Metallothioneins amino acid sequence

Fig. 23. The amino acid sequence of the brain-specific protein metallothionein III.

Comparative studies on amino acid sequences of 12 mammalian metallothioneins have been reported.1457 They are all similar and contain 61 residues JV-acetylmethionine and alanine are at the N and C termini, respectively. The cysteinyl residues are distributed fairly uniformly along the polypeptide chain with a predominance of —Cys—X—Cys— sequences. 

Since aromatic amino acids and cysteine are absent, there is no protein absorption above 270 nm. Metallothioneins exhibit a broad absorption peak, with the maximum at 190 mn. Absorptions due to the metal-thiolate complexes show as shoulders at 250 nm (Cd), 220 nm (Zn) and 270 nm (Cu).1458,1459 Theoretical predictions based on the amino acid sequence of the peptide chain indicate that the or-helical conformation is forbidden, and /3-structure is almost impossible to attain. CD and NMR studies on both the metal-containing and metal-free protein confirmed the predictions.1459 1460 However, metallothioneins are stable to tryptic digestion and the slow exchange of many peptide hydrogens of metallothionein with those of the solvent suggest that the protein has a compact and well-defined tertiary structure. 

Figure 44 The amino acid sequence in equine metallothionein...

The amino acid sequences of many mammalian metallothioneins are known.1152 They all contain 61 residues with the cysteine residues distributed along the full length of the polypeptide chain, and with N-acetylmethionlne and alanine at the N- and C-termini, respectively. All 20 cysteine residues are involved in the binding of up to seven metal ions via thiolate linkages. A typical amino acid sequence is shown in Figure 44. 

Capillary electrophoresis has proven to be useful in characterizing different molecular forms of various metalloproteins like metallothionein, transferrin, and conalbumin [2-5]. Molecular forms arise from differences in the amino acid sequence of proteins (isoforms), differences in the amount or type of metal bound (metalloforms), or from differences in the type and amount of carbohydrate side chains linked to the protein (glycoforms). CZE was used to follow the formation of the oligomeric iron core and its incorpora-... 

Overnell, J., C. Berger and K.J. Wilson. Partial amino acid sequence of metallothionein from plaice (Pleuronectes platessa). Biochem. Soc. Trans. 593rd Meet. 217-218, 1981. 

Fig. 36. Amino acid sequence of equine class I metallothioneins. From Kojima and Kagi 1979 [316] with permission...

The amino acid sequences of many mammalian metallothioneins are known. They all contain... 

Wei, D. Andrews, G.K. (1988). Molecular cloning of chicken metallothionein. Deduction of the complete amino acid sequence and analysis of expression using cloned cDNA. Nucl Acids Res., 16, 537-53. 

D structure is assumed to be similar to a domain of human metallothionein. The MerR regulatory protein is encoded by the mer operon of Tn507 (5) and is highly specific for Hg in its biological response. The amino acid sequence is known,... 

The recently identified brain-specific isoform of metallothionein, MT-III (neuronal growth inhibitory factor, GIF), has been linked with its potential role in neurophysiological and neuromodulatory functions (484). The human form of MT-III contains 68 amino acids with a 70% sequence (Fig. 23) similarity to other mammalian MTs and a... 

Metallothioneins are evolutionarily conserved in that they contain a high cysteine content and lack of aromatic amino acids. However, few invertebrate MTs have been characterized, and these can exhibit wide variation in noncysteine amino acid residues. Initially, MTs were classified according to their structural characteristics. Class I MTs consist of polypeptides with highly conserved cysteine residue sequences and closely resemble the equine renal MT. Mammalian MTs consist of 61-68 amino acids residues and the sequence is highly conserved with respect to the position of the cysteine residues (e.g., cys-x-cys, cys-x-y-cys, and cys-cys sequences, where x and y are noncysteine, non-aromatic amino acids). Class II MTs have less conserved cysteine residues and are distantly related to mammalian MTs. Class III MTs are defined as atypical and consist of enzymatically synthesized peptides such as phy-tochelatins and cadystins. This former classification scheme has been replaced by a more complex system to include the increasing number of identified isoforms. 

Metallothioneins of mammals are single-stranded low molecular proteins with 61 - 68 amino acids in the structure. Cysteine accounts for one third of amino acid moieties eysteine moieties are located in the conservative sequences such as cys-x-cys, cys-x-y-cys and cys-cys (x and y are amino acids different from cysteine). Metal ions are boimd to sulfhydiyl groups of cysteine moieties that create tetraedric conformation of thiolated clusters in the ease of divalent ions [12]. MT has the highest affinity to Cu (stability constant lO - lO ), then to Cd (lO - lO ) and Zn (lO " - lO") and it is not able to bind Cu. Eighteen metal ions that can be bound by MT have been shown however, only Cu", Cd, Pb, Hg, Ag and Bi are able to displace boimded Zn from the structure of MT. Overall, it can coordinate up to 12 mono- or 7 divalent ions. Its tertiary structure is based on the presence of two domains, a and (3 (Fig. 1). a-domain (C-terminal) is more stable and contains four binding sites for divalent ions of heavy metals, P-domain (N-terminal) can bind three divalent metal ions [13]. 

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