Microalgal Metallothioneins and Phytochelatins and Their Potential Use in Bioremediation

Abstract

The persistence of heavy metals (HMs) in the environment causes adverse effects to all living organisms; HMs accumulate along the food chain affecting different levels of biological organizations, from cells to tissues. HMs enter cells through transporter proteins and can bind to enzymes and nucleic acids interfering with their functioning. Strategies used by microalgae to minimize HM toxicity include the biosynthesis of metal-binding peptides that chelate metal cations inhibiting their activity. Metal-binding peptides include genetically encoded metallothioneins (MTs) and enzymatically produced phytochelatins (PCs). A number of techniques, including genetic engineering, focus on increasing the biosynthesis of MTs and PCs in microalgae. The present review reports the current knowledge on microalgal MTs and PCs and describes the state of art of their use for HM bioremediation and other putative biotechnological applications, also emphasizing on techniques aimed at increasing the cellular concentrations of MTs and PCs. In spite of the broad metabolic and chemical diversity of microalgae that are currently receiving increasing attention by biotechnological research, knowledge on MTs and PCs from these organisms is still limited to date.

Keywords: cysteine; glutathione; heavy metals; metal-binding proteins; metallothioneins; microalgal biotechnologies; phycoremediation; phytochelatins.

FIGURE 1

(A) Tertiary structure and active sites for metal binding in Type 4 plant metallothioneins from wheat. Adapted from Figure 9 in Leszczyszyn et al. (2013). (B) Occurrence of cysteine-rich motifs in the different metallothionein (MT) families described to date. Adapted from Figure 1D in Ziller et al. (2017). License to publish these figures has been obtained from the original publishers, the Royal Society of Chemistry (Metallomics) and Elsevier (Journal of Inorganic Biochemistry).

FIGURE 2

Current knowledge about metallothioneins and phytochelatins. (A,B) Taxonomic origin of the GenBank proteins annotated as metallothionein (MT), glutamate–cysteine ligase (GCL), glutathione synthase (GSH), and phytochelatin synthase (PCS) for (A) all eukaryotic organisms and (B) within eukaryotic microalgae. Please note the different scale on the y-axis (C,D) number of papers, as reported by Web of Sciences (September 2019), containing the keywords (C) “metallothionein*” and (D) “phytochelatin*” alone or in association with the keywords “microalga*/phytoplankt*/phytobenth*.” Keyword searches were case insensitive.

FIGURE 3

Structure and biosynthetic pathways of glutathione (GSH) and phytochelatin (PC). Three-letter abbreviations correspond to amino acid codes. (A) The biosynthetic pathway of GSH and PC consists of two and three reactions, respectively. First, a cysteine unit binds to the carboxylic group of the side chain of glutamic acid residue to form γ-glutamylcysteine, and this reaction is catalyzed by glutamate–cysteine ligase (GCL), then γ-glutamylcysteine binds to a glycine residue to form glutathione, and glutathione synthetase (GSHS) catalyses this reaction. PC-synthase (PCS) might then bind two or more GSH units to form phytochelatins. (B) Example of PC–metal complexes for divalent cations. M²⁺ denotes bivalent metal cations, whereas cysteine residues are indicated in red.