Pigment-free NADPH : protochlorophyllide oxidoreductase from Avena sativa L - Purification and substrate specificity

The enzyme NADPH:protochlorophyllide oxidoreductase (POR) is the key enzyme for light-dependent chlorophyll biosynthesis. It accumulates in dark-grown plants as the ternary enzyme-substrate complex POR-protochlorophyllide a-N ADPH. Here, we describe a simple procedure for purification of pigment-free FOR from etioplasts of Avena sativa seedlings. The procedure implies diffe rential solubilization with n-octyl-beta-D-glucoside and one chromatographi c step with DEAE-cellulose. We show, using pigment and protein analysis, th at etioplasts contain a one-to-one complex of FOR and protochlorophyllide a . The preparation of 13 analogues of protochlorophyllide a is described. Th e analogues differ in the side chains of the macrocycle and in part contain zinc instead of the central magnesium. Six analogues with different side c hains at rings A or B are active substrates, seven analogues with different side chains at rings D or E are not accepted as substrates by FOR. The kin etics of the Light-dependent reaction reveals three groups of substrate ana logues with a fast, medium and slow reaction. To evaluate the kinetic data, the molar extinction coefficients in the reaction buffer had to be determi ned. At concentrations above 2 mole substrate/mole enzyme, inhibition was f ound for protochlorophyllide a and for the analogues.