Biosynthesis of the 3‐Acetyl and 13¹‐Oxo Groups of Bacteriochlorophyll a in the Facultative Aerobic Bacterium, Rhodovulum sulfidophilum

Abstract

Using 18 O-labelling and mass spectrometry, we have examined bacteriochlorophyll a formation in Rhodovulum sulfidophilum, formerly known as Rhodobacter sulfidophilus, which forms large amounts of BChl a both aerobically in the dark and anaerobically in the light.R. sulfidophilum, growing under strict anaerobiosis in the light, possesses hydratases which incorporate 18 O label from H 2 18 O into both the 13 1 -oxo and 3-acetyl oxygens; in addition, the four carboxyl oxygens at C13 3 and C17 3 were labelled by H2 18 O. Under aerobic conditions in the dark, the labelling of the 13 1 -oxo group by H 2 18 O was reduced indicating that 16 O was being incorporated into this group from air.R. sulfidophilum, grown in the dark under an atmosphere initially containing 50% 18 O2 in Ar, possessed an oxygenase which incorporated 18 O label from 18 O 2 specifically into the 13 1 -oxo group; under these conditions the acetyl and carboxyl groups remained unlabelled.Thus, both an oxygenase and hydratase operate in R. sulfidophilum to form the 13 1 -oxo group of ring E of BChl a; the 3-acetyl group oxygen, however, arises only from water via a hydratase.Keywords : biosynthesis, bacteriochlorophyll a; isocyclic ring E formation ; oxygenase-catalyzed and hydratase-catalyzed cyclization; 3-acetyl formation.Bacteriochlorophyll (BChl) a, an antenna and reaction-centre pigment of many photosynthetic bacteria, possesses six oxygen atoms : in addition to four carboxyl oxygens at C13 3 and C17 3 there are two carbonyl oxygens at C3 1 and C13 1 (see formula in Scheme 1). The incorporation of 18 O label into these oxygens from 18 O 2 and H 2 18 O during pigment biosynthesis has recently been reviewed [1].During BChl a biosynthesis, the first of the two carbonyl groups formed is the 13 1 -oxo group which arises during isocyclic ring E formation from the 13-propionic acid methyl ester side chain of Mg-protoporphyin IX monomethylester: this oxo group is present in all known functional chlorophylls [2].The second carbonyl group is formed several biosynthetic steps later when the 3-acetyl group is formed by enzymic hydration of the 3-vinyl group of chlorophyllide a followed by dehydrogenation [3]. Consistent with this view, the 3-acetyl oxygen of BChl a was derived from water via a hydratase mechanism in both the obligate aerobe, Dedication. This article is dedicated to Prof. Wolfhart Rüdiger on the occasion of his 65th birthday. the facultative photosynthetic bacterium, Rhodobacter sphaeroides, which forms BChl a in both strictly anaerobic conditions in the light and in atmospheres containing up to 6% oxygen in the dark [5].The 13 1 -oxo group of BChl a was also derived from water via a hydratase mechanism in R. sphaeroides [4,6]. In R. denitrificans, however, the 13 1 -oxo group was derived only from molecular oxygen via an oxygenase [4] as also occurs in chlorophyll (Chl) a and Chl b formation in green plants [7Ϫ9] and green algae [10].We have now examined BChl a formation in Rhodovulum sulfidophilum, a purple non-sulphur bacterium and a membe...

Rubrivivax gelatinosus acsF (Previously orf358 ) Codes for a Conserved, Putative Binuclear-Iron-Cluster-Containing Protein Involved in Aerobic Oxidative Cyclization of Mg-Protoporphyrin IX Monomethylester

“…In purple bacteria, H 2 O has been shown to be the source of oxygen for the cyclization reaction in R. sphaeroides cells shifted from respiratory to photosynthetic growth conditions (28). In addition, two different cyclization mechanisms have been shown to coexist in Rhodovulum sulfidophilum where both an oxygenase and a hydratase operate to form the 13(1)-oxo group of the Bchl a isocyclic ring (29).…”

18O Labeling of Chlorophyll d in Acaryochloris marina Reveals That Chlorophyll a and Molecular Oxygen Are Precursors

“…4B). The lag phase (48 h) in the incorporation of 18 O label from H 2 18 O into Chl d can be explained with the proposed model of Chl d biosynthesis in Acaryochloris if the cultural doubling time is ϳ48 h. Oxygen atoms originating from water are incorporated into the C13 3 and C17 3 -carboxyl groups of the tetrapyrrole molecule in the first steps of the long Chl biosynthetic pathway (15)(16)(17). Prior to observing isotopic 18 O-labeled Chl d in these H 2 18 O labeling experiments, all unlabeled intermediates of the Chl biosynthetic pathway are metabolized into (unlabeled) Chl a and then into (unlabeled) Chl d. After 48 h of incubation time, almost all Chl a of Acaryochloris was isotopic 18 O-labeled because of the low Chl a/d ratio (0.03-0.05:1), resulting in a high turnover of Chl a as a precursor of Chl d (Fig.…”

“…All four oxygen atoms of the C13 3 -and C17 3 -carboxyl groups arise from the precursor ␦-aminolevulinic acid and thus originate from water during ␦-aminolevulinic acid synthesis, which was confirmed by Porra et al in 1995 and 1996 (15, 16). The fifth oxygen atom of the C13 1 -oxo group is derived from molecular oxygen in most aerobic photosynthetic organisms and is thus an oxygenase-type reaction (9,11,(15)(16)(17). Whereas this oxygen is derived from water using a hydratase reaction mechanism in anaerobic photosynthetic organisms, some exceptions to this generalization exist (9,16,18 (12,19,20).…”

“…The fifth oxygen atom of the C13 1 -oxo group is derived from molecular oxygen in most aerobic photosynthetic organisms and is thus an oxygenase-type reaction (9,11,(15)(16)(17). Whereas this oxygen is derived from water using a hydratase reaction mechanism in anaerobic photosynthetic organisms, some exceptions to this generalization exist (9,16,18).…”

Protein S-Acyltransferase 14: A Specific Role for Palmitoylation in Leaf Senescence in Arabidopsis

“…To estimate chlorophyll content, the methods described by Porra et al (1998) and Katsiarimpa et al (2013) were followed. Briefly, seedlings (10 mg) or fresh rosette leaves (20 mg) were homogenized in 500 mL of 96% ethanol in preweighed 1.5 mL microfuge tubes.…”