From Wikipedia,
the free encyclopedia.
The Accessible Surface Area
(ASA) is the
surface area of a
biomolecule (protein,
DNA, etc.) that is accessible
to the
solvent. Is usually quoted in
square
ångstrom (a standard
unit of
measurement in
molecular biology). ASA was
first described by Lee & Richards
in
1971, and is typically
calculated using the 'rolling
ball' algorithm developed by
Shrake & Rupley in
1973. This algorithm uses a
sphere (of
solvent) of a particular
radius to 'probe' the surface
of the
molecule. The choice of the
'probe radius' does have an effect
on the observed surface area as
using a smaller probe radius
detects more surface details and
therefore reports a larger
surface. A typical value is 1.4Å,
which approximates the radius of a
water molecule.
ASA is calculated differently
in the AREAIMOL program (in the
CCP4 Program Suite). AREAIMOL
calculates the solvent accessible
surface area by generating surface
points on an extended sphere about
each atom (at a distance from the
atom centre equal to the sum of
the atom and probe radii), and
eliminating those points that lie
within equivalent spheres
associated with neighbouring
atoms. The number of remaining
points is then multiplied by the
point density to obtain an
estimate of the ASA.
There is a simple (average)
relationship between the ASA and
the
molecular weight of
monomeric and
oligomeric
proteins.
