The sulfhydryl groups of ferredoxin-NADP+ oxidoreductases: is a disulfide bond really present?

The sulfhydryl groups of three ferredoxin-NADP+ oxidoreductases [EC 1.18.1.2] (FNR) two from blue-green algae, Spirulina sp. and Synechococcus sp., respectively, and one from spinach, were analyzed by carboxymethylation and titration with sulfhydryl reagents under different conditions. Five sulfhydryl groups in Spirulina and spinach enzymes and four in Synechococcus enzyme were found to be in reduced forms. There was no disulfide bond in any FNR tested. The results in the present experiment contradict those reported by Karplus et al. [Biochemistry 23, 6576-6583 (1984)].