Metal-catalyzed oxidation of phenylalanine-sensitive 3-deoxy-D-arabino- heptulosonate-7-phosphate synthase from Escherichia coli: Inactivation and destabilization by oxidation of active-site cysteines

The in vitro instability of the phenylalanine-sensitive 3-deoxy-D- arabino-heptulosonate-7-phosphate synthase [DAHPS(Phe)] from Escherichia coli has been found to be due to a metal-catalyzed oxidation mechanism. DAHPS(Phe) is one of three differentially feedback-regulated isoforms of the enzyme which catalyzes the first step of aromatic biosynthesis, the formation of DAHP from phosphoenolpyruvate and D-erythrose-4-phosphate. The activity of the apoenzyme decayed exponentially, with a half-life of about 1 day at room temperature, and the heterotetramer slowly dissociated to the monomeric state. The enzyme was stabilized by the presence of phosphoenolpyruvate or EDTA, indicating that in the absence of substrate, a trace metal(s) was the inactivating agent. Cu²⁺ and Fe²⁺, but none of the other divalent metals that activate the enzyme, greatly accelerated the rate of inactivation and subunit dissociation. Both anaerobiosis and the addition of catalase significantly reduced Cu²⁺-catalyzed inactivation. In the spontaneously inactivated enzyme, there was a net loss of two of the seven thiols per subunit; this value increased with increasing concentrations of added Cu²⁺. Dithiothreitol completely restored the enzymatic activity and the two lost thiols in the spontaneously inactivated enzyme but was only partially effective in reactivation of the Cu²⁺-inactivated enzyme. Mutant enzymes with conservative replacements at either of the two active-site cysteines, Cys⁶¹ or Cys³²⁸, were insensitive to the metal attack. Peptide mapping of the Cu²⁺-inactivated enzyme revealed a disulfide linkage between these two cysteine residues. All results indicate that DAHPS(Phe) is a metal- catalyzed oxidation system wherein bound substrate protects active-site residues from oxidative attack catalyzed by bound redox metal cofactor. A mechanism of inactivation of DAHPS is proposed that features a metal redox cycle that requires the sequential oxidation of its two active-site cysteines.