Structure of a serpin-enzyme complex probed by cysteine substitutions and fluorescence spectroscopy

Recombinant human alpha-1 proteinase inhibitor: towards therapeutic use

Human alpha-1-proteinase inhibitor is a well-characterized protease inhibitor with a wide spectrum of anti-protease activity. Its major physiological role is inhibition of neutrophil elastase in the lungs, and its deficiency is associated with progressive ultimately fatal emphysema. Currently in the US, only plasma-derived human alpha-1-proteinase inhibitor is available for augmentation therapy, which appears to be insufficient to meet the anticipated clinical demand. Moreover, despite effective viral clearance steps in the manufacturing process, the potential risk of contamination with new and unknown pathogens still exists. In response, multiple efforts to develop recombinant versions of human alpha-1-proteinase inhibitor, as an alternative to the plasma-derived protein, have been reported. Over the last two decades, various systems have been used to express the human gene for alpha-1-proteinase inhibitor. This paper reviews the recombinant versions of human alpha-1-proteinase inhibitor produced in various hosts, considers current major safety and efficacy issues regarding recombinant glycoproteins as potential therapeutics, and the factors that are impeding progress in this area(1).

Resolution of Cys and Lys labeling of alpha-crystallin with site-sensitive fluorescent 3-hydroxyflavone dye

Ratiometric fluorescent probes based on 3-hydroxyflavone (3HF) are highly sensitive tools for studying polarity, hydration, electronic polarizability, and electrostatics in different microheterogeneous systems, including protein molecules. In the present work, a reactive derivative of 3HF, 6-bromomethyl-4'-diethylamino-3-hydroxyflavone, recently synthesized in our group, was applied to label covalently bovine lens alpha-crystallin. The labeling of SH and NH(2) groups are clearly distinguished by spectroscopic criteria. We observe that the NH(2) labeling creates the positive charge in the proximity to fluorophore, which results in strong internal Stark effect producing the shift in excitation spectrum by ca. 15 nm. Analysis of excitation-dependent fluorescence spectra allows separation of the emission profiles of these SH- and NH(2)-labeled species. Applying recently developed multiparametric analysis of the obtained emission spectra, we described the physicochemical properties of the sites of SH and NH(2) labeling in alpha-crystallin. The site of SH labeling has medium-low polarity (dielectric constant, epsilon = 4.9 +/- 0.9) is protic, and does not contain proximal aromatic residues (according to the obtained refractive index, n = 1.41 +/- 0.14). The site of NH(2) labeling is also of medium-low polarity. The novel label due to its two-wavelength ratiometric response and high sensitivity to the type of labeling may offer new possibilities in the studies of structure, dynamics, and interactions of proteins by probing their SH- and NH(2)-labeling sites.

Dissecting substrate recognition by thrombin using the inactive mutant S195A

The catalytically inactive mutant S195A was used to study the interaction of thrombin with substrates under equilibrium conditions. By monitoring changes in intrinsic fluorescence, we measured dissociation constants for a variety of synthetic substrates, PAR peptides and the inhibitor PPACK. The S195A mutant retains the Na(+)-binding properties of the wild type, and substrate binding to the mutant is enhanced by the presence of Na(+). Temperature dependence studies allowed calculation of the thermodynamic parameters of substrate binding at the active site and showed a negligible deltaC(p). Titration of synthetic substrates carrying substitutions at the P1-P3 positions revealed energetics consistent with the specificity hierarchy identified in hydrolysis by the wild type. Titration with PAR peptides, which interact with both the active site and exosite I of thrombin, also showed consistency with the results obtained with the wild type at steady state. These findings demonstrate that inactive mutants of enzymes make it possible to dissect the equilibrium components linked to substrate binding and complement information on the kinetic properties of the wild type.

Mapping of the epitope of a monoclonal antibody protecting plasminogen activator inhibitor-1 against inactivating agents

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serpin family of serine proteinase inhibitors. Serpins inhibit their target proteinases by an ester bond being formed between the active site serine of the proteinase and the P1 residue of the reactive centre loop (RCL) of the serpin, followed by insertion of the RCL into beta-sheet A of the serpin. Concomitantly, there are conformational changes in the flexible joint region lateral to beta-sheet A. We have now, by site-directed mutagenesis, mapped the epitope for a monoclonal antibody, which protects the inhibitory activity of PAI-1 against inactivation by a variety of agents acting on beta-sheet A and the flexible joint region. Curiously, the epitope is localized in alpha-helix C and the loop connecting alpha-helix I and beta-strand 5A, on the side of PAI-1 opposite to beta-sheet A and distantly from the flexible joint region. By a combination of site-directed mutagenesis and antibody protection against an inactivating organochemical ligand, we were able to identify a residue involved in conferring the antibody-induced conformational change from the epitope to the rest of the molecule. We have thus provided evidence for communication between secondary structural elements not previously known to interact in serpins.

Comparative trajectories of active and S195A inactive trypsin upon binding to serpins

Serpins inhibit proteinases through a complicated multistep mechanism. The precise nature of these steps and the order by which they occur are still debated. We compared the fate of active and S195A inactive rat trypsin upon binding to alpha(1)-antitrypsin and P(1)-Arg-antichymotrypsin using stopped-flow kinetics with fluorescence resonance energy transfer detection and time-resolved fluorescence resonance energy transfer. We show that inhibition of active trypsin by these serpins leads to two irreversible complexes, one being compatible with the full insertion of the serpin-reactive site loop but not the other one. Binding of inactive trypsin to serpins triggers a large multistep reversible rearrangement leading to the migration of the proteinase to an intermediate position. Binding of inactive trypsin, unlike that of active trypsin, does not perturb the rhodamine fluorescence at position 150 on the helix F of the serpin. Thus, inactive proteinases do not migrate past helix F and do not trigger full serpin loop insertion.

Intrinsic fluorescence changes and rapid kinetics of proteinase deformation during serpin inhibition

The X-ray crystal structure of the serpin-proteinase complex suggested that the serpin deformed the proteinase thereby inactivating the molecule. Using a variant of alpha(1)-antitrypsin in which both tryptophan residues have been replaced by phenylalanine, we have shown that the proteinase becomes partially unfolded during serpin inhibition. The tryptophan free variant, alpha(1)-antitrypsin((FF)), is fully active as an inhibitor of thrombin. Thrombin has a fluorescence emission maximum of 340 nm which blue shifts to 346 nm, concomitant with a 40% increase in intensity, upon formation of the serpin-proteinase complex indicative of substantial conformational change within the proteinase. Stopped-flow analysis of the fluorescence changes within the proteinase indicated a two-step mechanism. A fast bimolecular reaction with a rate constant of 2.8x10(6) M(-1) s(-1) is followed by a slow unimolecular process with a rate of 0.26 s(-1) that is independent of concentration. We propose that the first rate is formation of an initial complex which is then followed by a slower process involving the partial unfolding of the proteinase during its translocation to the opposite pole of the serpin.