Turkey ovomucoid third domain (OMTKY3) is a canonical
inhibitor of serine proteinases. Upon complex formation,
the inhibitors fully exposed P1 residue becomes
fully buried in the preformed cavity of the enzyme. All
20 P1 variants of OMTKY3 have been obtained
by recombinant DNA technology and their equilibrium association
constants have been measured with six serine proteinases.
To rationalize the trends observed in this data set, high
resolution crystal structures have been determined for
OMTKY3 P1 variants in complex with the bacterial
serine proteinase, Streptomyces griseus proteinase
B (SGPB). Four high resolution complex structures are being
reported in this paper; the three β-branched variants,
Ile18I, Val18I, and Thr18I, determined to 2.1, 1.6, and
1.7 Å resolution, respectively, and the structure
of the Ser18I variant complex, determined to 1.9 Å
resolution. Models of the Cys18I, Hse18I, and Ape18I variant
complexes are also discussed. The β-branched side chains
are not complementary to the shape of the S1
binding pocket in SGPB, in contrast to that of the wild-type
γ-branched P1 residue for OMTKY3, Leu18I.
χ1 angles of approximately 40° are
imposed on the side chains of Ile18I, Val18I, and Thr18I
within the S1 pocket. Dihedral angles of +60°,
−60°, or 180° are more commonly observed
but 40° is not unfavorable for the β-branched side
chains. Thr18I Oγ1 also forms a hydrogen
bond with Ser195 Oγ in this orientation.
The Ser18I side chain adopts two alternate conformations
within the S1 pocket of SGPB, suggesting that
the side chain is not stable in either conformation.