Zinc mediated protection to the conformation of carbonic anhydrase in cadmium exposed Ceratophyllumdemersum L.

The mechanism by which Zn protects the conformation of carbonic anhydrase (CA; carbonate hydrolyase, E.C. 4.2.1.1) in Cd-exposed Ceratophyllum demersum L. was analyzed. Cd 10 μM, Zn (200 μM) supplemented Cd (10 μM) and Zn 200 μM alone treated plants of C. demersum were compared with the control plants. The circular dichroism spectra of native CA from control plants indicated α-helix secondary conformation, whereas CA from Cd 10 μM treatments indicated unfolding of the enzyme to a random coil, denaturation, protein-cross linking and aggregation as depicted by the decrease in ellipticities and diminishing of distinct α-helical peaks. Cd-induced protein unfolding lead to the quenching of tryptophan and tyrosine intrinsic fluorescence subsequently reflected in ultraviolet (UV) spectra also, decreased thiol (-SH) groups, increased the hydrophobicity measured as 8-anilino-1-naphthalene sulfonic acid (ANS)-protein fluorescence and enhanced the formation of oxidised components, dityrosine and carbonyls. Zn (200 μM) supplementation to Cd 10 μM treatments restored α-helical conformation with increased helical ellipticities, -SH groups, tryptophan and tyrosine intrinsic fluorescence, as well as UV spectra, decreased ANS-protein fluorescence and formation of oxidised dityrosine and carbonyl components. Zn alone treatments did not change the conformation or properties of the native CA. The clear inhibition of distortion in secondary structure, protection of hydrophobic core in the protein, inhibition of residual side chain oxidation or formation of insoluble aggregates by supplemented Zn indicates the potential role of Zn in combating Cd-induced oxidative damage to CA.