The self-association of chymotrypsinogen A

Using the water signal to detect invisible exchanging protons in the catalytic triad of a serine protease

Chemical Exchange Saturation Transfer (CEST) is an MRI approach that can indirectly detect exchange broadened protons that are invisible in traditional NMR spectra. We modified the CEST pulse sequence for use on high-resolution spectrometers and developed a quantitative approach for measuring exchange rates based upon CEST spectra. This new methodology was applied to the rapidly exchanging Hδ1 and Hε2 protons of His57 in the catalytic triad of bovine chymotrypsinogen-A (bCT-A). CEST enabled observation of Hε2 at neutral pH values, and also allowed measurement of solvent exchange rates for His57-Hδ1 and His57-Hε2 across a wide pH range (3-10). Hδ1 exchange was only dependent upon the charge state of the His57 (k (ex,Im+) = 470 s(-1), k (ex,Im) = 50 s(-1)), while Hε2 exchange was found to be catalyzed by hydroxide ion and phosphate base (k(OH)⁻ = 1.7 × 10(10) M(-1) s(-1), K(HPO)²⁻₄ = 1.7 × 10(6) M(-1) s(-1)), reflecting its greater exposure to solute catalysts. Concomitant with the disappearance of the Hε2 signal as the pH was increased above its pK (a), was the appearance of a novel signal (δ = 12 ppm), which we assigned to Hγ of the nearby Ser195 nucleophile, that is hydrogen bonded to Nε2 of neutral His57. The chemical shift of Hγ is about 7 ppm downfield from a typical hydroxyl proton, suggesting a highly polarized O-Hγ bond. The significant alkoxide character of Oγ indicates that Ser195 is preactivated for nucleophilic attack before substrate binding. CEST should be generally useful for mechanistic investigations of many enzymes with labile protons involved in active site chemistry.

Aggregation of chymotrypsinogen: portrait by infrared spectroscopy

Changes in the secondary structure and aggregation of chymotrypsinogen were investigated by infrared difference spectroscopy in conjunction with temperature and pressure tuning IR spectroscopy; both the amide I' band and side chain bands were studied. A prominent component of the amide I' band in the difference spectrum obtained upon cooling a chymotrypsinogen solution, or increasing the hydrostatic pressure, was observed in the region between 1627 and 1622 cm-1. Under denaturing conditions a white gel was formed, which is attributed to irreversible self-association or aggregation. This process was accompanied by the appearance of two new amide I' bands in the infrared spectrum of the protein: a very strong band at 1618 cm-1 and a weak band at 1685 cm-1. These bands are assigned to peptide segments with anti-parallel aligned beta-strands.

The self-association of zinc-free bovine insulin. A single model based on interactions in the crystal that describes the association pattern in solution at pH 2, 7 and 10

Sedimentation equilibrium studies are used to establish that a new pattern for the self-association of zinc-free insulin in solution is applicable over a wide range of conditions of pH, ionic strength and temperature. In this pattern, which is based on information from the existing literature on the X-ray crystal structure of insulin, the insulin monomer is viewed as having two distinct faces both capable of self-interaction. Sedimentation equilibrium experiments were analysed using expressions formulated for this association pattern that describe the dependence of weight average molecular weight and monomer concentration on total protein concentration. It has thereby been possible to obtain values for the two association constants which govern the system for each set of conditions studied, due allowance having been made for composition dependent non-ideality effects. Furthermore, by relating the pH, temperature and ionic strength dependence of the association constants with properties of various amino acid residues on the surface of the insulin monomer, it has also been possible to assign tentatively each constant to a particular reaction domain.

Effect of hydrostatic pressure on lysozyme and chymotrypsinogen detected by fluorescence polarization

The effect of hydrostatic pressure upon solutions of chymotrypsinogen and lysozyme at room temperature has been followed by employing a new technique [Chryssomallis, G. S., Drickamer, H. G., & Weber, G. (1978) J. Appl. Phys. 49, 3084] that permits the measurement of fluorescence polarization at pressures of up to 10 kbar. Lysozyme shows a stable, reversible 60% increase in apparent volume when the pressure is raised to 9 kbar. This can be given a simple interpretation in terms of solvent penetration of the structure at higher pressures. In contrast, the results with chymotrypsinogen are time dependent and only partially reversible on release of the pressure. They involve conversion (tl/e = 5 min) to a form with a lower rotational rate at approximately 6 kbar and return to a fast-rotating form at higher pressure. This latter form persists on pressure release. The possibility of generating what are clearly metastable conformations, not only in chymotrypsinogen but also in flavodoxins [Visser, A. J. W. G., Li, T. M., Drickamer, H. G., & Weber, G. (1977) Biochemistry 16, 4879], indicates that there are unresolved questions about the relative stability of protein conformations which can be profitably investigated by high-pressure experiments.

The indefinite self-association of lysozyme: consideration of composition-dependent activity coefficients

An improved iterative method for computing association constants from sedimentation equilibrium results obtained with self-interacting protein systems is presented which accounts for the composition-dependence of the activity coefficients of all oligomeric species. The method is based on the calculation of viral coefficients from covolume and charge considerations, the statistical mechanical basis of which is discussed in relation to the DLVO theory. The method is applied to results obtained with lysozyme in diethylbarbiturate buffer of pH 8.0 and ionic strength 0.15 at 15 degrees C. It is shown that these results, encompassing a range of total solute concentration up to 19.7 g/liter are consistent with self-association patterns comprising either a monomer--dimer--trimer system or an isodesmic indefinite self-association of the monomer, the latter being favored. A firmer distinction between these possibilities is sought on the basis of the dependence of the weight-average partition coefficient, determined by frontal gel chromatography, on total solute concentration (up to 56.6/liter). This analysis accounts for the composition-dependence of the ratio of the activity coefficients of partitioning monomer in mobile and stationary phases. It is concluded that all results are consistent with an indefinite self-association of lysozyme governed by a single association constant of 4.61 x 10(2) liter/mole.