Derivation of the formyl‐group oxygen of chlorophyll b from molecular oxygen in greening leaves of a higher plant (Zea mays)

Abstract

Using mass spectroscopy, we demonstrate as much as 93% enrichment of the 7-formyl group oxygen of chlorophyll b when dark-grown, etiolated maize leaves are greened under white light in the presence of "OZ. This suggests that a mono-oxygenase is mvolved in the oxidation of tts methyl group precursor.The concomitant enrichment of about 75% of the 13'-oxygen confirms the well-documented finding that this 0x0 group, in both chlorophyll (I and b, also arises from 0,. High "0 enrichment into the 7-formyl oxygen relative to the substrate "0, was achieved by optimization of the greenmg conditions in combination with a reductive extraction procedure. It indicates not only a single pathway for Chl b formyl group formation, but also unequivocally demonstrates that molecular oxygen is the sole precursor of the 'I-formyl oxygen.

Origin of the Two Carbonyl Oxygens of Bacteriochlorophyll a

“…The cost of this approach, however, prevented direct confirmation of the labelling of the 3-acetyl group by a hydratase mechanism in R. denitrqicum because at least 60 ml of medium prepared in H,I80 would be needed to produce sufficient pigment for the required analyses. Porra et al (1993, 1994) Porra et al (1993, 1994 Schneegurt and Beale (1992) Schneegurt and Beale (1992) Bolivar and Beale (1995) this paper this paper " Etioplasts from darkgrown wheat leaves " Cotyledons from greening cucumber seedlings. Greening maize leaves.…”

“…The uniformly high labelling at both C13' and C172, however, suggests that a common exchange step is likely. Therefore, it is interesting, :irstly, that H2"0 labels all four oxygens at C13' and C17' in BChl a but none in Chls a and b higher plants (Porra et al, 1993(Porra et al, , 1994 and, secondly, that bacteriochlorophyll and chlorophyll biosynthesis in R. sphaeroides and higher plants, respectively, prior to the 13'-methylation reaction, differ only in the reactions leading to 5-aminolaevulinate formation. Thus, it is attractive to suggest that the labelling of the carboxyl oxygens of BChl (1 in R. sphaeroides occurs during or prior to 5-aminolaevulinate formation from succinyl-CoA and glycine via the 5-aminolaevulinate synthase route ; in higher plants, 5-aminolaevulinate is derived from glutamate via glutamate-l-semialdehyde and 4,5-diaminovalerate by the C, pathway.…”

“…1) was proposed by the latter authors. The 7-formyl oxygen of Chl b also derived from molecular oxygen in both higher plants (Porra et al, 1993(Porra et al, , 1994 and in green algae (Schneegurt and Beale, 1992).…”

The Regulation of Enzymes Involved in Chlorophyll Biosynthesis

“…1). It is generally believed that Chl b is formed from Chl a by oxidation using molecular oxygen as the precursor of the 7-formyl group (Schneegurt and Beale, 1992;Porra et al, 1993) (Fig. 4).…”

“…It has been proposed that at least three different enzymes might catalyze this reaction : a mixed-function oxygenase of unusual reaction specificity, a dioxygenase, or a combination of two enzymes, a monooxygenase plus a dehydrogenase (Rudiger and Schoch, 1988;Porra et al, 1993Porra et al, , 1994. Porra et al (1994) further proposed that oxidation of the methyl group occurs in a very hydrophobic location within the thylakoid membranes and speculated that Chl a already bound to the pigment-protein complexes may serve as the substrate. In contrast, Shlyk (1971) had assembled extensive evidence that Chl b may be formed from easily extractable Chl a.…”