There are no human enzymes that are specifically designed to break down fat cells; instead, the process is a complicated one and involves many different enzymes. Humans store fat for energy and break it down only when they are at an energy deficit. In other words, the enzymes responsible for breaking down fat cells and their components come into play during periods of hunger or fasting.
While the well-known hormone insulin helps lower blood sugar and encourages the cells to take up nutritional molecules after a meal, a competing hormone, called glucagon, has the opposite effect. In her book, "Human Physiology," Lauralee Sherwood explains that during periods of hunger, the pancreas releases glucagon into the bloodstream to signal the body to use fat as an energy source. An enzyme called hormone-sensitive lipase responds to the glucagon by causing fat cells to release fatty acids, or fats, into the bloodstream. This gives most body cells access to fat, which is a valuable source of energy, despite the fact that only specialized cells, called adipose cells, store the compound. As fat cells release fatty acids, they shrink in size and become depleted.
Fatty Acyl-CoA Synthetase
From the bloodstream, fatty acid molecules can move freely across cell membranes into body cells. Once inside the cells, they must be metabolized, or chemically burned, for energy. In the same way that burning a log releases heat energy, burning a molecule releases energy that a cell can use to fuel its functions. Similar to the way in which it takes a match to start a fire, there is a chemical "starter" step, called an activation step, that initiates the process of burning a fatty acid. Drs. Reginald Garrett and Charles Grisham, in their book "Biochemistry", explain that the enzyme responsible for activating fatty acid metabolism is called fatty acyl-CoA synthetase.
Drs. Mary Campbell and Shawn Farrell, in their textbook "Biochemistry," explain that a further enzyme transports activated fatty acids into a cellular subdivision called a mitochondrion, where chemical metabolism takes place. This enzyme, called carnitine acyltransferase, moves the activated fatty acid across the borders of the mitochondrion and into its interior. Once inside, chemicals break down the fatty acid into smaller molecular parts, releasing large quantities of energy in addition to the waste product carbon dioxide, which exits the cell.