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Rubredoxins (Rds) and their derivatives have been extensively used, in the last few decades, in order to elucidate structure and functional aspects of metal sites involving rich sulphur coordination spheres. The derivatives have been designed with two main purposes: (a) replacement of selective and specific amino acid residues in native systems by site direct mutagenesis, and (b) replacement of native metal ion (iron), producing novel metal sites. We will highlight in this review the key amino acid residues, recognized in native Rds, directly involved in electron transfer mechanisms. By protein template assisted synthesis, metal-substituted Rds are used as structural probes and bio-models. The tetra-cysteinyl metal coordination site in Rd has the surprising capacity of chelating a wide variety of metal ions (other than native iron), and this chemistry has been witnessing significant growth based on two main interests: (i) metals such as 57 Fe, Zn, Co, Cd, Ga, In, and Hg were introduced and derivatives were synthesized in order to specifically probe structural determinants imposed by the protein, and (ii) Zn, Ni, Cu, as well as Mo (and most recently, W), were used with a particular interest: to model complex metal centres in sulphur-rich enzymes. The latter, in particular, is discussed in the context of Rd serving as a model complex of native enzymes, as well as the synthesis of small inorganic complexes. The review aims to bring synthetic biochemistry and synthetic inorganic chemistry together in a synergistic way. [ABSTRACT FROM AUTHOR]