The absence of tightly bound copper, iron, and flavin nucleotide in crystalline ribulose 1,5-bisphosphate carboxylase-oxygenase from tobacco.

Crystalline ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) was isolated from tobacco ($\text{Nicotiana tabacum}$ L.) leaf homogenates and analyzed for several characteristic oxygenase prosthetic groups. Analyses by atomic absorption and emission spectroscopy and neutron activation indicated that the crystalline protein contains less than 0.2 g-atoms of tightly bound copper or iron per mole (550,000 g) of enzyme. In addition, the absorption and fluorescence spectra of concentrated solutions of the crystalline protein gave no indication of the presence of a flavin nucleotide. Thus, the enzymatic oxygenation of ribulose 1,5-bisphosphate to yield P-glycolate, which is believed to comprise the initial reaction in the photorespiratory metabolism of higher plants, appears not to involve these cofactors in the catalytic mechanism.