Biology subjects

Pintscher, S.

Publications and source records attributed to Pintscher, S..

3 recordsLinked to original sources

bioRxiv2026

A 13-subunit c-ring in the Chlamydomonas chloroplast ATP synthase lowers the H⁺/ATP cost of carbon fixation

The chloroplast F1Fo ATP synthase is a rotary motor that converts the light-driven proton-motive force into the chemical energy of ATP. The number of c-subunits in its rotor fixes the number of protons translocated per ATP formed, a fundamental parameter of bioenergetic systems. The reference spinach enzyme possesses fourteen c-subunits and a H+/ATP ratio of 4.67. Green algae additionally operate a carbon-concentrating mechanism that sustains CO2 fixation in water at a substantial cost in ATP, yet the structure of the algal motor, and whether its bioenergetic parameters differ from those of vascular plants, remains unresolved. Here, a 2.2 [A] structure of the ATP synthase of Chlamydomonas reinhardtii reveals that the enzyme carries a thirteen-membered c-ring, the first departure from c14 in a chloroplast, and with a lower predicted H+/ATP ratio of 4.33. Ordered waters trace a Grotthuss proton relay through the membrane, where an insulating triad separates the proton loading and unloading sites and couples flux to rotation. A single substitution in the redox switching {gamma}-subunit abolishes the contact with the catalytic {beta}-subunit that idles the enzyme in darkness in vascular plants. These unique features of the algal ATP synthase lower the H+/ATP cost of carbon fixation in the light and facilitate acetate metabolism in the dark.

biochemistry↗

bioRxiv2026

Filament Formation by ChlI Challenges the Current View of Magnesium Chelatase Architecture

Magnesium chelatase (MgCh) catalyzes the first committed step in chlorophyll biosynthesis by inserting Mg2+ into the tetrapyrrole ring. The enzyme comprises three core subunits: ChlI, ChlD, and ChlH, and requires the auxiliary factor GUN4 for full activity. Despite extensive investigation, the structural organization of the active holoenzyme and the mechanistic coupling between ATP hydrolysis and Mg2+ insertion remain poorly defined. Here, we used cryo-electron microscopy to investigate MgCh architecture. We show that both cyanobacterial and plant ChlI homologs assemble into filamentous helical structures in the presence of Mg2+ and either ATP or ADP. However, only ATP-induced oligomers are susceptible to disassembly by ChlD. Structural analysis of the ATP-driven assemblies reveals compact inter-subunit packing, with ADP and Mg2+ coordinated at the interfacial regions. These findings suggest that ATP hydrolysis promotes subunit compaction and may facilitate partial dehydration of the Mg2+ hydration shell. Low-resolution reconstructions further provide a structural framework for ChlD engagement with ChlI filaments. Finally, we demonstrate that phosphatidylglycerol enhances catalytic activity, supporting a role for membrane lipids in modulating MgCh function. Significance statementMagnesium chelatase catalyzes the first committed step of chlorophyll biosynthesis, yet the structural basis of its activation and coupling to ATP hydrolysis remains unclear. Using cryo-electron microscopy, we show that the ATPase subunit ChlI forms filamentous helical oligomers in the presence of Mg2+ and nucleotide, a property conserved across cyanobacterial and plant homologs but not previously recognized. Only oligomers generated through ATP hydrolysis interact efficiently with the ChlD subunit, indicating that hydrolysis produces a distinct, recognition-competent conformation. Structural analysis further suggests that ATP hydrolysis promotes subunit compaction and partial dehydration of Mg2+. Together, these findings reveal a previously unrecognized oligomeric state of ChlI and provide a structural framework for understanding how ATP hydrolysis regulates magnesium chelatase activity.

biochemistry↗

bioRxiv2025

Structures of the LPOR-Chlide Complexes Imply the Basis of Membrane Remodeling and a Photocatalytic Mechanism

Flowering plants rely on the photocatalytic enzyme light-dependent protochlorophyllide oxidoreductase (LPOR) to synthesize chlorophyllide (Chlide), a chlorophyll intermediate, and at the same time to remodel lipid membranes into the cubic phase required for chloroplast development. Yet neither the mechanism of this light-driven catalysis nor the structural basis of its membrane remodeling activity is well understood, largely due to the lack of high-resolution structural information. To address these questions, we analyzed Chlide:LPOR:NADPH oligomeric assemblies by cryo-electron microscopy. Eleven maps were obtained, enabling the reconstruction of nine distinct LPOR oligomer models. Most assemblies adopt helical or stacked-ring forms, whereas one displays a segmented string-of-dimers architecture, representing a previously unobserved structural organization. Strings of LPOR dimers induce distinct periodic deformations of the lipid bilayer that stabilize multiple complex architectures. These architectures are maintained by three different inter-string interfaces occurring in various combinations. The plasticity of these interactions suggest that the filaments are capable of twisting and sliding relative to each other. Our highest-resolution map (2.55 [A]) provided a detailed view of the pigment-binding site, revealing a water channel that connects the central magnesium ion of the pigment to the bulk solvent. Unexpectedly, the propionate group of the pigment protrudes out of the porphyrin plane and arches back toward NADPH, ideally positioned for hydride transfer. These structural insights allowed us to propose a novel reaction mechanism for LPOR photocatalysis. Together, our data highlight the critical role of Chlide:LPOR:NADPH complexes in delaying prolamellar body disassembly and point to their possible regulatory function in mature leaves.

biochemistry↗