Promotion of oxidative phosphorylation by complex I-anchored carbonic anhydrases?

The mitochondrial NADH-dehydrogenase complex of the respiratory chain, known as complex I, includes a carbonic anhydrase (CA) module attached to its membrane arm on the matrix side in protozoans, algae, and plants. Its physiological role is so far unclear. Recent electron cryo-microscopy (cryo-EM) structures show that the CA module may directly provide protons for translocation across the inner mitochondrial membrane at complex I. CAs can have a central role in adjusting the proton concentration in the mitochondrial matrix. We suggest that CA anchoring in complex I represents the original configuration to secure oxidative phosphorylation (OXPHOS) in the context of early endosymbiosis. After development of 'modern mitochondria' with pronounced cristae structures, this anchoring became dispensable, but has been retained in protozoans, algae, and plants.

Cryo-EM structure of the respiratory I + III2 supercomplex from Arabidopsis thaliana at 2 Å resolution.

Protein complexes of the mitochondrial respiratory chain assemble into respiratory supercomplexes. Here we present the high-resolution electron cryo-microscopy structure of the Arabidopsis respiratory supercomplex consisting of complex I and a complex III dimer, with a total of 68 protein subunits and numerous bound cofactors. A complex I-ferredoxin, subunit B14.7 and P9, a newly defined subunit of plant complex I, mediate supercomplex formation. The component complexes stabilize one another, enabling new detailed insights into their structure. We describe (1) an interrupted aqueous passage for proton translocation in the membrane arm of complex I; (2) a new coenzyme A within the carbonic anhydrase module of plant complex I defining a second catalytic centre; and (3) the water structure at the proton exit pathway of complex III2 with a co-purified ubiquinone in the QO site. We propose that the main role of the plant supercomplex is to stabilize its components in the membrane.

A ferredoxin bridge connects the two arms of plant mitochondrial complex I.

Mitochondrial complex I is the main site for electron transfer to the respiratory chain and generates much of the proton gradient across the inner mitochondrial membrane. Complex I is composed of two arms, which form a conserved L-shape. We report the structures of the intact, 47-subunit mitochondrial complex I from Arabidopsis thaliana and the 51-subunit complex I from the green alga Polytomella sp., both at around 2.9 Šresolution. In both complexes, a heterotrimeric γ-carbonic anhydrase domain is attached to the membrane arm on the matrix side. Two states are resolved in A. thaliana complex I, with different angles between the two arms and different conformations of the ND1 (NADH dehydrogenase subunit 1) loop near the quinol binding site. The angle appears to depend on a bridge domain, which links the peripheral arm to the membrane arm and includes an unusual ferredoxin. We propose that the bridge domain participates in regulating the activity of plant complex I.

Rotary substates of mitochondrial ATP synthase reveal the basis of flexible F1-Fo coupling.

F1Fo-adenosine triphosphate (ATP) synthases make the energy of the proton-motive force available for energy-consuming processes in the cell. We determined the single-particle cryo-electron microscopy structure of active dimeric ATP synthase from mitochondria of Polytomella sp. at a resolution of 2.7 to 2.8 angstroms. Separation of 13 well-defined rotary substates by three-dimensional classification provides a detailed picture of the molecular motions that accompany c-ring rotation and result in ATP synthesis. Crucially, the F1 head rotates along with the central stalk and c-ring rotor for the first ~30° of each 120° primary rotary step to facilitate flexible coupling of the stoichiometrically mismatched F1 and Fo subcomplexes. Flexibility is mediated primarily by the interdomain hinge of the conserved OSCP subunit. A conserved metal ion in the proton access channel may synchronize c-ring protonation with rotation.

Noncovalent Functionalization of Carbon Substrates with Hydrogels Improves Structural Analysis of Vitrified Proteins by Electron Cryo-Microscopy.

In electron cryo-microscopy, structure determination of protein molecules is frequently hampered by adsorption of the particles to the support film material, typically amorphous carbon. Here, we report that pyrene derivatives with one or two polyglycerol (PG) side chains bind to the amorphous carbon films, forming a biorepulsive hydrogel layer so that the number of protein particles in the vitreous ice drastically increases. This approach could be extended by adding a hydrogel-functionalized carbon nanotube network (HyCaNet, the hydrogel again being formed from the PG-pyrene derivatives), which stabilized the protein-containing thin ice films during imaging with the electron beam. The stabilization resulted in reduced particle motion by up to 70%. These substrates were instrumental for determining the structure of a large membrane protein complex.

Structural basis of proton translocation and force generation in mitochondrial ATP synthase.

ATP synthases produce ATP by rotary catalysis, powered by the electrochemical proton gradient across the membrane. Understanding this fundamental process requires an atomic model of the proton pathway. We determined the structure of an intact mitochondrial ATP synthase dimer by electron cryo-microscopy at near-atomic resolution. Charged and polar residues of the a-subunit stator define two aqueous channels, each spanning one half of the membrane. Passing through a conserved membrane-intrinsic helix hairpin, the lumenal channel protonates an acidic glutamate in the c-ring rotor. Upon ring rotation, the protonated glutamate encounters the matrix channel and deprotonates. An arginine between the two channels prevents proton leakage. The steep potential gradient over the sub-nm inter-channel distance exerts a force on the deprotonated glutamate, resulting in net directional rotation.

Self-Perforated Hydrogel Nanomembranes Facilitate Structural Analysis of Proteins by Electron Cryo-Microscopy.

We developed a method to improve specimen preparation for electron cryo-microscopy of membrane proteins. The method features a perforated hydrogel nanomembrane that stabilizes the thin film of aqueous buffer spanning the holes of holey carbon films, while at the same time preventing the depletion of protein molecules from these holes. The membrane is obtained by cross-linking of thiolated polyglycerol dendrimer films on gold, which self-perforate upon transfer to holey carbon substrates, forming a sub-micron-sized hydrogel network. The perforated nanomembrane improves the distribution of the protein molecules in the ice considerably. This facilitates data acquisition as demonstrated with two eukaryotic membrane protein complexes.

Horizontal membrane-intrinsic α-helices in the stator a-subunit of an F-type ATP synthase.

ATP, the universal energy currency of cells, is produced by F-type ATP synthases, which are ancient, membrane-bound nanomachines. F-type ATP synthases use the energy of a transmembrane electrochemical gradient to generate ATP by rotary catalysis. Protons moving across the membrane drive a rotor ring composed of 8-15 c-subunits. A central stalk transmits the rotation of the c-ring to the catalytic F1 head, where a series of conformational changes results in ATP synthesis. A key unresolved question in this fundamental process is how protons pass through the membrane to drive ATP production. Mitochondrial ATP synthases form V-shaped homodimers in cristae membranes. Here we report the structure of a native and active mitochondrial ATP synthase dimer, determined by single-particle electron cryomicroscopy at 6.2 Å resolution. Our structure shows four long, horizontal membrane-intrinsic α-helices in the a-subunit, arranged in two hairpins at an angle of approximately 70° relative to the c-ring helices. It has been proposed that a strictly conserved membrane-embedded arginine in the a-subunit couples proton translocation to c-ring rotation. A fit of the conserved carboxy-terminal a-subunit sequence places the conserved arginine next to a proton-binding c-subunit glutamate. The map shows a slanting solvent-accessible channel that extends from the mitochondrial matrix to the conserved arginine. Another hydrophilic cavity on the lumenal membrane surface defines a direct route for the protons to an essential histidine-glutamate pair. Our results provide unique new insights into the structure and function of rotary ATP synthases and explain how ATP production is coupled to proton translocation.