PNNL

The Science

Plants and algae use light to make energy through photosynthesis, but when darkness falls, they must switch to other metabolic processes to survive. Cyanobacteria, tiny microbes also known as blue-green algae, experience this transition every day as they move between light and dark conditions. Researchers from Pacific Northwest National Laboratory (PNNL) found that small chemical changes called redox post-translational modifications (PTMs) on a helper protein named OpcA can alter the shape and motion of its partner enzyme G6PDH under dark conditions. These changes reshape a nearby "gate" region near the enzyme's active site and shift it toward a more active form. This may help the cell produce more NADPH, an energy-rich molecule that powers many important reactions. Understanding how cyanobacteria adjust their metabolism to changing light conditions could help scientists improve bioenergy production and engineer more resilient photosynthetic organisms.

The Impact

Can researchers learn to control metabolic reactions by flipping a single protein "switch" in microbes? This study suggests they can. A research team found that small chemical changes to cysteines—sulfur-containing and key building blocks in proteins—on a single regulatory protein can rewire how cyanobacteria control energy and carbon use. The team combined laboratory methods that spot chemical protein modifications with computer models that show how changes alter protein shape and motion. This links cellular redox signals to specific changes in enzyme activity and therefore connects cell signals to a clear cause. These findings improve scientists' understanding of how photosynthetic microbes adapt to changing environmental conditions and could support efforts to engineer cyanobacteria for better biofuels and other useful bioproducts.

Summary

In cyanobacteria, the redox-sensitive protein OpcA acts as a metabolic switch for G6PDH, an enzyme that helps control the generation of reducing power and the carbon flux through a cell. However, although it is known that redox-sensitive cysteines in OpcA regulate G6PDH, exactly how redox PTMs change G6PDH's structure and activity is not fully understood. Through this study, a team of researchers from PNNL applied a combination of redox proteomics with computational modeling to a cyanobacterium called Synechococcus elongatus PCC 7942 to examine this mechanism. Redox proteomics revealed site-specific cysteine thiol modifications on OpcA during transitions between light and dark conditions and circadian cycling, including modifications near the region where OpcA binds to G6PDH. Atomistic simulations using a computational modeling tool called PTM-Psi showed that thiol PTMs near the interface reshape the OpcA-G6PDH complex and affect a gate-like region in G6PDH that controls how molecules enter and leave the enzyme's active site, where chemical reactions occur. These PTMs consequently altered active-site hydrogen-bond networks and reorganized conformational dynamics and allosteric communication (structural changes in one part of the protein affected the activity in another part). Together, the results suggest that PTMs on OpcA tune G6PDH toward functionally relevant states, providing a possible molecular mechanism and explanation for how cyanobacteria rapidly rewire metabolism under changing light conditions. In the future, this knowledge could help scientists engineer photosynthetic microbes with improved carbon conversion for energy applications, stress tolerance, and bioproduct production capabilities.

The Environmental Molecular Sciences Laboratory (EMSL) contributed computational resources and expertise through the Molecular Sciences Computing Facility, which supported the molecular simulations used to study PTM-driven structural and dynamic effects.

Contacts

Hoshin Kim, Pacific Northwest National Laboratory, hoshin.kim@pnnl.gov

Margaret S. Cheung, Pacific Northwest National Laboratory and Environmental Molecular Sciences Laboratory, margaret.cheung@pnnl.gov

Funding

This work is mainly supported by the BRaVE project award "Enhancing biopreparedness through a model system to understand the molecular mechanisms that lead to pathogenesis and disease transmission" funded by the Department of Energy (DOE) Office of Science's Biological and Environmental Research program (FWP 81832). A portion of this research is supported by the Predictive Phenomics Initiative at Pacific Northwest National Laboratory (PNNL). An award for computer time was provided by the Advanced Scientific Computing Research Leadership Computing Challenge program. This research also used resources of the Oak Ridge Leadership Computing Facility, which is a DOE Office of Science user facility supported under contract DE-AC05-00OR22725. Specifically, part of the computational work in this paper was performed using the Molecular Sciences Computing Facility at the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility located on the campus of PNNL. PNNL is a multi-program national laboratory operated by Battelle for the DOE under Contract No. DE-AC05-76RL01830.