In a cold room, something remarkable happens inside your brown fat. Glycerol—a molecule released as your body breaks down stored fat for warmth—slips into a tiny pocket on an enzyme called TNAP and flips a switch that sets off a hidden calorie-burning system. This discovery by Lawrence Kazak and his team at McGill University's Rosalind and Morris Goodman Cancer Institute, published in Nature, reveals how the body orchestrates multiple energy-burning pathways to keep itself warm, a secret that scientists have chased for years.
Brown fat has long fascinated researchers because it works differently from the white fat most people think of. While white fat stores energy, brown fat burns it to generate heat—a process known as thermogenesis. For decades, scientists believed a single biological pathway powered this heat production. But in recent years, evidence of a second, alternative pathway emerged, called the futile creatine cycle. The problem was nobody knew what activated it. Kazak's discovery changes that.
Working with structural biologist Alba Guarne, Canada Research Chair in Macromolecular Machines in DNA Damage and Repair at McGill, the researchers pinpointed the mechanism: when cold exposure triggers fat breakdown, the resulting glycerol molecules bind to TNAP in what the team calls the glycerol pocket. That binding activates the alternative heat pathway. "This is the first time we've identified how an alternative heat-producing pathway is activated, independent of the classic system," Kazak said. The finding opens a window into how these multiple energy-burning systems work together to maintain body temperature with precision.
But the most immediate impact may lie far from metabolism research. TNAP already has a well-established role in bone formation—it's essential for calcification, the process that builds and maintains strong bones. Mutations that reduce TNAP activity cause hypophosphatasia, a rare disorder sometimes called "soft bones." The condition leads to fractures, chronic pain, and skeletal abnormalities, with certain inherited mutations making it more prevalent in parts of Canada, including Quebec and Manitoba.
By studying TNAP mutations in laboratory experiments, the team discovered something striking: the same molecular switch that regulates energy-burning in fat cells directly affects cells responsible for bone mineralization. This connection builds on earlier work by McGill's Marc McKee and Jose-Luis Millan of the Sanford Burnham Prebys Medical Discovery Institute, whose previous research contributed to developing a first-in-class enzyme replacement therapy for hypophosphatasia patients with defective TNAP enzymes.
The implications are substantial. "This finding opens the door to a new kind of treatment, where increasing the activity of the TNAP enzyme through its glycerol pocket by natural or synthetic bioactive compounds could potentially boost the beneficial actions of the enzyme in patients, to help restore deficient bone mineralization to healthy levels," McKee explained. Rather than replacing the enzyme entirely, researchers could now work to enhance its activity through the glycerol pocket—a more precise, potentially more effective approach to restoring bone health in patients who desperately need it.