Question

Metallothioneins are a class of compounds that are cysteine-rich and low molecular weight proteins which bind to heavy metals. It is suggested that cadmium (Cd) concentrations in oysters bioaccumulate because Cd binds to metallothioneins present in the gills of the oysters (Lekhi et al., 2008). The amino acid cysteine has the following chemical structure:

HS OH NH2

a. Circle all the possible ligand atoms (donor atoms) in cysteine.

b. Of the ligand atoms you circled, identify whether they are A-type or B-type.

c. Of the ligand atoms you circled, which one(s) is most likely to bind to Cd^2+?

Answer 1

Part a.

The ligand atoms (donor atoms) in cysteine: S (of SH group), N (of NH2 group), O (of C=O groups), O (of OH-group).

Part b.

S, N, and O (of C=O) group are L-type, i.e. they bind to metal by coordinate or dative-bond.

Whereas O (of OH-group) is X-type, i.e. it binds to the metal by a covalent bond.

Part c.

The ligand 'S' is most likely to Cd²⁺ because Cd²⁺ is a soft metal ion and S is a soft ligating atom, as a result, their soft-soft interaction is favorable.