Binding of HasA by its transmembrane receptor HasR follows a conformational funnel mechanism

Structural Basis and Mechanism of a Unique Haemophore in the Haem-Iron Acquisition by Riemerella anatipestifer

Several bacterial pathogens employ haemophores to scavenge haem from host haemoprotein to obtain an iron source. However, no homologues of well-characterized haemophores are found in Riemerella anatipestifer, a bacterium belonging to the order Flavobacteriales that encodes haem uptake systems. Herein, a unique haemophore RhuH is characterized in this bacterium. R. anatipestifer used RhuH to grow when duck hemoglobin serves as the sole iron resource. RhuH is secreted as a component of outer membrane vesicles. Recombinant RhuH exhibited a high binding affinity for haem (Kd of 3.44 × 10-11 m) and can extract haem from duck hemoglobin. X-ray crystallography elucidated the 3D structure of RhuH at 2.85 Å resolution, showing a dimeric conformation with each monomer exhibiting a unique structure. Structure modeling of RhuH-haem, coupled with mutagenesis, haemin utilization, and binding affinity assays, show that haem is captured in the β-barrel-like region, displaying the classic iron coordination. The RhuH homologues are predominantly distributed in Weeksellaceae and Flavobacteriaceae. Finally, the homologues of RhuH in Riemerella columbina, Flavobacterium columnare, and Flavobacterium soli are used as a proof of concept, demonstrating that these homologues exhibit conserved structures and functions.

The modification of heme special importer to improve the production of active hemoglobins in Escherichia coli

To enhance the import of heme for the production of active hemoproteins in Escherichia coli C41 (DE3) lacking the special heme import system, heme receptor ChuA from E. coli Nissle 1917 was modified through molecular docking and the other components (ChuTUV) for heme import was overexpressed, while heme import was tested through growth assay and heme sensor HS1 detection. A ChuA mutant G360K was selected, which could import 3.91 nM heme, compared with 2.92 nM of the wild-type ChuA. In addition, it presented that the expression of heme transporters ChuTUV was not necessary for heme import. Based on the modification of ChuA (G360K), the titer of human hemoglobin and the peroxidase activity of leghemoglobin reached 1.19 μg g-1 DCW and 24.16 103 U g-1 DCW, compared with 1.09 μg g-1 DCW and 21.56 103 U g-1 DCW of the wild-type ChuA, respectively. Heme import can be improved through the modification of heme receptor and the engineered strain with improved heme import has a potential to efficiently produce high-active hemoproteins.

Getting Deeper into the Molecular Events of Heme Binding Mechanisms: A Comparative Multi-level Computational Study of HasAsm and HasAyp Hemophores

Many biological systems obtain their activity by the inclusion of metalloporphyrins into one or several binding pockets. However, decoding the molecular mechanism under which these compounds bind to their receptors is something that has not been widely explored and is a field with open questions. In the present work, we apply computational techniques to unravel and compare the mechanisms of two heme-binding systems, concretely the HasA hemophores from Gram negative bacteria Serratiamarcescens (HasAsm) and Yersinia pestis (HasAyp). Despite the high sequence identity between both systems, the comparison between the X-ray structures of their apo and holo forms suggests different heme-binding mechanisms. HasAyp has extremely similar structures for heme-free and heme-bound forms, while HasAsm presents a very large displacement of a loop that ultimately leads to an additional coordination to the metal with respect to HasAyp. We combined Gaussian accelerated molecular dynamics simulations (GaMDs) in explicit solvent and protein–ligand docking optimized for metalloligands. GaMDs were first carried out on heme-free forms of both hemophores. Then, protein–ligand dockings of the heme were performed on cluster representatives of these simulations and the best poses were then subjected to a new series of GaMDs. A series of analyses reveal the following: (1) HasAyp has a conformational landscape extremely similar between heme-bound and unbound states with no to limited impact on the binding of the cofactor, (2) HasAsm presents as a slightly broader conformational landscape in its apo state but can only visit conformations similar to the X-ray of the holo form when the heme has been bound. Such behavior results from a complex cascade of changes in interactions that spread from the heme-binding pocket to the flexible loop previously mentioned. This study sheds light on the diversity of molecular mechanisms of heme-binding and discusses the weight between the pre-organization of the receptor as well as the induced motions resulting in association.

Heme-containing enzymes and proteins are important for many biological and biotechnological processes. However, very little is known about heme-binding mechanisms. To shed light on this, we report a multi-level approach combining Gaussian accelerated molecular dynamics and protein−ligand dockings optimized for metallic moieties. The protocol unveils the difference in heme recruitment between HasAsm and HasAyp hemophores and shows its possible applicability to other heme-binding proteins.