1H Nuclear Magnetic Resonance Studies on Metallothionein from Scylla serrata

The 1H NMR spectrum of 11³Cd₆ Scylla serrata metallothionein isoform 1 (MT-1) is presented and analyzed in considerable detail by using two-dimensional chemical shift correlated (COSY) methods. Using these methods, it has been possible to associate most ¹H resonances with amino acid type. The ¹H chemical shifts for all 18 cysteine residues present in the protein have been identified, as has the range of heteronuclear ¹H-¹¹³Cd scalar coupling between the ¹¹³Cd ions and the cysteine β-CH₂ protons, 10 Hz < ³J_113Cd-1H < 65 Hz. The relationship of these ¹H NMR data to the proposed cluster structures derived from ¹¹³Cd NMR is discussed with some emphasis being placed on the identification of bridging cysteine residues, required by the proposed cluster structures, via the analysis of the heteronuclear scalar coupling present in the cysteine αβ cross-peaks in the ¹H COSY spectrum. It is concluded that, in the absence of a calibrated Karplus equation for ³J_113Cd-1H and in the presence of short ¹¹³Cd T₁ values, identification of the 1H resonances arising from bridging cysteine residues using heteronuclear scalar coupling data alone is not readily achievable.