Abstract
Licanantase (Lic) is the major component of the secretome of Acidithiobacillus thiooxidans when grown in elemental sulphur. When used as an additive, Lic improves copper recovery from bioleaching processes. However, this recovery enhancement is not fully understood. In this context, our aim is to predict the 3D structure of Lic, to shed light on its structure-function relationships. Bioinformatics analyses on the amino acid sequence of Lic showed a great similarity with Lpp, an Escherichia coli Lipoprotein that can form stable trimers in solution. Lic and Lpp share the secretion motif, intracellular processing and alpha helix structure, as well as the distribution of hydrophobic residues in heptads forming a hydrophobic core, typical of coiled-coil structures. Cross-linking experiments showed the presence of Lic trimers, supporting our predictions. Taking the in vitro and in silico evidence as a whole, we propose that the most probable structure for Lic is a trimeric coiled-coil. According to this prediction, a suitable model for Lic was produced using the de novo algorithm “Rosetta Fold-and-Dock”. To assess the structural stability of our model, Molecular Dynamics (MD) and Replica Exchange MD simulations were performed using the structure of Lpp and a 14-alanine Lpp mutant as controls, at both acidic and neutral pH. Our results suggest that Lic was the most stable structure among the studied proteins in both pH conditions. This increased stability can be explained by a higher number of both intermonomer hydrophobic contacts and hydrogen bonds, key elements for the stability of Lic’s secondary and tertiary structure.
Highlights
Bioleaching is a process with increasing interest for the mining industry
The authors suggested a possible participation of Lic as a surfactant agent, by removing the hydrophobic barrier formed by elemental sulphur over the surface of ores during chalcopyrite bioleaching
Despite a low sequence identity, Lic shows a great similarity with a Lipoprotein (Lpp) of Escherichia coli and with its engineered mutants that contain larger alanine-zipper domains (Liu, Cao & Lu, 2002; Liu, Dai & Lu, 2003; Liu & Lu, 2002; Shu et al, 2000)
Summary
Bioleaching is a process with increasing interest for the mining industry. It consists of the release of heavy metals from insoluble ores through biological oxidation. Despite the authors reporting the amino acid sequence of Lic, the lack of structural information impeded further insights about its function. Lpp forms trimeric coiled-coils in solution; its poly-alanine mutants lose stability, as their alanine zipper domains are larger Based on this evidence, we propose that the most probable structure for Lic is a trimeric coiled-coil. Our findings suggest that Lic exhibits sequence and structural features that improve its stability at the extreme low-pH environment where it performs its function These features could be relevant to produce engineered versions of this protein in order to enhance the recovery of copper by bioleaching processes based on the presence of Acidithiobacillus thiooxidans
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