  The contribution of quarks with masses m >> Lambda_QCD is the only part of
the structure functions in deep-inelastic scattering (DIS) which is not yet
known at the next-to-next-to-leading order (NNLO) of perturbative QCD. We
present improved partial NNLO results for the most important structure function
F_2(x,Q^2) near the partonic threshold, in the high-energy (small-x) limit and
at high scales Q^2 >> m^2; and employ these results to construct approximations
for the gluon and quark coefficient functions which cover the full kinematic
plane. The approximation uncertainties are carefully investigated, and found to
be large only at very small values, x smaller about 10^-3, of the Bjorken
variable.
