The kinetics of binding of chicken cystatin to papain were studied by stopped-flow fluorometry under pseudo-first-order conditions, i.e., with an excess of inhibitor. All reactions showed first-order behavior, and the observed pseudo-first-order rate constant increased linearly with the cystatin concentration up to the highest concentration that could be studied, 35 microM. The analyses thus provided no evidence for a limiting rate resulting from a conformational change stabilizing an initial encounter complex, in contrast with previous studies of reactions between serine proteinases and their protein inhibitors. The second-order association rate constant for complex formation was 9.9 X 10(6) M-1 s-1 at 25 degrees C, pH 7.4, I = 0.15, for both forms of cystatin, 1 and 2. This value approaches that expected for a diffusion-controlled rate. The temperature dependence of the association rate constant gave an enthalpy of activation at 25 degrees C of 31.5 kJ mol-1 and an entropy of activation at 25 degrees C of -7 J K-1 mol-1, compatible with no appreciable conformational change during the reaction. The association rate constant was independent of pH between pH 6 and 8 but decreased at lower and higher pH in a manner consistent with involvement of an unprotonated acid group with a pKa of 4-4.5 and a protonated basic group with a pKa of 9-9.5 in the interaction. The association rate constant was unaffected by ionic strengths between 0.15 and 1.0 but decreased somewhat at lower ionic strengths. Incubation of the complex between cystatin 2 and papain with an excess of cystatin 1 resulted in slow displacement of cystatin 2 from the complex.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction between five N-terminally truncated forms of chicken cystatin (starting at Leu-7, Leu-8, Gly-9, Ala-10 and Asp-15) and the cysteine proteinases papain and actinidin was studied by spectroscopic, kinetic and equilibrium methods. The u.v. absorption, near-u.v. c.d. and fluorescence emission difference spectra for the interactions with papain were all similar to the corresponding spectra for intact cystatin. The second-order association rate constants at 25 degrees C, pH 7.4, I 0.15, for the binding of the truncated forms to papain varied about 2-fold, from 6 x 10(6) to 1.5 x 10(7) M-1.s-1, and were comparable to the value of 9.9 x 10(6) M-1.s-1 for intact cystatin. In contrast, the rate constants for the dissociation of the complexes with papain increased markedly with increasing extent of truncation, from 7.5 x 10(-6)s-1 for Leu7 cystatin (a truncated form of cystatin having Leu-7 as its N-terminal amino acid) to 1.6s-1 for Ala10-cystatin, whereas the dissociation rate constants for the latter form and Asp15-cystatin were similar. Consequently, the binding affinities between the truncated cystatins and papain decreased in an analogous manner, as was also shown for the interaction with actinidin by equilibrium measurements. Studies of the binding of the truncated cystatins to inactivated papains indicated that small substituents on the active-site cysteine of the enzyme can be accommodated in the complex without any loss of affinity when the N-terminal segment of the inhibitor is removed. Taken together, the results suggest that in the N-terminal region of chicken cystatin only residues preceding Ala-10 participate in the interaction with proteinases. Of these residues, Leu-7 and Leu-8 together account for about two-thirds of the unitary free energy of binding contributed by the N-terminal region, the relative importance of the two residues being dependent on the target proteinase. Both Gly-9 and residues N-terminal of Leu-7 further stabilize the interaction but contribute substantially smaller binding energies than do the two leucine residues.

Recombinant antithrombin produced by baby hamster kidney (BHK) or Chinese hamster ovary (CHO) cells was separated into two fractions, containing comparable amounts of protein, by affinity chromatography on matrix-linked heparin. Fluorescence titrations showed that the more tightly binding fraction had a heparin affinity similar to that of plasma antithrombin (Kd 20 nM), whereas the affinity of the more weakly binding fraction was nearly 10-fold lower (Kd 175 nM). Analyses of the heparin-catalysed rate of inhibition of thrombin further showed that the fractions differed only in their affinity for heparin and not in the intrinsic rate constant of either the uncatalysed or the heparin-catalysed inactivation of thrombin. The recombinant antithrombin fraction with lower heparin affinity migrated more slowly than both the fraction with higher affinity and plasma antithrombin in SDS/PAGE under reducing conditions, consistent with a slightly higher apparent relative molecular mass. This apparent size difference was abolished by the enzymic removal of the carbohydrate side chains from the proteins. Such removal also increased the heparin affinity of the weakly binding fraction, so that it eluted from matrix-linked heparin at a similar position to the deglycosylated tightly binding fraction or plasma antithrombin. Analyses of N-linked carbohydrate side chains showed that the weakly binding fraction from CHO cells had a higher proportion of tetra-antennary and a lower proportion of biantennary oligosaccharides than the tightly binding fraction. We conclude that the recombinant antithrombin produced by the two cell lines is heterogeneously glycosylated and that the increased carbohydrate content of a large proporti6n of the molecules results in a substantial decrease in the affinity of these molecules for heparin. These findings are of particular relevance for studies aimed at characterizing the heparin-binding site of recombinant antithrombin by site-directed mutagenesis.

Papain which was inactivated by covalent attachment of small substituents to the active-site cysteine, up to the size of a carbamoylmethyl group, bound with high affinity to chicken cystatin (Kd < -15 pM), although less tightly than did active papain (Kd 60 fM). However, as the size of the substituent was increased further, the affinity decreased appreciably, generally in proportion to the size of the inactivating group. For instance the dissociation constants for papain inactivated with N-ethylmaleimide and

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The Cys-71-Cys-81 disulfide bond of the cysteine proteinase inhibitor, chicken cystatin, was specifically reduced by thioredoxin or low concentrations of dithiothreitol. This cleavage, followed by S-carbamoylmethylation, induced a conformational change of the protein, as evidenced by changes in isoelectric point and circular dichroism spectra and by an increased susceptibility to digestion by nontarget proteinases. The proteinase binding ability and the immunological properties of the inhibitor, however, were not detectably altered, indicating that the conformational change was limited to the region around the disrupted bond. In contrast, reduction of both disulfide bonds of cystatin by higher concentrations of dithiothreitol and subsequent alkylation led to the slow conversion of the inhibitor into two forms lacking proteinase binding ability, indicative of more extensive conformational changes. Together, these results suggest that the less accessible Cys-95-Cys-115 disulfide bond of chicken cystatin, but not the more accessible Cys-71-Cys-81 bond, is of importance for maintaining the conformation of the inhibitor required for binding of target proteinases.

A phage-display library of the cysteine-proteinase inhibitor, cystatin A, was constructed in which variants with the four N-terminal amino acids randomly mutated were expressed on the surface of filamenteous phage. Screening of this library for binding to papain gave predominantly variants with a glycine residue in position 4. This finding is in agreement with previous conclusions that glycine in this position is essential for tight binding of cystatin A to cysteine proteinases by allowing optimal interaction of the N-terminal region of the inhibitor with the enzyme. In contrast, the first three residues of the variants obtained by the screening were more variable. Two variants were identified with similar affinities for papain as the wild-type inhibitor, but with these residues, Val-Phe-Thr-or Ile-Leu-Leu, differing appreciably from those of the wild-type, Met-Ile-Pro. Other sequences of the N-terminal region, presumably mainly hydrophobic, can thus substitute for the wild-type sequence and contribute similar energy to the inhibitor±proteinase interaction. The two variants binding tightly to papain differed in their affinity for cathepsin B, demonstrating that cystatin variants with increased selectivity for a particular target cysteine proteinase can be obtained by phage-display technology.