Convergent evolution enables eicosanoid precursor biosynthesis by ‘first’ fatty acyl desaturases in insects

Imagine the body as a bustling city where tiny chemical messengers, called eicosanoids, zip around delivering urgent orders. These messengers tell cells when to fight an infection, when to grow, or even when to reproduce. To make these messengers, the city needs specific raw materials: long, wiggly chains of fat called polyunsaturated fatty acids (PUFAs). In most animals, including humans, there's a specialized factory line that builds these chains. It uses a set of molecular machines called "front-end" desaturases. These machines are like skilled welders that can snap a double bond into a fat chain at a very specific spot near the end, turning a simple fat into a complex, signaling-ready one.

However, insects have a mystery. For a long time, scientists looked inside insect genomes and couldn't find these specific "front-end" welders. It was as if the insect city had a delivery service but no factory to build the packages. This left a huge gap in our understanding: How do insects make the essential fats they need to survive and communicate? The answer wasn't obvious, because insects seemed to be missing the very tools required for the job. This paper dives into that mystery, asking if insects might have found a clever workaround, perhaps using a different kind of machine that usually does a different job, to build these critical chemical messengers from scratch.

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The Great Insect Heist: How Termites Built a New Factory

In the world of insect biochemistry, a team of researchers led by Robert Hanus and Michal Tupec has uncovered a stunning case of evolutionary improvisation. They discovered that insects, specifically termites and their relatives, didn't just lose the ability to make essential fats; they actually stole a tool from a different toolbox and rewired it to do the job.

The Missing Blueprint
In most animals, the process of making arachidonic acid (a crucial fat for eicosanoids) follows a strict, well-known recipe. It starts with a basic fat, adds a double bond near the end (a "front-end" move), stretches the chain, and adds another double bond. The enzymes that do this "front-end" work are called FADS-like desaturases. But when scientists scanned the DNA of insects, these specific enzymes were nowhere to be found. It was a biological dead end.

The Unlikely Heroes: The "First" Desaturases
The researchers suspected that insects might have recruited a different type of enzyme, one usually known as a "first" desaturase (SCD-like). Normally, these enzymes are the entry-level workers; they take a saturated fat (one with no double bonds) and add the very first double bond. They are like the construction crew that lays the foundation. The paper suggests that in insects, some of these foundation-layers evolved to become master welders, capable of adding double bonds near the end of the chain, just like the missing "front-end" machines.

The Termite Breakthrough
The team focused on termites, specifically a species called Prorhinotermes simplex. They found that termites possess a family of these "first" desaturases, which they call the FAD-B subfamily. Through a mix of computer modeling (using a tool called AlphaFold to predict the 3D shape of the proteins) and real-world experiments, they proved that these enzymes had undergone a remarkable transformation.

• The Shape Shift: The researchers looked at the "tunnel" inside these enzymes where the fat chain sits. Most insect FAD-Bs have a long, open tunnel, like a straight highway. This shape allows them to hold long fat chains (up to 28 carbons long).
• The New Skill: When they tested these enzymes in yeast cells, the termites' FAD-Bs didn't just add a double bond at the start; they added it at position 5 (counting from the end). This is the exact "front-end" move needed to make eicosanoid precursors.
• The Versatility: Even more impressively, two specific versions of these enzymes in termites (FAD-Bb and FAD-Bc) could handle not just simple fats, but also complex, pre-existing polyunsaturated chains. They could turn a fat called eicosadienoic acid (EDA) into sciadonic acid (ScA), a key stepping stone toward arachidonic acid.

The New Pathway
The paper rules out the standard "front-end" route entirely. In the termite tissues, the researchers found no traces of the intermediate fats (like GLA or DGLA) that would be required if the insects were using the old, standard recipe. Instead, they found a different set of clues: high levels of EDA and sciadonic acid.

This led the authors to propose a noncanonical pathway:

1. The termite takes a standard fat (Linoleic acid) and stretches it to make EDA.
2. The FAD-B enzyme (specifically FAD-Bc) steps in and adds a double bond at position 5, creating sciadonic acid.
3. A final, still-mysterious step (likely a "Δ8" desaturase) adds one last double bond to turn sciadonic acid into arachidonic acid.

Why It Matters
This discovery is a masterclass in convergent evolution. It shows that nature doesn't always need the exact same tool to solve a problem. While mammals use a specific "front-end" welder, termites evolved a "first" welder to do the same job. The researchers found that this ability isn't unique to termites; they tested similar enzymes from damselflies, aphids, bees, and moths, and found that many of them also have this "position 5" skill, even if they haven't yet mastered the complex polyunsaturated chains like the termites have.

What's Still a Mystery
While the paper proves that termites use this new FAD-B pathway to make the precursors, one piece of the puzzle remains unsolved. The final step—converting sciadonic acid into arachidonic acid—requires an enzyme that adds a double bond at position 8. The researchers tested several other enzymes in the termite, but none of them could perform this final step in their lab experiments. They suspect the enzyme exists, perhaps hiding in a different family of proteins, but for now, the identity of the "Δ8 desaturase" remains a secret.

The Big Picture
The study also revealed a fascinating social twist. The gene for the most important enzyme (FAD-Bc) is turned on much higher in the reproductive kings and queens of the termite colony than in the sterile workers or soldiers. This suggests that the ability to make these signaling fats is tightly linked to reproduction and longevity, ensuring that the colony's leaders have the chemical tools they need to keep the family growing.

In short, this paper shows that insects didn't just lose a vital metabolic pathway; they reinvented it. By repurposing an old enzyme and reshaping its internal tunnel, termites and their cousins built a new factory line, proving that in the world of evolution, if you don't have the right tool, you can always build a new one from the scraps you have.