. Here
we report the first high resolution x-ray structures of
mammalian sialidase, human Neu2, in its apo form and
in complex with an inhibitor, 2-deoxy-2,3-dehydro-Nacetylneuraminic
acid (DANA). The structure shows the
canonical six-blade -propeller observed in viral and
bacterial sialidases with its active site in a shallow crevice.
In the complex structure, the inhibitor lies in the
catalytic crevice surrounded by ten amino acids. In particular,
the arginine triad, conserved among sialidases,
aids in the proper positioning of the carboxylate group
of DANA within the active site region. The tyrosine residue,
Tyr334, conserved among mammalian and bacterial
sialidases as well as in viral neuraminidases, facilitates
the enzymatic reaction by stabilizing a putative carbonium
ion in the transition state. The loops containing
Glu111 and the catalytic aspartate Asp46 are disordered
in the apo form but upon binding of DANA become ordered
to adopt two short -helices to cover the inhibitor,
illustrating the dynamic nature of substrate recognition.
The N-acetyl and glycerol moieties of DANA are
recognized by Neu2 residues not shared by bacterial
sialidases and viral neuraminidases, which can be regarded
as a key structural difference for potential drug
design against bacteria, influenza, and other viruses.