The polythiol peptides in Table 1 belong to a group of molecules designated metallothioneins. These are polypeptides sharing a low molecular mass, high cysteine content with absence of aromatic amino acids and his-tidine, high metal content, an abundance of CysXaaCys sequences where Xaa is an amino acid other than cysteine, spectroscopic features characteristic of metal thiolates, and metal thiolate clusters. Metallothioneins are now subdivided into three classes based on their structure (Fowler et al. 1987).
Class I: polypeptides with locations of cysteine closely related to those in equine renal metallothioneins;
Class II: polypeptides with locations of cysteine only distantly related to those in equine renal metallothioneins;
Class III: atypical, nontranslationally synthesized metal thiolate polypep-tides.
The polythiol peptides in Table 1 are class III metallothioneins because ribosomal synthesis produces only bonds and not the carboxyamide linkage. It follows that no genes directly define the primary structure of the polythiol peptides, rather gene-controlled non-ribosomal enzyme(s) are involved. The polythiols are structurally related to the respective monothiols, whether a biosynthetic relationship exists remains to be defined for most polythiols. No concensus is available concerning trivial names for the polythiols. The family with a carboxy-terminal Gly was first characterized for the fission yeast Schizosaccharomyces pombe exposed to Cd, the two peptides produced were called cadystins A and B (Kondo et al. 1984). Shortly thereafter a larger series within the same peptide family was found in several plants exposed to various metals (Grill et al. 1985). The name phytochelatin was proposed because this series of polythiols was not restricted to Cd nor to fungi. Since phytochelatins had a structural relationship to glutathione, the homologues related to homoglutathione were called homophytochelatins (Grill et al. 1986a) and those related to hydroxymethyl-glutathione were designated hydroxymethyl-phytochelatins (Klapheck et al. 1994) (Table 1). In a recent review, Zenk (1996) proposed a unified nomenclature based on phytochelatin (PC). Those peptides having a carboxy-terminal amino acid other than Gly are named iso-phytochelatin (iso-PC) with the parenthetic addition of the carboxy-terminal amino acid abbreviation [e.g. wo-PC(pAla)]. The listing for the six families of polythiol peptides is given in Table 1. This new terminology leaves room for new iso-PCs should they be discovered (Zenk 1996). The prefix iso was chosen to signify the equal function of the peptides. In this chapter the polythiol peptides are collectively called peptides based on the common structural ele-
ment. Specific thiols are named according to the sequence of amino acids (Table 1). Since most specific biochemical data are confined to the (yGluCy)nGly family the original name phytochelatin is used to ease consultation with original articles.
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