The tricarboxylic acid cycle produces NADH for oxidative phosphorylation and fumarase [EC 4.2.1.2] is a critical enzyme in this cycle, catalysing the reversible conversion of fumarate and l- malate. Fumarase is applied to industrial l -malate production as a biocatalyst. l -malate is used in a wide range of industries such as food and beverage, pharmacy chemistry. Although the biochemical properties of fumarases have been studied in many organisms, they have not been investigated in cyanobacteria. In this study, the optimum pH and temperature of Synechocystis 6803 fumarase C ( Sy FumC) were 7.5 and 30 °C, respectively. The K m of Sy FumC for l- malate was higher than for fumarate. Furthermore, Sy FumC activity was strongly inhibited by citrate and succinate, consistent with fumarases in other organisms. Substitution of alanine by glutamate at position 314 of Sy FumC changed the k cat for fumarate and l- malate. In addition, the inhibitory effects of citrate and succinate on Sy FumC activity were alleviated. Phylogenetic analysis revealed cyanobacterial fumarase clades divided in non-nitrogen-fixing cyanobacteria and nitrogen-fixing cyanobacteria. Sy FumC was thus biochemically characterised, including identification of an amino acid residue important for substrate affinity and enzymatic activity.

Direct conversion of carbon dioxide to valuable compounds is a desirable way to reduce the environmental burden and switch from fossil to renewable fuels. Cyanobacteria are photosynthetic bacteria that perform oxygenic photosynthesis and are able to produce valuable compounds from carbon dioxide in the air. Synechocystis and Synechococcus species, model unicellular cyanobacteria, can produce succinate and lactate, which are commodity chemicals used to generate bioplastics. Several cyanobacteria are also able to produce polyhydroxybutyrate, a biodegradable polyester that accumulates under nitrogen or phosphorus starvation. Genetic manipulation succeeded in increasing the productivity of succinate, lactate, and polyhydroxybutyrate from cyanobacteria. We summarize the recent findings in this review.

A unicellular cyanobacterium Synechocystis sp. PCC 6803 possesses a unique tricarboxylic acid (TCA) cycle, wherein the intracellular citrate levels are approximately 1.5–10 times higher than the levels of other TCA cycle metabolite. Aconitase catalyses the reversible isomerisation of citrate and isocitrate. Herein, we biochemically analysed Synechocystis sp. PCC 6803 aconitase (SyAcnB), using citrate and isocitrate as the substrates. We observed that the activity of SyAcnB for citrate was highest at pH 7.7 and 45 °C and for isocitrate at pH 8.0 and 53 °C. The Km value of SyAcnB for citrate was higher than that for isocitrate under the same conditions. The Km value of SyAcnB for isocitrate was 3.6-fold higher than the reported Km values of isocitrate dehydrogenase for isocitrate. Therefore, we suggest that citrate accumulation depends on the enzyme kinetics of SyAcnB, and 2-oxoglutarate production depends on the chemical equilibrium in this cyanobacterium.

Oxygenic photoautotrophic bacteria, namely, cyanobacteria, have the tricarboxylic acid (TCA) cycle. Recently, metabolite production using the cyanobacterial TCA cycle has been well studied.

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