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Figure 3.
FIG:. 3. Difference electron density maps computed with coefficients IE',\ - IP,I. phases hC in the region
ofthe active ite of SGPA. These are fragment maps in which a portin of the molecule was not included in.~
the structure factor calculation.
(a) AcPro-Ala-Pro-Phe-OH, peptide 1, the atoms excluded from the phase calculation were 0' of
Ser195 and th terminal carboxyl oxygen atom of the tetrapeptide product which binds in the oxyanion
bindin site. The 2 largest positive peaks correspond to these expected atomic position. The 3rd peak
just below that for OY of Ser195 corresponds to a water molecule not included in the phasng.
(b) The corresponding atomic model ofthe tetrapeptide aldehyde (onl P, Pro and P, Phe shown) with
the electron ensity for Oy Her195 and the two water moleules 0366 and 0361 which were not mcluded in
the calculation.
In both (a) and (b) the electron density cotour surfaces are kO.23 eA3 and the negative density is
represented by broken lines. This and subsequent Figures have been made with the aid of the MMS-X
interactive graphics (Barry et al., 1976).
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