Scientists at Marburg University have resolved the detailed architecture of one of the largest enzyme complexes ever found in nature, offering a clearer picture of how single-celled microbes generate methane and manage energy in oxygen-free surroundings.
The work, published in Nature, was led by Sophia Paul, a doctoral researcher at the Center for Synthetic Microbiology (SYNMIKRO), under the supervision of Jan Schuller. Using cryo-electron microscopy, the team pieced together a molecular machine known as the heterodisulfide reductase super-assembly, or Hdr–Vhu–Fwd complex.
The numbers are striking. The assembly weighs roughly eight megadaltons and measures about 50 nanometres across, built from 252 protein subunits and more than 600 cofactors, the small helper molecules essential to its work. By comparison, many familiar enzymes — including those cells use to break down sugar — are around 120 kilodaltons, dozens of times smaller.
The complex comes from Methanococcus maripaludis, a member of the methanogenic archaea. These microbes live without oxygen in extreme settings, from hot springs and deep sediments to the salt marshes along Germany's North Sea coast, where they use hydrogen to convert carbon dioxide into methane.
Wiring electrons with precision
The structure explains how the machine channels electrons so efficiently. Two ring-shaped modules are linked by a central core into a continuous, circular chain, allowing several reaction steps to be joined together and electrons to be passed quickly and precisely from one site to the next. In effect, the assembly connects the last and first steps of methane formation into a single loop.
The team also caught the complex adapting on the fly. In about 18 percent of the particles they examined, a formate dehydrogenase had taken the place of the usual hydrogen-processing hydrogenase — a swap that lets the microbes keep producing energy when hydrogen runs short. "We have not only been able to elucidate the structure of this enormous system, but also to see how flexibly microorganisms adapt their energy metabolism to their environment," Schuller said.
Beyond the laboratory, the researchers used cryo-electron tomography to image the enzymes inside intact cells, where the super-assemblies appear at high density and seem central to the cell's energy supply.
Because methanogens are among the largest biological sources of methane, a potent greenhouse gas, understanding their machinery helps scientists gauge their role in the carbon cycle and in climate change. The same microbes are also a promising route to renewable energy, making their inner workings doubly worth decoding.