How Zinc-Binding Systems, Expressed by Human Pathogens, Acquire Zinc from the Colonized Host Environment: A Critical Review on Zincophores

Abstract

Some transition metals, like manganese, iron, cobalt, nickel, copper and zinc, required for the biosynthesis of metalloenzymes and metalloproteins, are essential micronutrients for the growth and development of pathogenic microorganisms. Among the defenses put in place by the host organism, the so-called "nutritional immunity" consists of reducing the availability of micronutrients and thus "starving" the pathogen. In the case of metals, microorganisms can fight the nutritional immunity in different ways, i.e. by directly recruiting the metal ion or capturing an extracellular metalloprotein or also through the synthesis of specific metallophores which allow importing the metal in the form of a chelate complex. The best known and most studied metallophores are those directed to iron (siderophores), but analogous chelators are also expressed by microorganisms to capture other metals, such as zinc. An efficient zinc recruitment can also be achieved by means of specialized zinc-binding proteins. A deep knowledge of the properties, structure and action mechanisms of extracytoplasmic zinc chelators can be a powerful tool to find out new therapeutic strategies against the antibiotic and/or antifungal resistance. This review aims to collect the knowledge concerning zincophores (small molecules and proteins in charge of zinc acquisition) expressed by bacterial or fungal microorganisms that are pathogenic for the human body.

Keywords: Zincophores; antimicrobial agents.; bacteria; fungi; metal binding proteins; nutritional immunity; zinc.

Fig. (1)

Schematic illustration of the defined zincophore systems. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (2)

Schematic representation of Zn²⁺ acquisition by means of ABC transporter related zincophores in Gram-negative (left) and Gram-positive (right) bacteria. The Substrate Binding Protein (SBP) contains two α/β domains, the rigid α-helical linker and the flexible loop facing the metal binding pocket. The flexible loop is long and rich in histidine and acidic residues in the case of G-negative bacteria (group II) and short with or without histidine residues in G-positive bacteria (group I). The additional SBP associated zincophores are different classes of proteins that may act as zinc-chaperone and deliver the Zn²⁺ ion to the SBP or help the SBP in the zinc recruitment process. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (3)

Simulated (Phyre2 [78]) three-dimensional structure of AdcA; green = ZnuA-like domain, pink = ZnuA-flexible loop, blue = ZinT- like domain. Zn²⁺ ions are represented as purple spheres. The proposed zinc binding mode is (3His, 1H₂O). Protein picture was generated using CCP4mg [79]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (4)

Schematic representation of S. agalactiae zinc acquisition by means of Adc/Lmb importer system and the associate histidine triad proteins Sht and ShtII. Adapted from [74]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (5)

Biosynthetic pathway for the assembly of staphylopine, pseudopaline, yersinopine and bacillopaline. Adapted from [93].

Fig. (6)

Model of staphylopine (StP) secretion and recovery of zinc ions. After Stp synthesis inside the cell, its export occurs by means of the MFS efflux pump CntE. The import of StP zinc complex is then mediated by the ABC import system CntABCDF. The complex dissociates in the cytoplasm and zinc is released [94]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (7)

Model of pseudopaline secretion and recovery of zinc ions. After pseudopaline synthesis inside the cell, its transfer across the inner membrane occurs by means of the DMT transporter CntI. The transport system in charge of its export from the periplasm to the extracellular space is the MexAB-OprM efflux pump. The import of pseudopaline zinc complex in the periplasm space is mediated by the TBDT transporter, CntO. The complex is later internalized by unknown system. The complex then dissociates and zinc is released [104]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (8)

Schematic model of C. albicans zincophore-based Zn²⁺ acquisition [116]; after host invasion, Pra1 is expressed due to alkaline pH and low amount of soluble Zn²⁺ in the intracellular environment. It is secreted from the fungal cell surface, predominantly in the hyphal form and it binds host cellular zinc (either free cytosolic or bound to host protein). Pra1 returns to the fungal cell via physical interaction with Zrt1, a membrane transporter, to deliver the bound Zn²⁺ ions. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (9)

Proposed Zn²⁺ binding sites on C. albicans a) Pra1 zincophore and b) Zrt1 zinc transporter [19]. Structures are based on coordinates simulated by Phyre2 [78]. The figure was generated using CCP4mg [79]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (10)

(Top) General design of potential Trojan Horse zincophore-based therapeutics. The conjugated system is constituted by four elements: (1) the zinc ion, (2) the zincophore molecule which is able to form a stable zinc complex, (3) a linker to covalently bind the zincophore to the pharmacophore, (4) the drug or imaging probe. (Bottom) Schematic representation of the Trojan Horse strategy. The transporter recognizes its substrate (the Zn²⁺-zincophore complex) and facilitates the transition of the conjugated pharmacophore across the membrane barrier. (A higher resolution / colour version of this figure is available in the electronic copy of the article).