Your liver is one of the most important organs in digestion and metabolism. Among its many other invaluable roles, the liver acts to maintain stable blood sugar levels. When your blood sugar levels start to fall, your liver counteracts the change by mobilizing stored sugar, synthesizing glucose and breaking down fatty acids for fuel.
Glucagon
A drop in blood sugar levels causes alpha cells in your pancreas to respond by releasing a small protein hormone called glucagon. Glucagon-sensitive tissues like the liver have receptors for this hormone, and when the hormone binds the receptor, it triggers a cascade of events inside the cell. This cascade begins with an enzyme called adenylate cyclase, which takes a molecule of ATP and converts it to cyclic AMP. Cyclic AMP or cAMP is a powerful "second messenger" that amplifies the response to the hormone.
Protein Kinase A
cAMP activates an enzyme called protein kinase A, or PKA, which can attach phosphate groups to other enzymes to turn them on or off. Two of PKA's targets are the bifunctional enzyme PFK 2/FBPase 2 and glycogen phosphorylase kinase. Adding a phosphate group to PFK 2/FBPase 2 or "phosphorylating" it alters it so it stops producing fructose 2,6-bisphosphate and instead begins breaking it down. Fructose 2,6-bisphosphate is an important regulator for two key enzymes in glycolysis, the process that breaks glucose down in liver cells, and gluconeogenesis, the process that manufactures glucose in liver cells.
Effects
The drop in fructose 2,6-bisphosphate levels causes the liver cells to switch from breaking glucose down to making it; the additional glucose can now be distributed into the bloodstream to bring glucose levels back up. Phosphorylating glycogen phosphorylase kinase also activates it so it can phosphorylate another enzyme called glycogen phosphorylase, the enzyme responsible for breaking down polymers of glucose molecules called glycogen. The breakdown of glycogen releases large amounts of glucose into the bloodstream for use by other tissues.
Fatty Acid Oxidation
Protein kinase A also phosphorylates enzymes called lipases that break up or hydrolyze fat molecules. The glycerol released by lipid hydrolysis can go straight into the bloodstream for use by other tissues. The fatty acids, meanwhile, can be oxidized to provide energy for liver cells. Fatty acid oxidation may also potentially serve as the precursor for ketone bodies, small molecules your liver sometimes produces to serve as an energy source for other tissues.
References
- "Lehninger Principles of Biochemistry"; David L. Nelson and Michael M. Cox; 2008
- "Biology"; Neil A. Campbell, et al.; 2008
