Phytochelatins and Related Peptides (Structure, Biosynthesis, and Function)

Abstract

Plants obtain micronutrients such as Cu and Zn from aquatic and terrestrial environments that may also provide the metals Cd, Pb, and Hg. These essential and nonessentia1 metals exist at low to high concentrations depending on natural and manmade disturbances. In a fluctuating environment the plants may experience shifting internal concentrations of these bioreactive metals. Therefore, it is beneficial for plants to have mechanisms that (a) maintain internal concentrations of essential metals between deficient and toxic limits and (b) keep nonessential metals below their toxicity thresholds. The proteins and peptides known as MTs sequester metals and thereby may accomplish cellular metal homeostasis and detoxification. These molecules are rich in Cys's that provide thiols for binding the metals mentioned so far.The early work on plants was modeled after equine renal MT, a metal-induced protein with 20 Cys's distributed uniquely within the sequence of 60 amino acids and M, of approximately 9000 (Robinson et al., 1993). Studies of yeasts, algae, and plants relied heavily on isolating the metal-induced components of M , of approximately 9000 that bound Cd or Cu. In many of these preparations Cys, Glu, and Gly accounted for 45 to 97% of the amino acids, which is inconsistent with a dose relationship to the archetypal form, equine renal MT. Two independent groups, one working with the fission yeast Schizosaccharomyces pombe (Kondo et al., 1984) and the other working with cultured cells of Rauvolfia serpentina (Grill et al., 1985), showed that the molecules binding Cd were a family of peptides with the primary structure (y-Glu-Cys),-Gly, where n = 2 to 7

Expression and RNA Splicing of the Maize Glutathione S-Transferase Bronze2 Gene Is Regulated by Cadmium and Other Stresses

“…With cadmium modulating the splicing decision to give the "correct" product for detoxification of heavy metals, Bz2 would thus be considered a metal-activated protein. Cadmium can also regulate enzymes posttranslationally; PC synthase is activated directly by cadmium (Rauser, 1995). In summary, the maize Bz2 gene and other type 111 plant GSTs may encode multifunctional stress proteins.…”

In seedlings of the heavy metal accumulator Brassica juncea Cu2+ differentially affects transcript amounts for γ‐glutamylcysteine synthetase (γ‐ECS) and metallothionein (MT2)

“…In plants heavy metal tolerance and hyperaccumulation are thought to be the result of complexation and sequestration of heavy metal ions by specific cysteine-rich polypeptides, known as metallothioneins (MTs) and phytochelatins (PCs), respectively [1,2]. While MTs are gene products, PCs are synthesized enzymatically from glutathione (or related pep tides).…”

Probing the coordination properties of glutathione with transition metal ions (Cr2+,Mn2+,Fe2+,Co2+, Ni2+,Cu2+,Zn2+,Cd2+,Hg2+) by density functional theory

“…Due to its intracellular abundance, GSH is a likely target for metal cations, especially those having high affinity for thiolate sulfur [3]. It has been suggested that GSH may serve as a primary defense against metal cation toxicity [4,5]. Reduced glutathione (GSH), present in nuclei at concentrations ranging from 1 and 10 mM, has been implied as a protective agent against heavy metal cation toxicity through forming complexes [6][7][8][9][10][11].…”