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Streptomyces griseus trypsin (SGT) is a bacterial serine proteinase that is more
homologous to mammalian than to other bacterial enzymes. The structure of SGT
has been solved primarily by molecular replacement, though some low-resolution
phase information was supplied by heavy-atom derivatives. The mammalian
pancreatic serine proteinases bovine trypsin (BT) and alpha-chymotrypsin (CHT)
were used as molecular replacement models. Because these proteins have low
homology with SGT compared to the majority of other successful replacement
models, new strategies were required for molecular replacement to succeed. The
model of SGT has been refined at 1.7 A resolution to a final R-factor of 0.161
(1 A = 0.1 nm); the correlation coefficient between all observed and calculated
structure factor amplitudes is 0.908. Solvent molecules located in the crystal
structure play an important role in stabilizing buried charged and polar groups.
An additional contribution to stability can be seen in the fact that the
majority of the charged side-chains are involved in ionic interactions,
sometimes linking the two domains of SGT. A comparison of SGT with BT shows that
the greatest similarities are in the active-site and substrate-binding regions,
consistent with their similar substrate specificities. The modeling of complexes
of SGT with two inhibitors of BT, pancreatic trypsin inhibitor (PTI) and the
third domain of Japanese quail ovomucoid (OMJPQ3), helps to explain why PTI
inhibits SGT but OMJPQ3 does not. Like BT, but unlike other bacterial serine
proteinases of known structure, SGT has a buried N terminus. SGT has also a
well-defined Ca2+-binding site, but this site differs in location from that of
BT.
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