Substrate Specificity of Chlorophyll(ide) b Reductase in Etioplasts of Barley (Hordeurn Vulgare L.)

Abstract

Enzyme activity of chlorophyll(ide) b reductase is present in etioplasts. Recently the conversion of chlorophyllide b to chlorophyll a via 7'-hydroxychlorophyll a was demonstrated in barley etioplasts. We used zinc pheophorbide b for a detailed investigation of the reduction of the 7-formyl group to the 7l-hydroxy compound in intact barley etioplasts. The reaction proceeded likewise before esterification and after esterification with phytyl diphosphate. The metal-free pheophorbide b, that is not accepted by chlorophyll synthase for esterification, is reduced to 71-hydroxypheophorbide a to a small extent. The zinc (13'S)-pheophorbide b is at least equally well accepted for reduction as the epimer with the 13'R configuration of natural chlorophyll b. The reaction requires NADPH or NADH, although the latter is less effective. ATP is not required for the first step to the 7I-hydroxy compound. The significance of chlorophyll b reduction for acclimation from shade to sun leaves and for chlorophyll degradation is discussed.

In Situ Conversion of Protochlorophyllideb to Protochlorophyllide a in Barley

“…This enzyme had originally been implicated in the Chl b to Chl a reaction cycle of chloroplasts during photosynthetic acclimation and leaf senescence (26). But it is also highly active in etioplasts, as shown in this and previous studies (22)(23)(24)(25)(26). We assume that 7-formyl reductase may be involved in fine tuning the amounts of Pchlide b and Pchlide a and thereby could regulate the steady-state level of LHPP in etioplasts.…”

“…By analogy, this enzyme activity was found to also convert the nonesterified precursor of Chl b, Chlide b, as well as pyrochlorophyllide b and the magnesium-free pheophorbide b into the respective Chl a and 7-hydroxy compounds (23)(24)(25)(26).…”

“…In a recent paper, Scheumann et al (21) even generally questioned the existence of Pchlide b, but at the same time demonstrated that barley etioplasts rapidly convert exogenously added zinc protopheophorbide b (ZnPPb) to ZnPPa. This prompted us to conclude that Chl(ide) b reductase, presumably responsible for this conversion (22)(23)(24)(25)(26), could also metabolize the endogenously occurring Pchlide b to Pchlide a.…”

LHPP, the light-harvesting NADPH:protochlorophyllide (Pchlide) oxidoreductase:Pchlide complex of etiolated plants, is developmentally expressed across the barley leaf gradient

“…A presumed intermediate in the conversión of Pchlide b to Pchlide a is 7-OH-Pchlide a detected in this (Fig. 3, peak 1) and previous work [77].…”

“…In vivo, Pchlide a may either be freshly synthesized from protoporphyrin IX via the Mg branch of the C5-pathway or could alternatively be formed from excess Pchlide b by virtue of 7-formyl reductase present in the prolamellar body. According to previous and recent work, barley etioplasts contain 7-formyl reductase activity which converts a wide range of 6-type porphyrins and chlorins to their a-type compounds [27,[74][75][76][77]. A presumed intermediate in the conversión of Pchlide b to Pchlide a is 7-OH-Pchlide a detected in this (Fig.…”