Synthesis of trypsin inhibitor CMTI III from squash seeds (Cucurbita maxima)

Selenopeptide analogs of EETI-II retain potent trypsin inhibitory activities

Three-disulfide-bridged Ecballium elaterium trypsin inhibitor II (EETI-II) is a 28-residue peptide that belongs to the squash family of canonical trypsin inhibitors. Herein, we report synthesis and biological activity of three EETI-II analogs. In each of analog, a pair of cysteine residues forming a native disulfide bridge was individually replaced by a pair of selenocysteine residues. All selenopeptide analogs were chemically synthesized using the Fmoc protocol and subsequently folded in the presence of oxidized and reduced glutathione. The analogs containing a diselenide bridge displayed association constants with trypsin that ranged from 2.6 x 10(9) to 5.1 x 10(9)  [M(-1) ]. Our results suggest that the selenopeptide analogs retained low nanomolar inhibitory potencies, and only the diselenide bridge adjacent to the inhibitory binding loop weakened the interactions with trypsin by approximately fivefold. We discuss these findings in the context of a broader use of selenopeptide analogs as proxies to study cysteine-rich peptides.

Selection of low-molecular-mass trypsin and chymotrypsin inhibitors based on the binding loop of CMTI-III using combinatorial chemistry methods

Using a combinatorial chemistry approach, a decapaptide library containing the N-terminal fragment of trypsin inhibitor CMTI-III was synthesized by the solid-phase method. The peptide library was screened for trypsin and chymotrypsin inhibitory activity applying the iterative method in solution. Two decapeptides were selected and resynthesized for each enzyme. The association equilibrium constants ((1.1+/-0.2)x10(8) and (7.3+/-1.6)x10(7)) determined for peptides with trypsin inhibitory activity indicate that they are 3-4-fold less active than the CMTI inhibitors. On the other hand, they are significantly more effective as compared with the starting sequence. Two peptides selected as chymotrypsin inhibitors displayed about 10 times higher activity (1.7+/-0.4)x10(7) and (1.1+/-0.2)x10(7), respectively) than those monosubstituted in position P(1) of the CMTI-III analogue. Considering low molecular weight of peptides selected and the lack of conformational constraints in their structures, the results are promising. They are good templates as starting sequences for further selection of small, peptidomimetic proteinase inhibitors.

Analogues of Cucurbita maxima trypsin inhibitor III (CMTI-III) with elastase inhibitory activity

Three new CMTI-III analogues containing the Val residue in the reactive site (position 5) were synthesized by the solid-phase method. The analogues displayed an elastase inhibitory activity. It is shown that the removal of the N-terminal Arg residue and the introduction of the Gly-Pro-Gln tripeptide in the region 23-25 decreases the antielastase activity by two orders of magnitude. The removal of the disulfide bridge in positions 16-28 and the substitution of Ala for Cys16 and Gly for Cys28 decreases the activity (measured as Ka with HLE) by five orders of magnitude as compared with [Val5]CMTI-III.

Inhibition of human beta-factor XIIa by squash family serine proteinase inhibitors

Many inhibitors of trypsin and human beta-factor XIIa have been isolated from squash and related seeds and sequenced (Wieczorek et al., Biochem. Biophys. Res. Comm. (1985) 126, 646-652). The association equilibrium constants (Ka) of several of these inhibitors have now been determined with human beta-factor XIIa using a modification of the method of Green and Work (Park et al., Fed. Proc. Fed. Am. Soc. Exp. Biol. (1984) 43, 1962). The Ka's range from 7.8 x 10(4) M-1 to 3.3 x 10(8) M-1. Two isoinhibitors from Cucurbita maxima seeds, CMTI-I and CMTI-III, differ in only a single glutamate to lysine change in the P'4 position. This results in a factor of 62 increase in the Ka of the lysine inhibitor, CMTI-III (Ka = 3.3 x 10(8) M-1). To our knowledge, this is the largest effect ever seen for a residue substitution at the P'4 position of a serine proteinase inhibitor. The result is even more surprising because beta-factor XIIa's natural substrate, Factor XI, contains Gly in the P'4 position.

1H-n.m.r. studies of squash seed trypsin inhibitor

1H-n.m.r. studies at 500 MHz have been performed on a trypsin inhibitor (CMTI-III) found in squash seed (Cucurbita maxima). The sequential resonance assignments have been made using two-dimensional techniques. The chemical shifts for the assigned protons are reported at 30 degrees, pH 2.8 and form a basis for the determination of the solution structure of CMTI-III. Analysis of the NOE data, NH-alpha CH vicinal coupling constants and pattern of slowly exchanging amide protons indicates that the predominant feature of the solution conformation is a triple stranded beta sheet consisting of residues 8-10, 21-23, and 26-29. Residues 12-15 appear to form a beta turn.