ACAD10 encodes two orphan enzymes in the ether lipid biosynthetic and salvage pathways

Inside the cells of almost every animal, from the tiny roundworm to the human being, there exists a specialized family of fats that act as the body's internal shield. These are known as ether lipids. Unlike the more common fats that make up cell membranes, which are held together by standard chemical links, ether lipids are connected by a stronger, more resilient bond. This unique structure allows them to perform critical jobs: they help cells communicate, protect tissues from the damaging effects of oxidation, and maintain the fluidity of the cell's outer layer. In humans, these molecules make up about one-fifth of all the fats in our cell membranes. When the body fails to produce or recycle them correctly, the consequences can be severe, leading to a range of serious conditions including neurodegenerative diseases, heart problems, and diabetes.

For decades, scientists understood the general map of how these fats are built from scratch inside the body and how they can be repaired after being broken down by digestion. However, the map had two glaring holes. Researchers knew that specific chemical steps were required to turn dietary fats back into usable cellular components, and they could detect the activity of the enzymes that performed these steps in test tubes. Yet, they could not identify the actual proteins responsible for doing the work. These missing pieces were known as "orphan enzymes," biological functions without a known genetic source. Without knowing which genes coded for these enzymes, it was impossible to fully understand how the body manages these vital fats or why certain genetic variations might lead to disease.

A team of researchers has now filled these gaps by identifying the specific protein that performs these missing tasks. The study centers on a gene called ACAD10, which had previously been linked to metabolic disorders but whose exact chemical role remained a mystery. By examining this gene in worms, mice, and humans, the scientists discovered that the protein it produces acts as a dual-purpose tool. One part of the protein functions as a kinase, an enzyme that adds a phosphate group to a specific type of dietary fat, effectively tagging it so the cell can reuse it. The other part acts as a phosphatase, an enzyme that removes a phosphate group from a different fat molecule, a necessary step for creating a potent signaling molecule called platelet-activating factor.

The researchers began their investigation by looking at the genetic blueprint of the roundworm C. elegans, which possesses a single version of the ACAD10 gene. They isolated the protein and tested its ability to interact with various fats. They found that the protein successfully added a phosphate tag to 1-O-alkylglycerol, a fat molecule derived from the diet that the body needs to recycle. This confirmed that ACAD10 is the long-sought enzyme responsible for the first step of the salvage pathway. To understand how this worked, the team determined the three-dimensional structure of the protein's active site, revealing the precise arrangement of atoms that allows it to grab the fat molecule and attach the phosphate tag. They also showed that the protein requires a specific helper molecule, acetyl-CoA, to function, explaining why this enzyme had been difficult to spot in previous studies.

Next, the team turned to the second missing piece: the enzyme that removes a phosphate group to help build platelet-activating factor. Using the same worm protein, they demonstrated that it could strip the phosphate group from a specific fat precursor that contains a short acetyl tail, but not from fats with longer tails. This selectivity matched the behavior of the mysterious orphan enzyme described in earlier decades. By creating worms that lacked the ACAD10 gene, the researchers observed that these animals could no longer incorporate dietary fats into their own cellular stores. The worms also lived shorter lives and produced fewer offspring, highlighting the biological importance of this recycling system.

To see if these findings applied to mammals, the researchers studied mice that had been genetically engineered to lack the ACAD10 gene. When these mice were fed a diet supplemented with a specific type of fat alcohol, their livers failed to increase their levels of ether lipids, whereas the livers of normal mice did. This confirmed that the protein is essential for the salvage pathway in mammals as well. Interestingly, the mice without the gene also showed lower levels of platelet-activating factor, suggesting that the same protein is also responsible for the phosphatase activity needed to build this signaling molecule from scratch. The study further clarified that while a related protein, ACAD11, shares some of these abilities, it cannot fully replace ACAD10, indicating that both proteins play distinct but overlapping roles in fat metabolism.

The research extended beyond the laboratory to real-world human health. The scientists analyzed blood samples from members of the Akimel O'odham tribe, a population with a high prevalence of type 2 diabetes. They focused on individuals carrying specific genetic variations in the ACAD10 gene that had previously been associated with diabetes risk. The analysis revealed that people with these variations had significantly lower levels of platelet-activating factor in their blood compared to those without the variations. This finding provides a direct molecular link between a genetic difference, a disruption in fat metabolism, and a major metabolic disease. It suggests that the inability to properly manage these specific fats may be a contributing factor to the development of diabetes in these individuals.

The study also addressed a competing theory about the function of this protein. Previous research had suggested that ACAD10 and its partner ACAD11 were primarily involved in breaking down a different type of fat called 4-hydroxy acids. However, when the researchers measured the levels of these acids in the blood of mice lacking the ACAD10 gene, they found no difference compared to normal mice. This result suggests that while the protein might have some ability to interact with these molecules in a test tube, its primary and most critical role in living animals is the management of ether lipids. The researchers concluded that the enzyme's main job is to act as a gatekeeper for the body's ether lipid supply, ensuring that dietary fats can be salvaged and that essential signaling molecules can be produced.

By identifying the two orphan enzymes hidden within the ACAD10 protein, this work resolves a long-standing puzzle in biochemistry. It reveals that a single protein can perform two opposing chemical tasks—adding and removing phosphate groups—to regulate the flow of fats in the body. This discovery not only completes the map of how ether lipids are made and recycled but also offers a new perspective on the causes of metabolic diseases. It suggests that variations in this specific genetic pathway could be a key factor in conditions like type 2 diabetes, opening the door for future research into how dietary fats and genetic makeup interact to influence human health.