Power of tiny molecular 'flycatcher' surprises through disorder

(Press-News.org) For decades, scientists assumed that order drives efficiency. Yet in the bustling machinery of mitochondria — the organelles that crank out adenosine triphosphate (ATP), the universal “energy currency” of cells — one of the most enigmatic components is a protein that appears anything but orderly.

ATP powers nearly every biological task, from muscle contraction to neural signaling, by breaking high-energy phosphate bonds and being continually recharged through metabolism. This life-sustaining energy cycle depends on highly coordinated flows of electrons within respiratory supercomplexes. And nestled within these mega-assemblies is QCR6, a tiny ubiquitous protein found in bacteria, yeast and humans whose acidic, floppy tail has remained structurally unresolved for decades. Traditional experimental methods simply couldn’t pin it down; it was too disordered, too mobile, too electrically charged to freeze into a single, clean conformation.

Yet this very disorder, a new study shows, may be the secret to making life run efficiently.

Abhishek Singharoy, associate professor in ASU’s School of Molecular Sciences and associate faculty in the Biodesign Center for Applied Structural Discovery is the senior author on the Nature Communications study published today.

“The biological significance of protein supercomplexes has remained contentious, particularly how they tune the shuttling of charge-carrier redox proteins along with cell membranes during biological energy conversion,” Singharoy said.

Supercomplexes — large assemblies of multiple protein complexes — were once primarily associated with photosynthesis, but are now known to appear across biology. Respiratory chains in bacteria, yeast, plants and humans all build these molecular megastructures. And intriguingly, their individual enzymes work just as well outside of these assemblies as within them.

So why form a supercomplex at all?

The answer, it turns out, doesn’t lie in enzymatic reaction speed. Instead, it lies in something more fundamental: how quickly substrates can find their way to the proteins that process them.

As Jon Nguyen, a former graduate student in the Singharoy lab and now a postdoctoral research associate at Michigan State University’s Plant Research Laboratory, and a shared first author of the study, explained, “For decades, a highly disordered protein, QCR6, in mitochondrial supercomplexes was thought to enhance electron transfer and ATP production. However, experimental methods have, to this day, been unable to resolve the structure of QCR6 because of its acidic and flexible region.

“Now, using computational methods informed by experimental data, we present a model. Our simulations reveal that this highly disordered protein actually lowers the energy barrier for the diffusion of electron carriers during electron transfer, thereby increasing overall energy-conversion efficiency.”

In discussing the main image of this story above, Chun Kit Chan, postdoctoral research associate and shared first author of this study said, “Efficient metabolism is key to an organism’s survival. In this study, we revealed that QCR6 (the red, shining tube in the above image), a tiny, acidic domain of a yeast respiratory complex, can leverage its intrinsic disorder to hover (highlighted by the white wireframe in the above image) over respiratory protein condensates and cooperate with the surrounding acidic membrane environment to provide a folding-unfolding-based, guided diffusion to electron-shuttling enzymes (the colorful, small proteins in the above image), speeding up intra-protein electron transfers — and thus metabolism-linked ATP production — by up to 30%.

“Homologs of QCR6 are also present in the human respiratory system, leading us to wonder if the seemingly chaotic dynamics from the protein's disorder might actually be the means to prompt our survival fitness.”

But this new work pushes the idea even further.

The team combined multi-resolution computational methods, entropy-maximizing molecular dynamics, Brownian diffusion simulations and cryo-EM data to build the first proposed structural ensemble for QCR6’s elusive tail. What emerged was startling.

Rather than being a passive, floppy ornament, the disordered acidic region behaves like a molecular flycatcher.

The simulations revealed that QCR6’s acidic, flexible region forms a shifting corona around the respiratory supercomplex. Positively charged electron carriers, like cytochrome c, are electrostatically attracted to this zone. The mobile tail reaches, hooks and shepherds the electron carriers toward the reaction centers. This reduces the energy barrier for carriers to arrive at the correct location — making electron transfer faster and more reliable.

In other words, QCR6 doesn’t improve the chemistry of electron transfer; it improves the recognition problem — getting the reactants to the right place at the right time.

This is a fundamentally different mode of efficiency — not catalytic, but logistical.

Across simulations and experimental constraints, the team found that the presence of QCR6’s disordered hook can accelerate substrate delivery and boost metabolism-linked ATP production by up to 30%. Intriguingly, this increase matches unexplained experimental observations reported years earlier but never mechanistically resolved.

Working with collaborator and School of Molecular Sciences Professor Kevin Redding, the team also mapped how QCR6-like proteins vary across evolution. Primitive organisms such as heliobacteria lack these highly acidic, mobile hooks. Instead, their cytochrome electron carriers are literally tethered to the membrane — like balloons tied to a string — to ensure they don’t drift away.

"The heliobacterial cytochrome c is linked directly to a membrane lipid, with a 'leash' between that attachment site and the cytochrome domain,” Redding explained. “Unlike mitochondria, these bacteria don't have an outer membrane to keep everything inside; they link it physically, so the cytochrome cannot wander off.

“When we compared these leashes between species, we saw poor conservation of the exact sequence — what is maintained is the length of the leash and its composition, which is dominated by amino acids that confer flexibility. So, like QCR6, a flexible polypeptide chain is used to keep the cytochrome near its partners to facilitate rapid electron transfer, just in a different way that matches the needs of these cells better."

This evolutionary comparison reinforces the central claim: The QCR6 region is not accidental disorder but functional disorder — an elegant evolutionary innovation for optimizing electron-transfer traffic.

From cryo-EM haze to high-resolution insight

Because QCR6’s tail refuses to sit still, cryo-EM has long captured it only as a blurry smear. The team’s strategy flipped the traditional workflow: Instead of fitting experimental haze into a structure, they generated a high-resolution structural ensemble computationally and then “blurred” it to see if it reproduced the cryo-EM noise signature.

It did.

That convergence offered the first plausible structural interpretation of QCR6 — a model consistent with both its disorder and the known electrostatic environment of the respiratory supercomplex.

This work required enormous computational power, drawing on DoD supercomputers, DOE’s Summit and Frontier, and years of simulation time. It also relied on key collaborations — cryo-EM expertise from Eugenia Mileykovskaya and Matthew Baker (UT Health, Houston, Texas), evolutionary analysis with Redding, and the efforts of first authors Chun Kit Chan and Jonathan Nguyen.

The central idea emerging from this work is striking: A protein can gain functional power precisely by not having a fixed structure.

QCR6’s mobility, electrostatics and ability to fold and unfold near the membrane create a guided-diffusion mechanism that brings order to electron-transfer chaos. And the consequences ripple upward: from molecular recognition to reaction efficiency to overall cellular fitness. Indeed, the team’s models predict — and experiments confirm — that cells can grow 30% faster when this mechanism is active.

The discovery reframes how we think about disorder in biology. Sometimes, chaos is not a flaw.
Sometimes, it is a design principle.

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