Physiological Significance of NAD Kinases in Cyanobacteria

Abstract

Unicellular cyanobacteria are thought to be the evolutionary ancestors of plant chloroplasts and are widely used both for chemical production and as model organisms in studies of photosynthesis. Although most research focused on increasing reducing power (that is, NADPH) as target of metabolic engineering, the physiological roles of NAD(P)(H) in cyanobacteria poorly understood. In cyanobacteria such as the model species Synechocystis sp. PCC 6803, most metabolic pathways share a single compartment. This complex metabolism raises the question of how cyanobacteria control the amounts of the redox pairs NADH/NAD + and NADPH/NADP + in the cyanobacterial metabolic pathways. For example, photosynthetic and respiratory electron transport chains share several redox components in the thylakoid lumen, including plastoquinone, cytochrome b 6 f (cyt b 6 f ), and the redox carriers plastocyanin and cytochrome c6 . In the case of photosynthesis, NADP + acts as an important electron mediator on the acceptor-side of photosystem I (PSI) in the linear electron chain as well as in the plant chloroplast. Meanwhile, in respiration, most electrons derived from NADPH and NADH are transferred by NAD(P)H dehydrogenases. Therefore, it is expected that Synechocystis employs unique NAD(P)(H) -pool control mechanisms to regulate the mixed metabolic systems involved in photosynthesis and respiration. This review article summarizes the current state of knowledge of NAD(P)(H) metabolism in Synechocystis . In particular, we focus on the physiological function in Synechocystis of NAD kinase, the enzyme that phosphorylates NAD + to NADP + .

Current knowledge and recent advances in understanding metabolism of the model cyanobacteriumSynechocystissp. PCC 6803

“…A second two-step pathway for NAD + biosynthesis from nicotinamide has also been proposed [107,108], although how nicotinamide is produced has not been determined. NAD + is converted into NADP + , required as an electron acceptor in linear photosynthetic electron transport, by NAD kinases, of which two are present in Synechocystis (NadK1, NadK2) [109]. The NAD + /NADP + ratio is regulated by pyridine nucleotide transhydrogenase (PntA/PntB), which catalyses electron transfer between the two compounds [110].…”

Time‑resolved transcriptome analysis during transitions of sulfur nutritional status provides insight into triacylglycerol (TAG) and astaxanthin accumulation in the green alga Chromochloris zofingiensis

“…The MEP pathway also requires NADPH to synthesis the terpenoid backbone for carotenoids [34]. In phototrophic microalgae, the photosynthesis pathway generates NADPH from the linear electron chain [46]. However, because the thylakoid membrane was severely degraded and the chlorophyll content was dramatically reduced, the photosynthesis pathway was unlikely the main source of NADPH and ATP for the biosynthesis of FAs and astaxanthin under SS.…”

Over-expression of an electron transport protein OmcS provides sufficient NADH for d-lactate production in cyanobacterium

“…As already illustrated, compared with the NADPH directly generated from light energy, the NADH generated from the catabolic metabolism of organic carbon is the minor form of reducing equivalent in cyanobacteria. Generally, the NADH will be consumed by RET for ATP generation, leaving little NADH available for chemical production using NADH-dependent dehydrogenases [ 28 , 29 ]. Therefore, increasing ATP generation from photosynthetic electrons in light reactions of photosynthesis might decrease ATP generation from RET and save NADH that otherwise would be consumed by RET.…”