Engineered staphylococcal protein A's IgG-binding domain with cathepsin L inhibitory activity

Small and Simple, yet Sturdy: Conformationally Constrained Peptides with Remarkable Properties

The sheer size and vast chemical space (i.e., diverse repertoire and spatial distribution of functional groups) underlie peptides’ ability to engage in specific interactions with targets of various structures. However, the inherent flexibility of the peptide chain negatively affects binding affinity and metabolic stability, thereby severely limiting the use of peptides as medicines. Imposing conformational constraints to the peptide chain offers to solve these problems but typically requires laborious structure optimization. Alternatively, libraries of constrained peptides with randomized modules can be screened for specific functions. Here, we present the properties of conformationally constrained peptides and review rigidification chemistries/strategies, as well as synthetic and enzymatic methods of producing macrocyclic peptides. Furthermore, we discuss the in vitro molecular evolution methods for the development of constrained peptides with pre-defined functions. Finally, we briefly present applications of selected constrained peptides to illustrate their exceptional properties as drug candidates, molecular recognition probes, and minimalist catalysts.

An engineered Staphylococcal Protein A based ligand: Production, characterization and potential application for the capture of Immunoglobulin and Fc-fusion proteins

In antibody purification processes, affinity chromatography has been used with Staphylococcus aureus protein A (SpA) as the main ligand. In this work, we present a novel Staphylococcal Protein A (AviPure thereafter), a synthetic ligand analogue based on native SpA B domain, with a molecular weight of approximately 14 kDa. The binding affinity of mAbs to AviPure was evaluated using Surface Plasmon Resonance (SPR) and affinity chromatography methods. The equilibrium dissociation constant (K_D) between the AviPure and mAbs was systematically measured using 1:1 (Langmuir) model and found to be 4.7 × 10^-8 M, with constant of dissociation at k_d ≤ 1.0 × 10^-3 s^-1 and k_a being 3.1 × 10⁴ M^-1 s^-1. When immobilized on Sepharose, the AviPure ligand density was 429 nmol/g moist weight resin and was able to effectively bind immunoglobulin and Fc fragment samples with higher affinity and the most effective flow rate when using ligand - Sepharose beads was at 75 cm/h giving the dynamic binding capacity of 53 mg/mL and 91% recovery of IgG. Suitable ligands used in affinity purification should have a K_D ≤ 10^-6 M and a dissociation rate (k_a) averaging 10^-3 M^-1 s^-1 with the k_d ranging between 10³ - 10⁸ M^-1. Therefore, the AviPure ligand can be used as an alternative to the standard protein A ligand in the purification of mAbs and Fc-fused proteins.

High-level fed-batch fermentative expression of an engineered Staphylococcal protein A based ligand in E. coli: purification and characterization

The major platform for high level recombinant protein production is based on genetically modified microorganisms like Escherichia coli (E. coli) due to its short dividing time, ability to use inexpensive substrates and additionally, its genetics is comparatively simple, well characterized and can be manipulated easily. Here, we investigated the possibilities of finding the best media for high cell density fermentation, by analyzing different media samples, focusing on improving fermentation techniques and recombinant protein production. Initial fermentation of E. coli BL21 DE3:pAV01 in baffled flasks showed that high cell density was achieved when using complex media, Luria–Bertani (LB) and Terrific medium broth (TB) (10 and 14 g/L wet weight, respectively), as compared to mineral media M9, modified minimal medium (MMM) and Riesenberg mineral medium (RM) (7, 8 and 7 g/L, respectively). However, in fed-batch fermentation processes when using MMM after 25 h cultivation, it was possible to yield an optical density (OD₆₀₀) of 139 corresponding to 172 g/L of wet biomass was produced in a 30 L TV Techfors-S Infors HT fermenter, with a computer controlled nutrient supply (glucose as a carbon source) delivery system, indicating nearly 1.5 times that obtained from TB. Upon purification, a total of 1.65 mg/g of protein per gram cell biomass was obtained and the purified AviPure showed affinity for immunoglobulin. High cell density fed batch fermentation was achieved by selecting the best media and growth conditions, by utilizing a number of fermentation parameters like media, fermentation conditions, chemical concentrations, pO₂ level, stirrer speed, pH level and feed media addition. It is possible to reach cell densities higher than shake flasks and stirred tank reactors with the improved oxygen transfer rate and feed.

Cysteine-terminated B-domain of Staphylococcus aureus protein A as a scaffold for targeting GABA(A) receptors

This study reports the preparation and characterization of cysteine-terminated B-domain (Bd-cys) of Staphylococcus aureus protein A, in combination with immunoglobulin G (IgG) antibodies directed against the ρ1 and α1 subunits of GABA(A) receptors, for localizing reagents of interest to the target receptor. A cysteine residue was inserted at the C terminus of the cysteine-lacking B-domain (Bd) and used for conjugating maleimide-containing compounds. As determined by enzyme-linked immunosorbent assay (ELISA), binding of a Bd-cys-S-fluorescein conjugate to polyclonal guinea pig anti-GABA(A)-ρ1 and rabbit anti-GABA(A)-α1 IgG was similar to that exhibited by full-length protein A. Surface plasmon resonance analysis of the interaction of Bd-cys-S-PEG3400-biotin conjugate (where PEG is polyethylene glycol) with anti-GABA(A)-ρ1 and anti-GABA(A)-α1 yielded K(D) values of 6.4 ± 1.9 and 0.4 ± 0.1 nM, respectively. Fluorescence anisotropy analysis of the binding of Bd-cys-S-fluorescein to the two antibodies yielded EC50 values of 65 and 18 nM, respectively. As determined with biotin-reactive fluorescent reagents, Bd-cys-S-PEG3400-biotin specifically bound to the plasma membrane of Xenopus laevis oocytes that expressed α1β2γ2 or homomeric ρ1 GABA(A) receptors and were pretreated with the corresponding anti-GABA(A) IgG. The IgG-binding specificity and high affinity of Bd-cys conjugates illustrate the potential of these conjugates, in combination with a selected IgG, to localize compounds of interest at specific cell surface proteins.

3D antibody immobilization on a planar matrix surface

The amount of active capture antibodies immobilized per unit square is crucial to developing effective antibody chips, biosensors, immunoassays and other molecular recognition technologies. In this study, we present a novel yet simple method for oriented antibody immobilization at high density based on the formation of an orderly, organized aggregation of immunoglobulin G (IgG) and staphylococcal protein A (SPA). Following the chelation of His-tag with Ni(2+), antibodies were immobilized on a solid surface in a three-dimensional (3D) manner and exposed the analyte receptor sites well, which resulted in a substantial enhancement of the analytical signal with more than 64-fold increase in detection sensitivity. Pull-down assays confirmed that IgG antibody can only bind to Ni(2+) matrix indirectly via a SPA linkage, where the His-tag is responsible for binding Ni(2+) and homologous domains are responsible for binding IgG Fc. The immobilization approach proposed here may be an attractive strategy for the construction of high performance antibody arrays and biosensors as long as the antibody probe is of mammalian IgG.

Engineered lactic acid bacterium Lactococcus lactis capable of binding antibodies and tumor necrosis factor alpha

We have optimized the display of the B domain of staphylococcal protein A on the surface of Lactococcus lactis. The maximum binding capacity was estimated at 0.146 μg of antibody per 10⁸ cells and was sustained at 86% after treatment with simulated gastric juice. A tumor necrosis factor alpha (TNF-α)-binding affibody was also displayed and bound TNF-α, which could be useful in the treatment of inflammatory bowel disease.

Calcium structural transition of troponin in the complexes, on the thin filament, and in muscle fibres, as studied by site-directed spin-labelling EPR

We have measured the intersite distance, side-chain mobility and orientation of specific site(s) of troponin (Tn) complex on the thin filaments or in muscle fibres as well as in solution by means of site-directed spin labeling electron paramagnetic resonance (SDSL-EPR). We have examined the Ca(2+)-induced movement of the B and C helices relative to the D helix in a human cardiac (hc)TnC monomer state and hcTnC-hcTnI binary complex. An interspin distance between G42C (B helix) and C84 (D helix) was 18.4 angstroms in the absence of Ca2+. The distance between Q58C (C helix) and C84 (D helix) was 18.3 angstroms. Distance changes were observed by the addition of Ca2+ and by the formation of a complex with TnI. Both Ca2+ and TnI are essential for the full opening -3 angstroms of the N-domain in cardiac TnC. We have determined the in situ distances between C35 and C84 by measuring pulsed electron-electron double resonance (PELDOR) spectroscopy. The distances were 26.0 and 27.2 A in the monomer state and in reconstituted fibres, respectively. The addition of Ca2+ decreased the distance to 23.2 angstroms in fibres but only slightly in the monomer state, indicating that Ca2+ binding to the N-lobe of hcTnC induced a larger structural change in muscle fibres than in the monomer state. We also succeeded in synthesizing a new bifunctional spin labels that is firmly fixed on a central E-helix (94C-101C) of skeletal(sk)TnC to examine its orientation in reconstituted muscle fibres. EPR spectrum showed that this helix is disordered with respect to the filament axis. We have studied the calcium structural transition in skTnI and tropomyosin on the filament by SDSL-EPR. The spin label at a TnI switch segment (C133) showed three motional states depending on Ca2+ and actin. The data suggested that the TnI switch segment binds to TnC N-lobe in +Ca2+ state, and that in -Ca2+ state it is free in TnC-I-T complex alone while fixed to actin in the reconstituted thin filaments. In contrast, the side chain spin labels along the entire tropomyosin molecule showed no Ca(2+)-induced mobility changes.