Figure 2.
Figure 2. Stereoview of the substrate bound to CGTase. The
maltononaose binds from subsites -7 to +2, but for clarity only
subsites -2, -1 and +1 are shown. The arrow indicates the
scissile bond. a, Showing how the substrate fits into the 2F[o]
- F[c] electron density (1 contoured),
which was calculated with F[c] and phases from unliganded CGTase
to avoid bias^16. b, The substrate distortion at the catalytic
subsite -1 (central sugar ring) is revealed by superposition
with the minimum energy conformation of maltose (orange)^15. The
superposition is based on the glucose C3, C4 and C5 atoms in
subsite -1. Comparing the substrate ring puckering parameters
with a potential map from molecular mechanics calculations
indicates that the glucose ring at the catalytic subsite is
strained by ~17 kJ mol^−1 and has a ^4C[1] chair conformation
distorted towards a ^2H[3] half chair^15. c, Undistorted (free)
maltose clearly does not fit the 2F[o] - F[c] electron density
at subsite -1. The glycosidic bond torsion angles of maltose
were adjusted to fit the density at subsite +1.
contoured),
which was calculated with F[c] and phases from unliganded CGTase
to avoid bias^16. b, The substrate distortion at the catalytic
subsite -1 (central sugar ring) is revealed by superposition
with the minimum energy conformation of maltose (orange)^15. The
superposition is based on the glucose C3, C4 and C5 atoms in
subsite -1. Comparing the substrate ring puckering parameters
with a potential map from molecular mechanics calculations
indicates that the glucose ring at the catalytic subsite is
strained by ~17 kJ mol^−1 and has a ^4C[1] chair conformation
distorted towards a ^2H[3] half chair^15. c, Undistorted (free)
maltose clearly does not fit the 2F[o] - F[c] electron density
at subsite -1. The glycosidic bond torsion angles of maltose
were adjusted to fit the density at subsite +1.