posted on 2020-03-24, 14:38authored byNorbert Lihi, Dóra Kelemen, Nóra V. May, István Fábián
Detailed equilibrium,
spectroscopic, and SOD activity studies are reported on nickel(II)
complexes formed with the N-terminally free HHDLPCGVY-NH2 (NiSODHH) and HCDLPHGVY-NH2 (NiSODHC) peptides mimicking the nickel binding loop in NiSOD.
In these model peptides, cysteine was incorporated in different positions
in order to gain better insight into the role of the cysteine residues
in NiSOD. The results are compared with those obtained with the wild-type
fragment of NiSOD. The complex formation equilibria of nickel(II)
with the two peptides exhibit different features. In the case of NiSODHH, the ligand field of the (NH2,NIm,NIm,S–) donor set is not strong enough
to cause spin pairing and an octahedral paramagnetic complex is formed
under physiological conditions. In contrast, NiSODHC forms
a square-planar diamagnetic complex with (NH2,N–,S–,NIm) donors which exhibits remarkable
SOD activity. Our results unambiguously prove that the presence of
cysteine in the secondary position of the peptide chain is crucial
to establish the square-planar geometry in the reduced form of NiSOD,
while the distant cysteine affects the redox properties of the Ni(II)/Ni(III)
couple. Compared to the model systems, the Ni(II) complex with the
wild-type fragment of NiSOD exhibits superior SOD activity. This confirms
that both cysteinyl residues are essential in the efficient degradation
of superoxide ion. The enzyme mimetic complexes are also capable of
assisting the decomposition of superoxide ion; however, they show
considerably smaller catalytic activity due to the absence of one
of the cysteine residues.