Stereochemical conversion of C3-vinyl group to 1-hydroxyethyl group in bacteriochlorophyll c by the hydratases BchF and BchV: adaptation of green sulfur bacteria to limited-light environments

Supramolecular chirality in self-assembly of zinc protobacteriochlorophyll-d analogs possessing enantiomeric esterifying groups

Zinc 3-hydroxymethyl-pyroprotopheophorbides-a esterified with a chiral secondary alcohol at the 17-propionate residue were prepared as bacteriochlorophyll-d analogs. The synthetic zinc 31-hydroxy-131-oxo-porphyrins self-aggregated in an aqueous Triton X-100 micellar solution to give red-shifted and broadened Soret and Qy absorption bands in comparison with their monomeric bands. The intense, exciton-coupled circular dichroism spectra of their self-aggregates were dependent on the chirality of the esterifying groups. The observation indicated that the self-aggregates based on the J-type stacking of the porphyrin cores were sensitive to the peripheral 17-propionate residues. The supramolecular structures of the present J-aggregates as models of bacteriochlorophyll aggregates in natural chlorosomes were remotely regulated by the esterifying groups.

In vitro reversible dehydration in C3-substituents of zinc chlorophyll analogs by BchF and BchV enzymes: Stereoselectivity and substrate specificity in the dehydration

In the biosynthetic pathway of bacteriochlorophyll(BChl)-a/b/c/d/e molecules, BchF and BchV enzymes catalyze the hydration of a C3-vinyl to C3-1-hydroxyethyl group. In this study, the in vitro reactions catalyzed by BchF and BchV partially afforded a C3¹-epimeric mixture of the hydrated products (secondary alcohols), with the primary recovery of the C3-vinylated substrate. The stereoselectivity and substrate specificity for the in vitro reverse enzymatic dehydration were examined using zinc chlorophyll analogs as model substrates by BchF and BchV, which were obtained from extracts of Escherichia coli overexpressing the respective genes from Chlorobaculum tepidum and used without further purification. Both BchF and BchV preferred dehydration of the (3¹R)-epimers over the (3¹S)-epimers. The (3¹R)-epimer was directly dehydrated by BchF and BchV to give the C3-vinylated product. By contrast, two reaction pathways for BchF and BchV dehydrations of the (3¹S)-epimer were proposed: (1) the (3¹S)-epimer would be directly dehydrated to C3-vinyl group. (2) the (3¹S)-epimer would be epimerized to the (3¹R)-epimer, and the resulting epimer was dehydrated. The results indicated that both BchF and BchV did function as a hydratase/dehydratase and could play a role in the C3¹-epimerization. An increase in the alkyl size at the C8-position gradually suppressed the BchF and BchV-catalyzed dehydration in vitro, while the C12¹- and C20-methylation only slightly affected the reaction. Using the BchF dehydration, a large amount of 3-vinyl-bacteriochlorophyllide-a was successfully prepared, with the retention of the chemically labile, central magnesium atom.

Biosynthesis of the modified tetrapyrroles-the pigments of life

Modified tetrapyrroles are large macrocyclic compounds, consisting of diverse conjugation and metal chelation systems and imparting an array of colors to the biological structures that contain them. Tetrapyrroles represent some of the most complex small molecules synthesized by cells and are involved in many essential processes that are fundamental to life on Earth, including photosynthesis, respiration, and catalysis. These molecules are all derived from a common template through a series of enzyme-mediated transformations that alter the oxidation state of the macrocycle and also modify its size, its side-chain composition, and the nature of the centrally chelated metal ion. The different modified tetrapyrroles include chlorophylls, hemes, siroheme, corrins (including vitamin B12), coenzyme F430, heme d1, and bilins. After nearly a century of study, almost all of the more than 90 different enzymes that synthesize this family of compounds are now known, and expression of reconstructed operons in heterologous hosts has confirmed that most pathways are complete. Aside from the highly diverse nature of the chemical reactions catalyzed, an interesting aspect of comparative biochemistry is to see how different enzymes and even entire pathways have evolved to perform alternative chemical reactions to produce the same end products in the presence and absence of oxygen. Although there is still much to learn, our current understanding of tetrapyrrole biogenesis represents a remarkable biochemical milestone that is summarized in this review.

BciC-Catalyzed C132 -Demethoxycarbonylation of Metal Pheophorbide a Alkyl Esters

Bacteriochlorophyll c molecules self-aggregate to form large oligomers in the core part of chlorosomes, which are the main light-harvesting antenna systems of green photosynthetic bacteria. In the biosynthetic pathway of bacteriochlorophyll c, a BciC enzyme catalyzes the removal of the C13² -methoxycarbonyl group of chlorophyllide a, which possesses a free propionate residue at the C17-position and a magnesium ion as the central metal. The in vitro C13² -demethoxycarbonylations of chlorophyll a derivatives with various alkyl propionate residues and central metals were examined by using the BciC enzyme derived from one green sulfur bacteria species, Chlorobaculum tepidum. The BciC enzymatic reactions of zinc pheophorbide a alkyl esters were gradually suppressed with an increase of the alkyl chain length in the C17-propionate residue (from methyl to pentyl esters) and finally the hexyl ester became inactive for the BciC reaction. Although not only the zinc but also nickel and copper complexes were demethoxycarbonylated by the BciC enzyme, the reactions were largely dependent on the coordination ability of the central metals: Zn>Ni>Cu. The above substrate specificity indicates that the BciC enzyme would not bind directly to the carboxy group of chlorophyllide a, but would bind to its central magnesium to form the stereospecific complex of BciC with chlorophyllide a, giving pyrochlorophyllide a, which lacks the (13² R)-methoxycarbonyl group.

In vitro demethoxycarbonylation of various chlorophyll analogs by a BciC enzyme

Unique light-harvesting antennas in the green sulfur bacterium Chlorobaculum tepidum, called chlorosomes, consist of self-aggregates of bacteriochlorophyll (BChl) c. In the biosynthesis of BChl c, BciC demethoxycarbonylase removes the C13²-methoxycarbonyl group to facilitate the self-aggregation of BChl c. We previously reported the in vitro BciC-enzymatic reactions and discussed the function of this enzyme in the biosynthesis of BChl c. This study aims to examine the substrate specificity of BciC in detail using several semi-synthetic (bacterio)chlorophyll derivatives. The results indicate that the substrate specificity of BciC is measurably affected by structural changes on the A/B rings including the bacteriochlorin π-systems. Moreover, BciC showed its activity on a Zn-chelated chlorophyll derivative. On the contrary, BciC recognized structural modifications on the D/E rings, including porphyrin pigments, which resulted in the significant decrease in the enzymatic activity. The utilization of BciC provides mild conditions that may be useful for the in vitro preparation of various chemically (un)stable chlorophyllous pigments.

Semi-synthesis and HPLC analysis of (bacterio)chlorophyllides possessing a propionic acid residue at the C17-position

Various chlorophyll and bacteriochlorophyll derivatives possessing a magnesium or zinc atom at the central position and a free carboxylic acid group at the C17[Formula: see text]-position, also known as (bacterio)chlorophyllides, were synthesized through a combination of organic synthesis techniques and enzymatic steps. The semi-synthetic (bacterio)chlorophyllides were purified and analyzed using reversed-phase high-performance liquid chromatography with UV-vis spectroscopy and mass spectrometry. These free propionic acid-containing chlorophyllous pigments can be useful research materials for the study of (bacterio)chlorophyll metabolisms.

Engineered biosynthesis of bacteriochlorophyll gF in Rhodobacter sphaeroides

Engineering photosynthetic bacteria to utilize a heterologous reaction center that contains a different (bacterio) chlorophyll could improve solar energy conversion efficiency by allowing cells to absorb a broader range of the solar spectrum. One promising candidate is the homodimeric type I reaction center from Heliobacterium modesticaldum. It is the simplest known reaction center and uses bacteriochlorophyll (BChl) g, which absorbs in the near-infrared region of the spectrum. Like the more common BChls a and b, BChl g is a true bacteriochlorin. It carries characteristic C3-vinyl and C8-ethylidene groups, the latter shared with BChl b. The purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides was chosen as the platform into which the engineered production of BChl g_F, where F is farnesyl, was attempted. Using a strain of Rba. sphaeroides that produces BChl b_P, where P is phytyl, rather than the native BChl a_P, we deleted bchF, a gene that encodes an enzyme responsible for the hydration of the C3-vinyl group of a precursor of BChls. This led to the production of BChl g_P. Next, the crtE gene was deleted, thereby producing BChl g carrying a THF (tetrahydrofarnesol) moiety. Additionally, the bchG^Rs gene from Rba. sphaeroides was replaced with bchG^Hm from Hba. modesticaldum. To prevent reduction of the tail, bchP was deleted, which yielded BChl g_F. The construction of a strain producing BChl g_F validates the biosynthetic pathway established for its synthesis and satisfies a precondition for assembling the simplest reaction center in a heterologous organism, namely the biosynthesis of its native pigment, BChl g_F.

Supramolecular Organogelation of Bacteriochlorophyll-c Possessing an Isobutyl Substituent at the 8-Position in Carbon Tetrachloride

The supramolecular organogelation of bacteriochlorophyll(BChl)-c carrying an isobutyl substituent at the 8-position was observed in carbon tetrachloride at a concentration of about 10 mm at room temperature. The BChl-c gel was evaluated by several spectroscopic measurements: the electronic absorption spectrum exhibited a far-red shift of the Qy-absorption from 660 to 748 nm and the FTIR spectrum showed a shorter frequency shift of the 13-C=O stretching from 1683 to 1643 cm^-1 compared to the shifts of the corresponding monomer solution in tetrahydrofuran. These observations strongly indicate that the gelating BChl-c molecules form self-aggregates that are reminiscent of light-harvesting chlorosomes of green photosynthetic bacteria. The present supramolecular organogel prepared from natural chlorophylls is promising for the creation of an intelligent soft material involving artificial photosynthesis.

BciD Is a Radical S-Adenosyl-l-methionine (SAM) Enzyme That Completes Bacteriochlorophyllide e Biosynthesis by Oxidizing a Methyl Group into a Formyl Group at C-7

Green bacteria are chlorophotorophs that synthesize bacteriochlorophyll (BChl) c, d, or e, which assemble into supramolecular, nanotubular structures in large light-harvesting structures called chlorosomes. The biosynthetic pathways of these chlorophylls are known except for one reaction. Null mutants of bciD, which encodes a putative radical S-adenosyl-l-methionine (SAM) protein, are unable to synthesize BChl e but accumulate BChl c; however, it is unknown whether BciD is sufficient to convert BChl c (or its precursor, bacteriochlorophyllide (BChlide) c) into BChl e (or BChlide e). To determine the function of BciD, we expressed the bciD gene of Chlorobaculum limnaeum strain DSMZ 1677^T in Escherichia coli and purified the enzyme under anoxic conditions. Electron paramagnetic resonance spectroscopy of BciD indicated that it contains a single [4Fe-4S] cluster. In assays containing SAM, BChlide c or d, and sodium dithionite, BciD catalyzed the conversion of SAM into 5'-deoxyadenosine and BChlide c or d into BChlide e or f, respectively. Our analyses also identified intermediates that are proposed to be 7¹-OH-BChlide c and d Thus, BciD is a radical SAM enzyme that converts the methyl group of BChlide c or d into the formyl group of BChlide e or f This probably occurs by a mechanism involving consecutive hydroxylation reactions of the C-7 methyl group to form a geminal diol intermediate, which spontaneously dehydrates to produce the final products, BChlide e or BChlide f The demonstration that BciD is sufficient to catalyze the conversion of BChlide c into BChlide e completes the biosynthetic pathways for all "Chlorobium chlorophylls."

In vitro stereospecific hydration activities of the 3-vinyl group of chlorophyll derivatives by BchF and BchV enzymes involved in bacteriochlorophyll c biosynthesis of green sulfur bacteria

The photosynthetic green sulfur bacterium Chlorobaculum (Cba.) tepidum produces bacteriochlorophyll (BChl) c pigments bearing a chiral 1-hydroxyethyl group at the 3-position, which self-aggregate to construct main light-harvesting antenna complexes, chlorosomes. The secondary alcoholic hydroxy group is requisite for chlorosomal aggregation and biosynthesized by hydrating the 3-vinyl group of their precursors. Using recombinant proteins of Cba. tepidum BchF and BchV, we examined in vitro enzymatic hydration of some 3-vinyl-chlorophyll derivatives. Both the enzymes catalyzed stereoselective hydration of zinc 3-vinyl-8-ethyl-12-methyl-bacteriopheophorbide c or d to the zinc 3¹ R-bacteriopheophorbide c or d homolog, respectively, with a slight amount of the 3¹ S-epimric species. A similar R-stereoselectivity was observed in the BchF-hydration of zinc 3-vinyl-8-ethyl- and propyl-12-ethyl-bacteriopheophorbides c, while their BchV-hydration gave a relatively larger amount of the 3¹ S-epimers. The in vitro stereoselective hydration confirmed the in vivo production of the S-epimeric species by BchV. The enzymatic hydration for the above 8-propylated substrate proceeded more slowly than that for the 8-ethylated, and the 8-isobutylated substrate was no longer hydrated. Based on these results, biosynthetic pathways of BChl c homologs and epimers are proposed.

In Vitro Assays of BciC Showing C132-Demethoxycarbonylase Activity Requisite for Biosynthesis of Chlorosomal Chlorophyll Pigments

A BciC enzyme is related to the removal of the C13(2)-methoxycarbonyl group in biosynthesis of bacteriochlorophylls (BChls) c, d and e functioning in green sulfur bacteria, filamentous anoxygenic phototrophs and phototrophic acidobacteria. These photosynthetic bacteria have the largest and the most efficient light-harvesting antenna systems, called chlorosomes, containing unique self-aggregates of BChl c, d or e pigments, that lack the C13(2)-methoxycarbonyl group which disturbs chlorosomal self-aggregation. In this study, we characterized the BciC derived from the green sulfur bacterium Chlorobaculum tepidum, and examined the in vitro enzymatic activities of its recombinant protein. The BciC-catalyzing reactions of various substrates showed that the enzyme recognized chlorophyllide (Chlide) a and 3,8-divinyl(DV)-Chlide a as chlorin substrates to give 3-vinyl-bacteriochlorophyllide (3V-BChlide) d and DV-BChlide d, respectively. Since the BciC afforded a higher activity with Chlide a than that with DV-Chlide a and no activity with (DV-)protoChlides a (porphyrin substrates) and 3V-BChlide a (a bacteriochlorin substrate), this enzyme was effective for diverting the chlorosomal pigment biosynthetic pathway at the stage of Chlide a away from syntheses of other pigments such as BChl a and Chl a The addition of methanol to the reaction mixture did not prevent the BciC activity, and we identified this enzyme as Chlide a demethoxycarbonylase, not methylesterase.

Origin of Bacteriochlorophyll a and the Early Diversification of Photosynthesis

Photosynthesis originated in the domain Bacteria billions of years ago; however, the identity of the last common ancestor to all phototrophic bacteria remains undetermined and speculative. Here I present the evolution of BchF or 3-vinyl-bacteriochlorophyll hydratase, an enzyme exclusively found in bacteria capable of synthetizing bacteriochlorophyll a. I show that BchF exists in two forms originating from an early divergence, one found in the phylum Chlorobi, including its paralogue BchV, and a second form that was ancestral to the enzyme found in the remaining anoxygenic phototrophic bacteria. The phylogeny of BchF is consistent with bacteriochlorophyll a evolving in an ancestral phototrophic bacterium that lived before the radiation event that gave rise to the phylum Chloroflexi, Chlorobi, Acidobacteria, Proteobacteria, and Gemmatimonadetes, but only after the divergence of Type I and Type II reaction centers. Consequently, it is suggested that the lack of phototrophy in many groups of extant bacteria is a derived trait.