Crystal structure of thioltransferase at 2.2 A resolution.

We report here the first three-dimensional structure of a mammalian thioltransferase as determined by single crystal X-ray crystallography at 2.2 A resolution. The protein is known for its thiol-redox properties and dehydroascorbate reductase activity. Recombinant pig liver thioltransferase expressed in Escherichia coli was crystallized in its oxidized form by vapor diffusion technique. The structure was determined by multiple isomorphous replacement method using four heavy-atom derivatives. The protein folds into an alpha/beta structure with a four-stranded mixed beta-sheet in the core, flanked on either side by helices. The fold is similar to that found in other thiol-redox proteins, viz. E. coli thioredoxin and bacteriophage T4 glutaredoxin, and thus seems to be conserved in these functionally related proteins. The active site disulfide (Cys 22-Cys 25) is located on a protrusion on the molecular surface. Cys 22, which is known to have an abnormally low pKa of 3.8, is accessible from the exterior of the molecule. Pro 70, which is in close proximity to the disulfide bridge, assumes a conserved cis-peptide configuration. Mutational data available on the protein are in agreement with the three-dimensional structure.     

Characterization of a quaternary-structured folding intermediate of an antibody Fab-fragment.

Antibody folding is a complex process comprising folding and association reactions. Although it is usually difficult to characterize kinetic folding intermediates, in the case of the antibody Fab fragment, domain-domain interactions lead to a rate-limiting step of folding, thus accumulating folding intermediates at a late step of folding. Here, we analyzed a late folding intermediate of the Fab fragment of the monoclonal antibody MAK 33 from mouse (kappa/IgG1). As a strategy for accumulation of this intermediate we used partial denaturation of the native Fab by guanidinium chloride. This denaturation intermediate, which can be populated to about 90%, is indistinguishable from a late-folding intermediate with respect to denaturation and renaturation kinetics. The spectroscopic analysis reveals a native-like secondary structure of this intermediate with aromatic side chains only slightly more solvent exposed than in the native state. The respective partner domains are weekly associated. From these data we conclude that the intramolecular association of the two chains during folding, with all domains in a native-like structure, follows a two-step mechanism. In this mechanism, presumably hydrophobic interactions are followed by rearrangements leading to the exact complementarity of the contact sites of the respective domains.     

Comparison of the crystal structure of bacteriophage T4 lysozyme at low, medium, and high ionic strengths

Crystals of bacteriophage T4 lysozyme used for structural studies are routinely grown from concentrated phosphate solutions. It has been found that crystals in the same space group can also be grown from solutions containing 0.05 M imidazole chloride, 0.4 M sodium choride, and 30% polyethylene glycol 3500. These crystals, in addition, can also be equilibrated with a similar mother liquor in which the sodium chloride concentration is reduced to 0.025 M. The availability of these three crystal variants has permitted the structure of T4 lysozyme to be compared at low, medium, and high ionic strength. At the same time the X-ray structure of phage T4 lysozyme crystallized from phosphate solutions has been further refined against a new and improved X-ray diffraction data set. The structures of T4 lysozyme in the crystals grown with polyethylene glycol as a precipitant, regardless of the sodium chloride concentration, were very similar to the structure in crystals grown from concentrated phosphate solutions. The main differences are related to the formation of mixed disulfides between cysteine residues 54 and 97 and 2-mercaptoethanol, rather than to the differences in the salt concentration in the crystal mother liquor. Formation of the mixed disulfide at residue 54 resulted in the displacement of Arg-52 and the disruption of the salt bridge between this residue and Glu-62. Other than this change, no obvious alterations in existing salt bridges in T4 lysozyme were observed. Neither did the reduction in the ionic strength of the mother liquor result in the formation of new salt bridge interactions. These results are consistent with the ideas that a crystal structure determined at high salt concentrations is a good representation of the structure at lower ionic strengths, and that models of electrostatic interactions in proteins that are based on crystal structures determined at high salt concentrations are likely to be relevant at physiological ionic strengths.     

Reductive cleavage and regeneration of the disulfide bonds inStreptomyces subtilisin inhibitor (SSI) as studied by the carbonyl13C NMR resonances of cysteinyl residues

Summary Four enhanced carbonyl carbon resonances were observed whenStreptomyces subtilisin inhibitor (SSI) was labeled by incorporating specifically labeled [1-¹³C]Cys. The¹³C signals were assigned by the¹⁵N,¹³C double-labeling method along with site-specific mutagenesis. Changes in the spectrum of the labeled protein ([C]SSI) were induced by reducing the disulfide bonds with various amounts of dithiothreitol (DTT). The results indicate that, in the absence of denaturant, the Cys⁷¹-Cys¹⁰¹ disulfide bond of each SSI subunit can be reduced selectively. This disulfide bond, which is in the vicinity of the reactive site scissile bond Met⁷³-Val⁷⁴, is more accessible to solvent than the other disulfide bond. Cys³⁵-Cys⁵⁰, which is embedded in the interior of SSI. This half-reduced SSI had 65% of the inhibitory activity of native SSI and maintained a conformation similar to that of the fully oxidized SSI. Reoxidation of the half reduced-folded SSI by air regenerates fully active SSI which is indistinguishable with intact SSI by NMR. In the presence of 3 M guanidine hydrochloride (GuHCl), however, both disulfide bonds of each SSI subunit were readily reduced by DTT. The fully reduced-unfolded SSI spontaneously refolded into a native-like structure (fully reduced-folded state), as evidenced by the Cys carbonyl carbon chemical shifts, upon removing GuHCl and DTT from the reaction mixture. The time course of disulfide bond regeneration from this state by air oxidation was monitored by following the NMR spectral changes and the results indicated that the disulfide bond between Cys⁷¹ and Cys¹⁰¹ regenerates at a much faster rate than that between Cys³⁵ and Cys⁵⁰.Nomenclature of the various states of SSI that are observed in the present study Fully oxidized-folded native or intact (without GuHCl or DTT) - half reduced-folded (Cys⁷¹-Cys¹⁰¹ reduced; DTT without GuHCl) - inversely half reduced-folded (Cys³⁵-Cys⁵⁰ reduced; a reoxidation intermediate from fully reduced-folded state) - fully reduced-unfolded (reduced by DTT in the presence of GuHCl) - fully reduced-folded (an intermediate state obtained by removing DTT and GuHCl from the fully reduced-unfolded SSI reaction mixture)     

β‐amyloid model core peptides: Effects of hydrophobes and disulfides

The mechanism by which a disordered peptide nucleates and forms amyloid is incompletely understood. A central domain of β‐amyloid (Aβ21–30) has been proposed to have intrinsic structural propensities that guide the limited formation of structure in the process of fibrillization. In order to test this hypothesis, we examine several internal fragments of Aβ, and variants of these either cyclized or with an N‐terminal Cys. While Aβ21–30 and variants were always monomeric and unstructured (circular dichroism (CD) and nuclear magnetic resonance spectroscopy (NMRS)), we found that the addition of flanking hydrophobic residues in Aβ16–34 led to formation of typical amyloid fibrils. NMR showed no long‐range nuclear overhauser effect (nOes) in Aβ21–30, Aβ16–34, or their variants, however. Serial ¹H‐¹⁵N‐heteronuclear single quantum coherence spectroscopy, ¹H‐¹H nuclear overhauser effect spectroscopy, and ¹H‐¹H total correlational spectroscopy spectra were used to follow aggregation of Aβ16–34 and Cys‐Aβ16–34 at a site‐specific level. The addition of an N‐terminal Cys residue (in Cys‐Aβ16–34) increased the rate of fibrillization which was attributable to disulfide bond formation. We propose a scheme comparing the aggregation pathways for Aβ16–34 and Cys‐Aβ16–34, according to which Cys‐Aβ16–34 dimerizes, which accelerates fibril formation. In this context, cysteine residues form a focal point that guides fibrillization, a role which, in native peptides, can be assumed by heterogeneous nucleators of aggregation.     

Role of a noncanonical disulfide bond in the stability,affinity, and flexibility of a VHH specific for the Listeria virulence factor InlB

A distinguishing feature of camel (Camelus dromedarius) VHH domains are noncanonical disulfide bonds between CDR1 and CDR3. The disulfide bond may provide an evolutionary advantage, as one of the cysteines in the bond is germline encoded. It has been hypothesized that this additional disulfide bond may play a role in binding affinity by reducing the entropic penalty associated with immobilization of a long CDR3 loop upon antigen binding. To examine the role of a noncanonical disulfide bond on antigen binding and the biophysical properties of a VHH domain, we have used the VHH R303, which binds the Listeria virulence factor InlB as a model. Using site directed mutagenesis, we produced a double mutant of R303 (C33A/C102A) to remove the extra disulfide bond of the VHH R303. Antigen binding was not affected by loss of the disulfide bond, however the mutant VHH displayed reduced thermal stability (T_m = 12°C lower than wild‐type), and a loss of the ability to fold reversibly due to heat induced aggregation. X‐ray structures of the mutant alone and in complex with InlB showed no major changes in the structure. B‐factor analysis of the structures suggested that the loss of the disulfide bond elicited no major change on the flexibility of the CDR loops, and revealed no evidence of loop immobilization upon antigen binding. These results suggest that the noncanonical disulfide bond found in camel VHH may have evolved to stabilize the biophysical properties of the domain, rather than playing a significant role in antigen binding.     

Redox biology of the intestine

Abstract

The intestinal tract, known for its capability for self-renew, represents the first barrier of defence between the organism and its luminal environment. The thiol/disulfide redox systems comprising the glutathione/glutathione disulfide (GSH/GSSG), cysteine/cystine (Cys/CySS) and reduced and oxidized thioredoxin (Trx/TrxSS) redox couples play important roles in preserving tissue redox homeostasis, metabolic functions, and cellular integrity. Control of the thiol-disulfide status at the luminal surface is essential for maintaining mucus fluidity and absorption of nutrients, and protection against chemical-induced oxidant injury. Within intestinal cells, these redox couples preserve an environment that supports physiological processes and orchestrates networks of enzymatic reactions against oxidative stress. In this review, we focus on the intestinal redox and antioxidant systems, their subcellular compartmentation, redox signalling and epithelial turnover, and contribution of luminal microbiota, key aspects that are relevant to understanding redox-dependent processes in gut biology with implications for degenerative digestive disorders, such as inflammation and cancer.     

In vitro Interactions of Thallium with Components of the Glutathione-dependent Antioxidant Defence System

We investigated the hypothesis that thallium (Tl) interactions with the glutathione-dependent antioxidant defence system could contribute to the oxidative stress associated with Tl toxicity. Working in vitro with reduced glutathione (GSH), glutathione reductase (GR) or glutathione peroxidase (GPx) in solution, we studied the effects of Tl⁺ and Tl³⁺ (1-25 μM) on: (a) the amount of free GSH, investigating whether the metal binds to GSH and/or oxidizes it; (b) the activity of the enzyme GR, that catalyzes GSH regeneration; and (c) the enzyme GPx, that reduces hydroperoxide at expense of GSH oxidation. We found that, while Tl⁺ had no effect on GSH concentration, Tl³⁺ oxidized it. Both cations inhibited the reduction of GSSG by GR and the diaphorase activity of this enzyme. In addition, Tl³⁺per se oxidized NADPH, the cofactor of GR. The effects of Tl on GPx activity depended on the metal charge: Tl⁺ inhibited GPx when cumene hydroperoxide (CuOOH) was the substrate, while Tl³⁺-mediated GPx inhibition occurred with both substrates. The present results show that Tl interacts with all the components of GSH/GSSG antioxidant defence system. Alterations of this protective pathway could be partially responsible for the oxidative stress associated with Tl toxicity.     

Monoclonal antibody disulfide reduction during manufacturing

Manufacturing-induced disulfide reduction has recently been reported for monoclonal human immunoglobulin gamma (IgG) antibodies, a widely used modality in the biopharmaceutical industry. This effect has been tied to components of the intracellular thioredoxin reduction system that are released upon cell breakage. Here, we describe the effect of process parameters and intrinsic molecule properties on the extent of reduction. Material taken from cell cultures at the end of production displayed large variations in the extent of antibody reduction between different products, including no reduction, when subjected to the same reduction-promoting harvest conditions. Additionally, in a reconstituted model in which process variables could be isolated from product properties, we found that antibody reduction was dependent on the cell line (clone) and cell culture process. A bench-scale model using a thioredoxin/thioredoxin reductase regeneration system revealed that reduction susceptibility depended on not only antibody class but also light chain type; the model further demonstrates that the trend in reducibility was identical to DTT reduction sensitivity following the order IgG1λ > IgG1κ > IgG2λ > IgG2κ. Thus, both product attributes and process parameters contribute to the extent of antibody reduction during production.     

Removal of a C-terminal serine residue proximal to the inter-chain disulfide bond of a human IgG1 lambda light chain mediates enhanced antibody stability and antibody dependent cell-mediated cytotoxicity

Optimization of biophysical properties is a critical success factor for the developability of monoclonal antibodies with potential therapeutic applications. The inter-domain disulfide bond between light chain (Lc) and heavy chain (Hc) in human IgG1 lends structural support for antibody scaffold stability, optimal antigen binding, and normal Fc function. Recently, human IgG1λ has been suggested to exhibit significantly greater susceptibility to reduction of the inter Lc-Hc disulfide bond relative to the same disulfide bond in human IgG1κ. To understand the molecular basis for this observed difference in stability, the sequence and structure of human IgG1λ and human IgG1κ were compared. Based on this Lc comparison, three single mutations were made in the λ Lc proximal to the cysteine residue, which forms a disulfide bond with the Hc. We determined that deletion of S214 (dS) improved resistance of the association between Lc and Hc to thermal stress. In addition, deletion of this terminal serine from the Lc of IgG1λ provided further benefit, including an increase in stability at elevated pH, increased yield from transient transfection, and improved in vitro antibody dependent cell-mediated cytotoxicity (ADCC). These observations support the conclusion that the presence of the terminal serine of the λ Lc creates a weaker inter-chain disulfide bond between the Lc and Hc, leading to slightly reduced stability and a potential compromise in IgG1λ function. Our data from a human IgG1λ provide a basis for further investigation of the effects of deleting terminal serine from λLc on the stability and function of other human IgG1λ antibodies.