Spectroscopic characterization of Co(II)-, Ni(II)-, and Cd(II)-substituted wild-type and non-native retroviral-type zinc finger peptides.

The nucleocapsid protein (NCP) from Mason-Pfizer monkey virus (MPMV) contains two evolutionary invariant Cys-X2-Cys-X4-His-X4-Cys retroviral-type zinc finger structures, where the Cys and His residues provide ligands to a tetrahedrally coordinated Zn(II) ion. The N-terminal zinc finger (F1) of NCP from MPMV contains an immediately contiguous Cys in the -1 position relative to the start of this conserved motif: Cys-Cys-X2-Cys-X4-His-X4-Cys. Metal complexes of 18-amino acid peptides which model the native zinc finger sequence, SER-Cys-X2-Cys-X4-His-X4-Cys (F1-SC), and non-native Cys-SER-X2-Cys-X4-His-X4-Cys (F1-CS) and SER-SER-X2-Cys-X4-His-X4-Cys (F1-SS) sequences have been spectroscopically characterized and compared to the native two-zinc-finger protein fragment, MPMV NCP 21-80. All Co(II)-substituted peptide complexes adopt tetrahedral ligand geometries and have S- -->Co(II) ligand-to-metal charge-transfer (LMCT) transition intensities consistent with three Co(II)-S bonds for F1-SC and F1-CS. The non-native F1-CS peptide binds Co(II) with KCo= 1.5 x 10(6) M(-1), comparable to that of the native complex, and approximately 100-fold tighter than F1-SS. Like the Co(II) derivative, the absorption spectrum of Ni(II)-substituted NCP 21-80 is most consistent with tetrahedral Ni(II) complexes with multiple thiolate donors. In contrast, Ni(II) complexes of F1-SC and F1-CS exhibit a single absorption band in the 400-550 nm region (epsilon approximately 200-300 M(-1) cm (-1), distinct in the two complexes, assignable to a degenerate d-d transition envelope characteristic of non-native square-planar coordination geometry, and an intense LMCT transition in the UV (epsilon255 approximately 14,000 M(-1) cm(-1)). Cd(II) complexes have intense absorption in the UV (lambda(max)=233nm), with absolute intensities consistent with approximately 5000 M(-1) cm(-1) per Cd(II)-S bond. 113Cd NMR spectroscopy of 113Cd MPMV NCP gives delta=649 ppm, consistent with S3N coordination. Co(II) and Cd(II) complexes of non-native F1-CS peptides are more sensitive to oxidation by O2, relative to F1-SC, suggestive of a higher lability in the non-native chelate. The implications of these findings for the evolutionary conservation of this motif are discussed.