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The regulation of recombinant plastidic glucose-6P dehydrogenase from  Populus trichocarpa  ( Pt P2-G6PDH - EC 1.1.1.49) was investigated by exposing wild type and mutagenized isoforms to heavy metals. Nickel and Cadmium caused a marked decrease in  Pt P2-G6PDH WT activity, suggesting their poisoning effect on plant enzymes; Lead (Pb ++ ) was substantially ineffective. Copper (Cu ++ ) and Zinc (Zn ++ ) exposition resulted in strongest decrease in enzyme activity, thus suggesting a physiological competition with Magnesium, a well-known activator of G6PDH activity. Kinetic analyses confirmed a competitive inhibition by Copper, and a mixed inhibition by (Cd ++ ). Mutagenized enzymes were differently affected by HMs: the reduction of disulfide (C 175 –C 183 ) exposed the NADP +  binding sites to metals; C 145  participates to NADP +  cofactor binding; C 194  and C 242  are proposed to play a role in the regulation of NADP + /NADPH binding. Copper (and possibly Zinc) is able to occupy competitively Magnesium (Mg ++ ) sites and/or bind to NADP + , resulting in a reduced access of NADP +  sites on the enzyme. Hence, heavy metals could be used to describe specific roles of cysteine residues present in the primary protein sequence; these results are discussed to define the biochemical mechanism(s) of inhibition of plant plastidic G6PDH.

Mechanism(s) of action of heavy metals to investigate the regulation of plastidic glucose-6-phosphate dehydrogenase