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

Abstract

Until recently, all oxygenic photosynthetic organisms had been found to contain chlorophyll (Chl) 3 a as their major photopigment (1). However, a novel cyanobacterium, Acaryochloris marina (Acaryochloris) that contains Chl d (Ͻ 95%) as its major photopigment (2), challenged the Chl a-centralized requirement in oxygenic photosynthesis. Chlorophyll d only differs from Chl a through one substitution at the C3 1 position: the vinyl group of Chl a is replaced by a formyl group in Chl d (Fig. 1). This substitution results in the following unique characteristics of Chl d: 1) its Q y absorption peaks (in vivo) lie between 690 and 740 nm (Fig. 1) where other oxygenic Chls (i.e.Chl a, b, or c) do not absorb (3) and 2) it is the only Chl found so far that can substitute for Chl a in charge separation in the reaction centers of oxygenic photosynthetic organisms (4 -6). However, the biosynthetic mechanism responsible for the formation of the C3 1 -formyl group of Chl d has not been determined.The known Chl biosynthetic pathway contains at least 17 enzymatic steps from the precursor ␦-aminolevulinic acid to Chl a. Eight molecules of ␦-aminolevulinic acid are condensed together to form four monopyrroles that condense to a linear tetrapyrrole, which in turn is cyclized to uropor-phyrinogen III (7). Metal-free protoporphyrin IX is formed after a number of decarboxylation and oxidation reactions. Protochlorophyllide is synthesized from protoporphyrin IX through magnesium insertion, methylation, and oxidative cyclization reactions forming a fifth ring (8). Reductions of ring D as well as of the vinyl group on ring B result in chlorophyllide a, which is converted to Chl a by an esterification of phytol. All of the intermediate products up to the step of chlorophyllide are common in the synthesis of all (bacterio-) chlorophylls (7,8). However, the enzymes carrying out most of the oxidation and reduction reactions are different in anaerobic and aerobic environments because they either are oxygen-sensitive or require different chemistry to catalyze the oxidations in the absence of oxygen (8,9). Genes homologous to the cyanobacterial genes encoding enzymes for each of the reaction steps up to Chl a are present in the genome of Acaryochloris (10).Radioisotopes and stable isotopes were widely applied in the elucidation of the biosynthetic pathway of Chls, the magnesium branch of tetrapyrrole biosynthesis (11-13). The oxygen atoms of Chl and BChl molecules have their origins in either molecular oxygen or water and thus are incorporated by an oxygenase or a hydratase reaction mechanism, respectively (9, 14). 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 oxyg...

Chlorophyll f and chlorophyll d are produced in the cyanobacterium Chlorogloeopsis fritschii when cultured under natural light and near‐infrared radiation

“…Seven CAO and CAO‐like genes of suitable length were identified in the genome of C. fritschii , with many closely related to those of A. marina (Fig. 5 C double bond of the C‐3 vinyl group [41]. Labelling studies have shown that the enzyme responsible for incorporation of the formyl group into chlorophyll d is an oxygenase, that the formyl group oxygen atom comes from molecular oxygen, and that in Acaryochloris , chlorophyll a is a biosynthetic precursor [41].…”

Non‐enzymatic conversion of chlorophyll‐a into chlorophyll‐d in vitro: A model oxidation pathway for chlorophyll‐d biosynthesis

“…Chl-d (or chlorophyllide-d) is thought to be derived from Chl-a (or chlorophyllide-a) on the basis of genome analysis of A. marina [7]: oxidative cleavage of the C3-vinyl group of the latter to the C3-formyl group of the former. The oxygen atom of the C3-formyl group would come from molecular oxygen [8]. An enzyme for such an oxidation in the biosynthesis of Chl-d may be P450-type [9], while Chl-b bearing the C7-formyl group is synthesized from Chl-a with Chl-a oxygenase (CAO) [10].…”

Biofilm Growth and Near-Infrared Radiation-Driven Photosynthesis of the Chlorophyll d -Containing Cyanobacterium Acaryochloris marina

“…In A. marina, Chl a has essentially been displaced by Chl d as the main photosynthetic pigment (10,11,24,34,(48)(49)(50) and its functional role appears more related to Chl d biosynthesis (45) and charge recombination in photosystem II (50). Photoacclimation in the A. marina type strain MBIC11017 appears to be performed through alterations of Chl a/d ratios under high-light (10, 32) and low-light (18,48) conditions, while chromatic photoacclimation in A. marina was reported to involve regulation of its phycobiliprotein content (17).…”

“…Due to its unique possession of Chl d, such A. marina cultures have been the subject of a number of detailed studies, including studies of its genome (32,47), its basic photophysiology (2,8,49,50), Chl d biosynthesis (45), chromatic photoacclimation (18), pigment composition (1,9), and ultrastructure (28,48). A recent study also demonstrated the presence and activity of N 2 fixation under hypoxic conditions in a culture of the A. marina strain HICR111A (36).…”