Phytochelatins binding

Grill, E., Winnacker, E.-L. Zenk, M.H. (1987). Phytochelatins, a class of heavy-metal-binding peptides from plants, are functionally analogous to metallo-thioneins. Proceedings of the National Academy of Sciences, USA, 84, 439-43. 

Within a relatively short period the purification of these metal-binding components had been completed by several groups (Grill et al., 1985 Steffens et al., 1986 Reese Wagner, 1987 Jackson et al., 1987). Unfortunately, this led to a series of different names for the same molecules (see Steffens, 1990) for clarity in this chapter, we are using the term phytochelatin as a contrast to gene-encoded MTs. 

FIGURE 7 Examples of Fe ligands. (A) The bacterial hexadentate siderophore enterobactin. (B) The planar porphyrin phytochelatin. M denotes the metal binding site. Reprinted with permission from Stumm W., and J. J. Morgan. 1996. Aquatic Chemistry Chemical Equilibria and Rates in Natural Waters. Copyright 1996 Wiley, New York. 

Maier, T., Yu, C., Kulleritz, G., and Clemens, S. (2003) Localization and functional characterization of metal-binding sites in phytochelatin synthases. Planta 218,... 

As an indirect effect of increased metal uptake, the physiological state of the cell can alter and defence mechanisms can be induced. Phytochelatin (metal binding proteins) synthesis and induction of free radical quenching enzymes and metabolites were frequently observed. Especially the latter can protect membranes against oxidative breakdown. 

CD spectrum including formation of a band at 255 nm (+), a shoulder at about 270 nm (+), a weaker signal at 300 nm (-), and a broad signal centered at 330 nm (+). The CD spectrum became saturated after addition of one equivalent of Pb + per phytochelatin molecule. The authors concluded from these spectral data that the bound Pb + adopts a two-coordinate binding geometry when bound to this phytochelatin. ... 

Figure 6.18. MacrcKyclic complex fonners. (a) Structure of a ferrichrome (desferri-ferrichrome), one of the strongest complex formers presently known for Fe(III). The iron-binding center is an octahedral arrangement of six oxygen donor atoms of trihy-droxamate. Such naturally occurring ferrichromes play an important role in the biosynthetic pathways involving iron. Complexing functionalities of some biogenic ligands (b) hydroxamate siderophores, (c) catechol siderophores, and (d) phytochelatines. For detailed structures see Neilands (1981).

Both plants and yeast are known to produce phytochelatins (PC ), peptide metal-binding ligands, in response to heavy metal (especially Cd2+) toxicity heavy metal ions activate the enzyme, PC synthase (PCS), which produces PC s from glutathione (GSH) see equation (7.1). 

Metallothionein, metal-binding proteins and phytochelatins (see Gagne and Blaise, Chapter 7 of this volume). Metallothioneins are low molecular weight, cysteine-rich proteins with a high affinity for transition metals. After they were first discovered in the kidney cortex of the horse they have been detected in a variety of animal species. It is widely accepted that metallothioneins are multifunctional proteins primarily involved in the homeostasis of essential trace metals, zinc-mediated gene regulation, and in the protection of cells against oxidative... 

In plants, two kinds of metal-binding peptides or proteins are synthesized. Plant metallothioneins are inducible cysteine-rich entities very like those found in animals. Differential expression (induction) of metallothionein genes can be due to both variation of external heavy metal concentrations and the influence of various environmental factors. The principle role of plant metallothioneins seems to be in homeostasis rather than in metal detoxification. Plants are also known to have so-called phytochelatins, which are non-protein thiols specifically induced upon exposure to heavy metals. A close positive relationship between the concentrations of cadmium and phytochelatins in the plant shoot material has been observed and linked to the degree of growth inhibition (Keltjens and Van Beu-sichem, 1998). These observations make the use of phytochelatins promising for the assessment of heavy metal effect on plants. 

Complexing and chelate forming of toxic ions are possible to prevent their direct contact with sensitive enzymes. For Ah, the complexes with organic acids are important within the plant (Ma et al. 1997, Wenzel et al. 2002). In other instances, proteins or phytochelatins (polypeptides consisting of repetitive glutamylcys-teine units) are formed which bind the toxic ion. While the role of an additional synthesized protein in Al-tolerant wheat genotypes merits further consideration (Taylor et al. 1997), the detoxification of Cd (Tukendorf and Rauser 1990) by phytochelatins is evident. 

Tolerance to metal stress relies on plant capacity to detoxify metals having entered cells. The postulated mechanisms involve biochemical detoxification, for example by binding to organic acids (especially citrate) or proteins like ferritin, metallothioneins and phytochelatins, and finally compart-mentalization of the metal within the cell. In most plant cells the vacuole comprises more than 80-90% of the cell volume and is acting as a central storage compartment for ions (Briat et al. 1999). 

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