  We define the reduced Khovanov homology of an open book (S,h), and we
identify a distinguished "contact element" in this group which may be used to
establish the tightness or non-fillability of contact structures compatible
with (S,h). Our construction generalizes the relationship between the reduced
Khovanov homology of a link and the Heegaard Floer homology of its branched
double cover. As an application, we give combinatorial proofs of tightness for
several contact structures which are not Stein-fillable. Lastly, we investigate
a comultiplication structure on the reduced Khovanov homology of an open book
which parallels the comultiplication on Heegaard Floer homology defined
previously by the first author.
