Heavy metal-binding proteins/peptides: Occurrence, structure, synthesis and functions. A review

Abstract

This review summarizes information on heavy metal-binding proteins/peptides other than animal metallothioneins. Metal-binding proteins which are produced in response to various heavy metals in higher plants, algae and fungi were initially thought to be similar to animal metallothioneins. However, later studies indicated they are different from metallothioneins in structure and synthesis. These proteins have been isolated and characterized from various organisms and are comprised mainly of three amino acids, i.e. cysteine, glutamic acid and glycine. HPLC and sequencing studies elucidated that these peptides have the general structure ( γ-glutamyl-cysteinyl) n -glycine where n = 2–11. These γ-glutamyl peptides have been reported to occur in algae, fungi and higher plants, and have been named as phytochelatins, cadystins and γ-glutamyl peptides. Free sulfide in these peptides is reported to stabilize the metal cysteinyl thiolate co-ordination. With γ-carboxamide linkages these are thought not to be synthesized via mRNA but studies with buthionine sulfoximine suggest glutathione is the precursor for these peptides. These peptides may be involved in metal ion homeostasis and detoxification of excess metals.