Spectroscopic investigations of interactions between metals and consensus peptide sequences from nitrile hydratase active sites

In this work, the metal coordination properties of peptides containing consensus sequences found at the active sites of nitrile hydratases have been studied with Fe(III), Co(II), and Ni(II) in both aqueous solution and DMF. The goal of this investigation is to understand the structural roles of highly conserved metal binding motifs in metalloenzymes.; Nitrile hydratases are nonheme iron-containing or noncorronoid cobalt-containing enzymes that catalyze the hydration of nitriles into corresponding amides. Both Co- and Fe-nitrile hydratases contain low spin trivalent metals bound to the highly conserved peptide sequence -Cys-(Thr/Ser)-Leu- Cys-Ser-Cys-, in which all Co-nitrile hydratases contain the Thr residue and all Fe-nitrile hydratases have the Ser residue. The metal coordination motif -Cys-X-X-Cys-X-Cys- in these sequences binds the protein metal center by 3 Cys-thiolates and 2 deprotonated peptide backbone amide nitrogens. The 2 cysteine thiolates and 2 amide nitrogens from the -Cys-X-Cys- section of this motif bind to metal forming a MS₂N₂ coordination plane.; In aqueous solution, peptides containing the -Cys-X-X- Cys-X-Cys- motif interact with Co(II) in 1:1 ratio forming tetrahedral Co(II) peptide complexes with 2 cysteine thiolates from the - Cys-X-X-Cys- sequence. By contrast, peptides containing the -Cys-X-Cys- motif coordinate to Co(II) in a 4:1 ratio, suggesting the -Cys-X-Cys- motif cannot chelate Co(II). However, the peptide containing this -Cys-X- Cys- motif can slowly form a square planar Ni(II)(peptide)₂ complex with a NiS₄ center. The metal controls the coordination geometry in the Ni(II)-[-Cys-X-Cys-]₂ complex forming a square planar geometry, and the peptide controls the coordination geometry in the Ni(II)-[-Cys-X-X-Cys-]₂ complexes, in which tetrahedral Ni(II) site are formed. The backbone amide nitrogens from the peptide do not coordinate to metal. Furthermore, a preformed β turn conformation is predicted based on CD spectra of the -Cys-X-X- Cys- section in solution when the X residues are Thr-Leu.; In DMF solution, peptides having the -Cys-X-X-Cys -X-Cys- or -Cys-X-Cys- motif can both interact with Fe(III) to form high spin complexes. Upon addition of cyanide, a low spin Fe(III) complex is formed that has spectroscopic properties very similar to those of the nitrile hydratase protein active site Fe(III) center. Further investigation reveals, however, a multimeric, spin coupled Fe-peptide structure in this complex.