The role of intra-domain disulfide bonds in heat-induced irreversible denaturation of camelid single domain VHH antibodies

Abstract

Camelid-derived single domain VHH antibodies are highly heat resistant, and the mechanism of heatinduced VHH denaturation predominantly relies on the chemical modification of amino acids. Although chemical modification of disulfide bonds has been recognized as a cause for heat-induced denaturation of many proteins, there have been no mutagenesis studies, in which the number of disulfide bonds was controlled. In this article, we examined a series of mutants of two different VHHs with single, double or no disulfide bonds, and scrutinized the effects of these disulfide bond modifications on VHH denaturation. With the exception of one mutant, the heat resistance of VHHs decreased when the number of disulfide bonds increased. The effect of disulfide bonds on heat denaturation was more striking if the VHH had a second disulfide bond, suggesting that the contribution of disulfide shuffling is significant in proteins with multiple disulfide bonds. Furthermore, our results directly indicate that removal of a disulfide bond can indeed increase the heat resistance of a protein, irrespective of the negative impact on equilibrium thermodynamic stability.Keywords: antibody engineering/disulfide bonds/protein chemical modification/protein engineering/protein stability.Abbreviations: bLA, Bacillus cereus b lactamase; CD, circular dichroism; hCG, human chorionic gonadotropin; SPR, surface plasmon resonance; t 1/2 90 C , halflife of VHH at 90 C; T m , mid-point temperature of thermal unfolding; VHH, variable domain of camelid heavy chain antibody.The variable domain of the camelid heavy chain antibody (VHH) is known to be highly heat resistant.VHHs remain active even after incubation at high temperatures (>80 C) because of their ability to refold to their native structure from a heat-induced unfolded state (1 6). However, even for VHHs, a long incubation finally leads to denaturation: e.g. the half-life of VHH at 90 C (t 1/2 90 C ) was 3 h at neutral pH (7). Residual activity of VHH depends only on the incubation time at 90 C, and not on the number of heating (90 C)-cooling (20 C) cycles, indicating that folding and unfolding intermediates are irrelevant to the loss of activity. Heat denaturation of VHHs is independent of protein concentration; therefore, VHHs lose their activity in a unimolecular manner, not by aggregation. This strongly suggests that chemical modifications play an important role during VHH denaturation. Indeed, replacing the Asn residues, which are prone to chemical modification, increase the heat resistance of VHHs (7).VHHs typically have only one intramolecular disulfide bond. The introduction of a second non-canonical artificial disulfide bond connecting the amino acids at positions 49 and 69, with numbering according to Kabat et al. (8), results in a significantly higher midpoint temperature of equilibrium thermal unfolding (T m ) in comparison to wild-type protein (9 13). In spite of its superior equilibrium thermodynamic stability, the mutant with a second disulfide bond exhibited lower residual activity ...

The structural basis of nanobody unfolding reversibility and thermoresistance

“…Therefore, three common strategies to obtain fully reversible nanobodies are suggested: (i) favoring long CDR3 loops which are stabilized by a disulfide bond; (ii) stabilizing long CDR3 loops by other, non-covalent interactions; and (iii) solubilizing the unfolded state, for example by the introduction of repulsive charges 23 , 25 . Importantly, the situation changes for two scenarios: First, in applications that could involve long incubation times at very high temperatures, disulfide bonds were shown to compromise nanobody refolding ability due to heat-induced disulfide shuffling and modifications of cysteine residues 21 , 65 . This phenomenon can compromise the effect of disulfide bonds on thermodynamic stability and aggregation behavior.…”

“…It seemed to be based on a simple two-state mechanism of folding, devoid of intermediate states and reversible even upon heat-denaturation 19 . If heat-induced irreversible inactivation of nanobodies was observed, it was suggested to be due to chemical modifications of amino acids and not due to aggregation 20 , 21 . These superior examples of nanobody binders tend to shape the conception of nanobody thermoresistance in the literature 4 , 6 , 12 .…”

Single-Domain Antibodies As Versatile Affinity Reagents for Analytical and Diagnostic Applications

“…In these cases, the mutation of Asn residues to Asp has also shown to increase the heat resistance of the Nb ( 61 ). Likewise, the removal of the extra disulfide bonds has proved beneficial for the thermal resistance of Nbs, although this modification has a negative impact in their thermodynamic stability ( 62 ).…”

VHH antibodies: emerging reagents for the analysis of environmental chemicals

“…Other researchers took a different approach and introduced a non-canonical disulfide bond into the hydrophobic core of llama VHHs between framework region 2 (FR2) and FR3, which proved to not only increase thermal stability at neutral pH, but also imparted resistance to proteolytic degradation and increased antibody stability at low pH [60, 61]. A recent thorough study of the relationship between number of disulfide bonds and the heat resistance of VHH concluded that while T m increased with the increase in number of disulfide bonds the half-life of VHH at 90 °C ( t 1/2 90 C ) decreased [62]. The researchers also showed that the thermal loss of activity of VHH is chemical in nature and proceeds through disulfide exchange and peptide cleavage near Cys and Asn amino acid residues.…”