PDBsum entry 1o8y

Protease inhibitor

PDB id: 1o8y

Contents

Protein chain

14 a.a.

References listed in PDB file

Key reference

Title

Enzymatic cyclization of a potent bowman-Birk protease inhibitor, Sunflower trypsin inhibitor-1, And solution structure of an acyclic precursor peptide.

Authors

U.C.Marx, M.L.Korsinczky, H.J.Schirra, A.Jones, B.Condie, L.Otvos, D.J.Craik.

Ref.

J Biol Chem, 2003, 278, 21782-21789. [DOI no: 10.1074/jbc.M212996200]

PubMed id

12621047

Abstract

The most potent known naturally occurring Bowman-Birk inhibitor, sunflower trypsin inhibitor-1 (SFTI-1), is a bicyclic 14-amino acid peptide from sunflower seeds comprising one disulfide bond and a cyclic backbone. At present, little is known about the cyclization mechanism of SFTI-1. We show here that an acyclic permutant of SFTI-1 open at its scissile bond, SFTI-1[6,5], also functions as an inhibitor of trypsin and that it can be enzymatically backbone-cyclized by incubation with bovine beta-trypsin. The resulting ratio of cyclic SFTI-1 to is approximately 9:1 regardless of whether trypsin is incubated with or SFTI-1. Enzymatic resynthesis of the scissile bond to form cyclic SFTI-1 is a novel mechanism of cyclization of SFTI-1[6,5]. Such a reaction could potentially occur on a trypsin affinity column as used in the original isolation procedure of SFTI-1. We therefore extracted SFTI-1 from sunflower seeds without a trypsin purification step and confirmed that the backbone of SFTI-1 is indeed revealed high heterogeneity, were identified. The main species closely resembles the structure of cyclic SFTI-1 with the broken binding loop able to rotate between a cis/trans geometry of the I7-P8 bond with the cis conformer being similar to the canonical binding loop conformation. The non-reactive loop adopts a beta-hairpin structure as in cyclic wild-type SFTI-1. Another species exhibits an iso-aspartate residue at position 14 and provides implications for possible in vivo cyclization mechanisms.

Figure 4.
FIG. 4. H secondary chemical shifts of cyclic SFTI-1 in 20% TFE (white) (pH 4.5) recorded at 750 MHz and 273 K, species A of SFTI-1[6,5] in water (black), species B of SFTI-1[6,5] in water (hatched lines sloping down to the left) and species C of SFTI-1[6,5] in water (hatched lines sloping down to the right) (pH 4.5) recorded at 750 MHz and 286 K. K5 is omitted, because it is the open COOH-terminal residue in SFTI-1[6,5], and therefore, a comparison to the closed K5 residue of cyclic SFTI-1 is not meaningful.

Figure 5.
FIG. 5. Ensemble of 20 structures of the most abundant species (species A) of SFTI-1[6,5]. Conformer A1 (cis I7-P8 conformation, pdb code 1O8Z [PDB] ) and conformer A2 (trans I7-P8 conformation, pdb code 1O8Y [PDB] ) superimposed over the backbone heavy atoms of all residues. The backbone of the conformers A1 and A2 are colored purple and pink, respectively, and the disulfide bond between C3 and C11 is colored yellow. The P1-P1' site occurs at the break in the backbone between residues K5 and S6.

The above figures are reprinted by permission from the ASBMB: J Biol Chem (2003, 278, 21782-21789) copyright 2003.