Optimal energy and redox metabolism in the cyanobacterium Synechocystis sp. PCC 6803

Abstract

Understanding energy and redox homeostasis and carbon partitioning is crucial for systems metabolic engineering of cell factories. Carbon metabolism alone cannot achieve maximal accumulation of metabolites in production hosts, since an efficient production of target molecules requires energy and redox balance, in addition to carbon flow. The interplay between cofactor regeneration and heterologous production in photosynthetic microorganisms is not fully explored. To investigate the optimality of energy and redox metabolism, while overproducing alkenes—isobutene, isoprene, ethylene and 1-undecene, in the cyanobacterium Synechocystis sp. PCC 6803, we applied stoichiometric metabolic modelling. Our network-wide analysis indicates that the rate of NAD(P)H regeneration, rather than of ATP, controls ATP/NADPH ratio, and thereby bioproduction. The simulation also implies that energy and redox balance is interconnected with carbon and nitrogen metabolism. Furthermore, we show that an auxiliary pathway, composed of serine, one-carbon and glycine metabolism, supports cellular redox homeostasis and ATP cycling. The study revealed non-intuitive metabolic pathways required to enhance alkene production, which are mainly driven by a few key reactions carrying a high flux. We envision that the presented comparative in-silico metabolic analysis will guide the rational design of Synechocystis as a photobiological production platform of target chemicals.

A quantitative description of light-limited cyanobacterial growth using flux balance analysis

“…The metabolic reconstruction of Synechocystis sp. PCC 6803 is based on previous reconstruction [5, 24], and revised according to current literature and other recent reconstructions [8]. SBO annotation was added using the SBOannotator version 0.9 [56].…”

“…Genome-scale metabolic reconstructions (GSMRs) are available for an increasing number of microbial organisms, including Synechosystis sp. PCC 6803 [3][4][5][6][7][8] and several other cyanobacteria [9][10][11][12][13].…”

Machine learning predicts system-wide metabolic flux control in cyanobacteria

“…The iJN678_AK genome-scale metabolic model of Synechocystis was used for the computational analysis (Kugler & Stensjö, 2023). For the simulation of recombinant Synechocystis producing metabolites of interest, the metabolic reconstruction was expanded with unidirectional pseudo-reactions allowing product export of the respective compounds (Supplementary Tables 1-3).…”

Regulation of organic acid and hydrogen production by NADH/NAD+ ratio in Synechocystis sp. PCC 6803

“…Alterations in the expression of ddh, citH, and hoxH result in changes in NADH utilization, leading to either an increase or decrease in hydrogen production under dark, anaerobic conditions (Chongsuksantikul et al, 2015;Iijima et al, 2016Iijima et al, , 2021. A flexible balance of ATP and NAD(P)H is crucial for metabolite production in Synechocystis 6803, whether in photoautotrophic or mixotrophic conditions (Kugler and Stensjö, 2023). In addition, lowering the adenylate energy charge has proven effective for succinate production under dark, anaerobic conditions in Synechocystis 6803 (Hasunuma et al, 2018).…”

“…In addition, lowering the adenylate energy charge has proven effective for succinate production under dark, anaerobic conditions in Synechocystis 6803 (Hasunuma et al, 2018). It is worth noting that the NADPH pool of Synechocystis 6803 is larger than the NADH pool (Takahashi et al, 2008), making the use of NADPH-dependent enzymes advantageous (Kugler and Stensjö, 2023). However, when transitioning to dark, anaerobic conditions, intracellular NADPH is entirely depleted (Osanai et al, 2015).…”