Abstract
The NADPH-protochlorophyllide oxidoreductase, an enzyme catalysing the light-driven conversion of protochlorophyllide to chlorophyllide, was studied in the greening mutant C- 2A′ of the unicellular green alga Scenedesmus obliquus. Studies of the enzyme activity in vitro showed strong dependence on the presence of glycerol and the detergent Triton X-100. Prerequisite for the formation of a photoactive enzyme complex is a sufficient preincubation time with the substrates PChlide and NADPH. A continuous assay system, reading the absorbance increase at the wavelength of chlorophyllide, was used to determine the kinetic constants. The Km value for NADPH is 4.2 μᴍ, the Vmax is 5.9 pmol · s-1. The Km and Vmax for protochlorophyllide are 0.19 μᴍ and 6.5 pmol · s-1, respectively. The pH-dependence of the reaction exhibits a broad maximum between pH 7-8.5 typically for an enzyme active during chloroplast development, when pH-changes might be expected. The obtained kinetic data outline that the light dependent formation of chlorophyll in vivo is not limited by the substrates PChlide and NADPH, indicating that only light is the triggering factor in the very early greening process.
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