The calcium ion binding site in bovine chymotrypsin A

Simplicity within the complexity: bilateral impact of DMSO on the functional and unfolding patterns of α-chymotrypsin

New understanding of the fundamental links between protein stability, conformational flexibility and function, can be gained through synergic studies on their catalytic and folding/unfolding properties under the influence of stabilizing/destabilizing additives. We explored an impact of dimethyl sulfoxide (DMSO), the moderate effector of multilateral action, on the kinetic (functional) and thermodynamic (thermal unfolding) patterns of a hydrolytic enzyme, α-chymotrypsin (α-CT), over a wide range of additive concentrations, 0-70% (v/v). Both the calorimetric and kinetic data exhibited rich behavior pointing to the complex interplay of global/local stability (and flexibility) patterns. The complex action of DMSO is explained through the negative and positive preferential solvation motifs that prevail for the extreme opposite, native-like and unfolded states, respectively, implying essential stabilization of compact domains by enhancement of interfacial water networks and destabilization of a flexible active site by direct binding of DMSO to the unoccupied specific positions intended for elongated polypeptide substrates.

Absorption spectroscopic investigations involving 4f-4f transitions to explore the kinetics of complexation of Zn(II) with Pr(III)-glutathione complex

The kinetics of complexation of Zn(II) with Pr2(GSH)2.(H2O)4 1 (GSH-glutathione) to form Pr2(GSH)2Zn(H2O)6 complex 2 has been investigated at 30 degrees C by following the changes in the intensity (absorbance, molar absorptivity) and intensity (oscillator strength P and Judd-Ofelt-T(lambda) intensity) parameters in equimolar DMF-Water. In order to support kinetics data, the complex 1 and 2 have been synthesized and characterized by elemental analysis, IR and 1H NMR spectral analysis. Molecular weights of 1 and 2 determined by ebullioscopic and cryoscopic methods and preliminary molecular modeling experiments have helped in proposing most probable structures for 1 and 2. Kinetics showed first order dependence, in terms of concentration of Zn(II) and complex 1, on the rate of formation of complex 2. Generally, hypersensitive transitions (the transitions obeying /DeltaJ/ selection rules) were used in spectra-structure correlation studies. We have used the four 4f-4f transitions of Pr(III), which do not obey selection rules, yet are substantially sensitive to coordination changes, for kinetic study. The rate constants evaluated from individual 4f-4f bands are quite close hence supporting our observations on ligand mediated pseudohypersensitivity.

Effect of calcium ion on the dimerization of alpha-chymotrypsin

Results of a sedimentation equilibrium study of the inhibitory effect of calcium ion on the dimerization of alpha-chymotrypsin (pH 3.9, I 0.2) are used to establish that the phenomenon does not reflect increased electrostatic repulsion between Ca2(+)-saturated enzyme molecules, but rather the displacement of the monomer-dimer equilibrium by the specific, weak interaction of metal ion with a single site on monomeric enzyme.

Long-range structural changes in proteinase K triggered by calcium ion removal

The X-ray crystal structure of the subtilisin-type enzyme proteinase K at 1.5 A resolution shows that is has two binding sites for Ca2+. Scatchard analysis indicates that one Ca2+ binds tightly, with pK 7.6 x 10(-8) M-1, and the other only weakly. Although Ca2+ is not directly involved in the catalytic mechanism and is 16.6 A away from the alpha-carbon atoms of the catalytic triad Asp 39-His 69-Ser 224, the activity of proteinase K towards the synthetic substrate succinyl-Ala-Ala-Ala-p-nitroanilide drops slowly to approximately 20% of its original value when it is depleted of Ca2+. This is not due to autolysis of the enzyme. The X-ray crystal structure of Ca2+-free proteinase K shows that removal of Ca2+ from the tight binding site triggers a concerted domino-like movement of five peripheral loops and of two alpha-helices. At a distance of 25 A from this calcium-binding site, the geometry of both the secondary substrate binding site and of the catalytic triad is affected by this movement thereby reducing the activity of the enzyme.

Effect of Cu2+ and Zn2+ on the inhibition of human leucocyte elastase by 6-alkyl-3-(omega-carboxyalkyl)-2-pyrone, oleic acid and sulindac sulfide

Inhibition of human leucocyte elastase by oleic acid and the structurally related 3-(1'-oxo-7'-carboxyheptyl)-4-hydroxy-6-octyl-2-pyrone is considerably enhanced by the addition of Cu2+, Zn2+ and, to a lesser extent, Co2+ and Ca2+. Sulindac sulfide and diflunisal also respond to changes in copper concentration, while Boc-Ala-Pro-Val-NH[CH2]10CO2H does not. Binding of the -CO2H group in the vicinity of the S5 subsite is proposed for all but the last compound to account for this effect. Incubation experiments indicate that Cu2+ binds more rapidly to the enzyme than does the inhibitor. Local changes in conformation result in improved binding of the inhibitor, but do not affect the substrate (Km unchanged). Chelation by EDTA is time-dependent, indicating that the Cu2+ is shielded by the inhibitor. The results may partially explain the well-known anti-arthritic and anti-inflammatory properties of copper and zinc and their organic salts.

The enzymatic activity of proteinase K is controlled by calcium

The fungal proteinase K (EC 3.4.21.14) is a very potent unusually stable member of the subtilisin family. Its X-ray structure determined at 0.15-nm resolution shows two bound Ca2+ ions. Ca1 is in near-ideal pentagonal bipyramidal configuration with Asp200 carboxylate and Pro175 peptide C = O in an apical, and Val177 peptide C = O and four water molecules in an equatorial position, whereas Ca2 displays incomplete octahedral coordination with the carboxylate of Asp260, the peptide C = O of Val16 and the two water molecules. Scatchard analysis of the titration of Ca2+-free proteinase K with Ca2+ yields a single dissociation constant (7.6 +/- 2.5) x 10(-8) M associated with the tightly bound Ca1 whereas Ca2 is so weakly bound that it cannot be titrated. If proteinase K is depleted of Ca2+ by treatment with EDTA, followed by gel filtration, its enzymatic activity drops within 6 h to 20% of its original value, without autolysis. Addition of excess Ca2+ immediately raises the residual activity to 28%, but full activity is not achieved. Removal of Ca2+ triggers a conformational change of the substrate recognition site because there is a direct connection, via secondary structure hydrogen bonds, between the Ca1 binding site and the substrate-recognition site. This is indicated further by circular dichroism and fluorescence-spectroscopic data, and by reversed-phase FPLC, carried out in the presence and absence of Ca2+, but the overall structure of the enzyme is not affected. Depletion of Ca2+ also influences binding of longer peptide inhibitors of the chloromethane type, it increases the rate of autolysis after about 48 h, it reduces the thermal stability (measured by activity tests from 65 degrees C to 46 degrees C), and it enhances the deactivation by 8 M urea which inactivates to only 65%, whereas sodium dodecyl sulfate totally inactivates at a concentration of 12.5%.

Influence of calcium binding on the thermal stability of 'thermitase', a serine protease from Thermoactinomyces vulgaris

'Thermitase' (EC 3.4.21.14), a thermostable extracellular serine protease from Thermoactinomyces vulgaris, binds one calcium ion with a dissociation constant of about 10-4M at 25 degrees C and pH 7.5 to 3.5. In addition, two calcium ions are bound more tightly to the enzyme, as shown by experiments with a calcium-selective electrode. The single most weakly bound calcium ion causes a slight quenching of the protein fluorescence emission, accompanied by a stabilization against thermal denaturation or autolysis and an increase of esterolytic activity of approx. 10%. The tightly bound calcium ions have only a slight influence on activity or on thermal denaturation or autolytic degradation. The activation parameters of thermal denaturation indicate that 'thermitase' belongs to the class of thermostable enzymes with a high intrinsic stability.

Lanthanides as probes for calcium in biological systems

Calcium ion plays an essential role in many biological processes. The environment about Ca2+may be probed by substitution of tripositive lanthanide ions, Ln3+. Ca2+proteins fall into two broad classes: those that are inhibited by Ln3+substitution and those that are not. It is suggested that although Ca2+undertakes a primarily structural role in the Ln3+non-inhibited proteins, Ca2+may be near the active site or participate in the mechanism of action of Ln3+inhibited proteins. Ca2+and Ln3+radii are similar; most Ln3+are slightly larger than Ca2+in complexes of the same coordination number, and substitution of Ln3+for Ca2+is accommodated by a slight decrease in bond distance or by an increase in coordination number. Luminescence from Tb3+has been demonstrated to be a sensitive environmental probe of Ca2+binding sites in proteins.

The effect of calcium ion on the urea denaturation of immobilized bovine trypsin

The effect of calcium ion on the urea denaturation of trypsin has been investigated. By using trypsin immobilized on glass beads, all possibilities of autolysis occurring during the denaturation process are eliminated. It was found that in 8 M urea calcium ion markedly decreases the denaturation rate of the immobilized trypsin. Conversely, the presence of calcium ion markedly accelerates the rate of renaturation of denatured immobilized trypsin. Calcium may exert its stabilizing effect on the tertiary structure of the protein by coordination to the side chains of Asp 194, Ser 190 and the carbonyl group of Ser 139 (using the chymotryptic numbering system).