Polycation interactions with spinach ferredoxin-nicotinamide adenine dinucleotide phosphate reductase

Polylysine stimulates the activity of ferredoxin-NADP reductase. The stimulation is observed with three different catalytic activities of this enzyme. The stoichiometry of interaction of polylysine and enzyme is a function of the size of the polylysine molecule. Polylysine alters the absorption and fluorescence emission spectra of the enzyme with the same stoichiometry as that affecting catalytic activity. Steady state initial velocity kinetics indicates that polylysine is a noncompetitive hyperbolic activator of the enzyme and is able to prevent substrate inhibition by NADPH. In the absence of polylysine, the specific activity of the enzyme increases upon dilution, and this effect is magnified in the presence of polylysine. These results suggest that the soluble enzyme is an aggregate, and gel filtration measurements indicate a molecular weight of 85,000 for the purified enzyme, which appears to consist of two subunits. This confirms the observations of Fredricks and Gehl (FREDRICKS, W. W., AND GEHL, J. M. (1973) Fed. Proc. 32, 477). Indirect evidence is presented suggesting the enzyme is bound to a cationic region on the chloroplast membrane.