Biosynthesis of delta-aminolevulinic acid in Rhodopseudomonas sphaeroides

Levulinic Acid Biorefineries: New Challenges for Efficient Utilization of Biomass

Levulinic acid is a sustainable platform molecule that can be upgraded to valuable chemicals and fuel additives. This article focuses on the catalytic upgrading of levulinic acid into various chemicals such as levulinate esters, δ-aminolevulinic acid, succinic acid, diphenolic acid, γ-valerolactone, and γ-valerolactone derivatives such valeric esters, 5-nonanone, α-methylene-γ valerolactone, and other various molecular-weight alkanes (C9 and C18-C27 olefins).

Isolation and characterization of an aminolevulinate-requiring Rhodobacter capsulatus mutant

Using transposon Tn5 mutagenesis, we isolated a mutant strain of Rhodobacter capsulatus that requires aminolevulinate for growth. Southern blot analysis indicated that this strain has a single Tn5 insertion. The addition of 0.1 mM aminolevulinate to the medium allowed the mutant to grow either aerobically or photosynthetically with generation times similar to those of the parental strain. When grown photosynthetically, bacteriochlorophyll accumulation increased with increasing aminolevulinate concentration. The mutant strain had only 10% of the normal aminolevulinate synthase activity, but it had a normal level of porphobilinogen synthase activity. The requirement for aminolevulinate could be satisfied by porphobilinogen, hemin, or protoporphyrin. While the mutant grew well on agar plates containing any of these substrates, growth in liquid media containing hemin or protoporphyrin was poor. Introduction of an R' factor containing all the known R. capsulatus bch genes into the mutant strain did not relieve the requirement for aminolevulinate, suggesting that the Tn5 insertion is not within the bch region.

Labelling of chlorophylls and precursors by [2-14C]glycine and 2-[1-14C]oxoglutarate in Rhodopseudomonas spheroides and Zea mays. Resolution of the C5 and Shemin pathways of 5-aminolaevulinate biosynthesis by thin-layer radiochromatography

The C-5 of 5-aminolaevulinate, a tetrapyrrole precursor which accumulates when inhibitory laevulinate is present, is derived from either the C-2 of glycine by the 5-aminolaevulinate-synthase-mediated Shemin pathway or the C-1 of 2-oxoglutarate by the C5 pathway. Thin-layer-radiochromatographic procedures are described for determining whether [2-14C]glycine or 2-[1-14C]oxoglutarate labelled the macrocycle of bacteriochlorophyll a, in addition to or rather than the methyl ester or phytyl ester moieties of the side-chains. The method was also used for detecting whether the same substrates label the formaldehyde (C-5) or the succinate (C-1 to C-4) fragments, obtained by periodate cleavage of 5-aminolaevulinate. These methods therefore can readily distinguish between the Shemin and C5 pathways as was demonstrated by using Rhodopseudomonas spheroides and Zea mays (maize), respectively, as examples of each pathway. Both [2-14C]glycine and, to a lesser extent 2-[1-14C]oxoglutarate labelled the macrocycle of bacteriochlorophyll a formed during adaptation of respiring R. spheroides cells to photosynthetic (anaerobic, illuminated) conditions. This and earlier evidence suggested augmentation of the Shemin pathway by a minor C5 pathway contribution. The present studies revealed only Shemin pathway activity: with laevulinate present, [2-14C]glycine formed 5-[5-14C]aminolaevulinate as proved by H14CHO production during periodate cleavage. These methods were sufficiently sensitive also to detect the incorporation of 14CO2, from degradation of either substrate, into 5-aminolaevulinate via the Shemin pathway thus labelling the succinate fragment produced with periodate: this explains bacteriochlorophyll a labelling by 2-[1-14C]oxoglutarate and proves double labelling of 5-aminolaevulinate by [2-14C]glycine. The same techniques were applied to etiolated maize leaves exposed to aerobic illuminated conditions with laevulinate and either 2-[1-14C]oxoglutarate or [2-14C]glycine as substrates. Only the C5 pathway was detected: 2-[1-14C]oxoglutarate was converted to 5-[5-14C]aminolaevulinate, which yielded H14CHO on periodate cleavage. This is not inconsistent with our earlier 13C-NMR studies [Porra, R.J., Klein, O. and Wright, P. E. (1983) Eur. J. Biochem. 130, 509-516] showing that the C5 pathway formed all the 5-aminolaevulinate for chlorophyll biosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

13C nuclear magnetic resonance studies on bacteriochlorophyll a biosynthesis in Rhodopseudomonas spheroides S

The 13C NMR spectra were analyzed in bacteriochlorophyll a and magnesium protoporphyrin methyl ester formed in Rhodopseudomonas spheroides S. in the presence of L-[1-13C]glutamate and [2-13C]glycine. After reassignment of three alpha-pyrrolic carbons (C-9, -14 and -16) of bacteriochlorophyll a, the spectra showed that C-2 of glycine was preferentially incorporated into the eight-carbon atoms in these tetrapyrrole macrocycles derived from C-5 of 5-aminolevulinic acid (ALA). C-2 of glycine was also incorporated specifically into methyl ester carbon of magnesium protoporphyrin IX methyl ester and methoxyl carbon of methoxycarbonyl group attached to isocyclic ring of bacteriochlorophyll a. No enrichment of these nine-carbon atoms was observed in the spectrum of bacteriochlorophyll formed in the presence of L-[1-13C]glutamate, showing exclusive operation of ALA synthase on bacteriochlorophyll biosynthesis.

The participation of the Shemin and C5 pathways in 5-aminolaevulinate and chlorophyll formation in higher plants and facultative photosynthetic bacteria

Chlorophyll and bacteriochlorophyll formation from 14C-labelled precursors was studied during the illumination of etiolated maize leaves excised from dark-grown seedlings and in cell suspensions of respiring, dark-, aerobically-grown Rhodopseudomonas spheroides adapting to the photosynthetic state in the light under anaerobic conditions. It was found that 1-14C-labelled glutamate and 2-oxoglutarate were incorporated into the tetrapyrrole moieties of chlorophyll and bacteriochlorophyll. This suggests that the C5 pathway of tetrapyrrole biosynthesis operates in both Zea mays and R. spheroides since in the alternative Shemin pathway the label would have been lost as 14CO2 during the formation of succinyl-CoA prior to the condensation with glycine to form 5-aminolaevulinate. It was also found that 5(-14)C-labelled glutamate and 2-oxoglutarate were incorporated into these chlorophylls which is consistent with the operation of both the C5 and Shemin pathways. That the Shemin pathway is also involved was confirmed by the incorporation of [2(-14)C]glycine into both chlorophylls. None of these substrates were incorporated into the phytol moieties of either plant or bacterial chlorophyll or into the carotenoids. However, when [1(-14)C]acetate was added to greening maize leaves not only the tetrapyrrole and phytol moieties were labelled but also the carotenoids: the labelling of these lipids is consistent with their formation from acetate via the isopentenyl pyrophosphate pathway. By comparing the incorporation of [1(-14)C]2-oxoglutarate with that of [5(-14)C]2-oxoglutarate the approximate relative contribution of each pathway to chlorophyll biosynthesis was determined. In maize leaves both pathways contributed almost equally but in R. spheroides the contribution by the Shemin and C5 pathways was 90 and 10%, respectively.

Biosynthetic and bioenergetic functions of citric acid cycle reactions in Rhodopseudomonas capsulata

Rhodopseudomonas capsulata can grow in a number of alternative modes, including (i) photosynthetic, defined here as anaerobic growth with light as the energy source, and (ii) heterotrophic, referring to aerobic heterotrophic growth in darkness. The functions of citric acid cycle sequences in these growth modes were investigated using wild-type and appropriate mutant strains. Results of growth tests and O(2) utilization experiments showed that in the heterotrophic mode, energy conversion is dependent on operation of the classical citric acid cycle. Alpha-ketoglutarate dehydrogenase (KGD) activity in wild-type strain B10 is substantially higher in cells grown heterotrophically than in cells grown photosynthetically. Molecular oxygen, even at low concentration, appears to be important in regulation of KGD synthesis and, thus, in expression of citric acid cycle activity. Extracts of (photosynthetically grown) mutant strain KGD11 lack demonstrable KGD activity, and in contrast to the wild type, KGD11 is unable to grow heterotrophically on succinate, malate, or pyruvate owing to failure of the energy conversion function of the citric acid cycle. KGD11, however, grows well photosynthetically on malate or on CO(2) + H(2). The KGD activity level required to support the bioenergetic function of the citric acid cycle is evidently much higher than that necessary to satisfy biosynthetic demands; thus, a very low rate of succinyl-coenzyme A formation (needed for biosynthesis) in the mutant would suffice for growth under photosynthetic conditions. In wild-type R. capsulata, the alpha-ketoglutarate required for glutamate synthesis is ordinarily generated via citric acid cycle reactions, which include the conversions catalyzed by citrate synthase and isocitrate dehydrogenase. Mutants blocked in the former or both of these enzymes can grow photosynthetically if provided with an exogenous source of alpha-ketoglutarate or glutamate, but grow very poorly (if at all) as heterotrophs since the energy supply under these conditions depends on operation of the complete citric acid cycle.