Of course, for many organisms, the food used by cells is not in the form of simple glucose solutions, but made up of various polymeric biomolecules. The breakdown of those molecules is described in the next sections.
Starch and Glycogen Depolymerization
Glycogen and starch are long branched polymers of glucose that provide a rapidly available source of glucose molecules for glycolysis. In omnivores and herbivores, the primary source of carbohydrates (and thus glucose) is dietary starch. The catabolism of the amylose and amylopectin in humans begins in the mouth with salivary a-amylase. This enzyme breaks α(1-4) bonds of both starch molecules except at the ends and near branch points (in the case of amylopectin). Though the salivary enzyme is inactivated by the acidity of the stomach, a pancreatic a-amylase goes to work on starch that has reached the small intestine. The product of these digestions includes maltose, maltotriose, and dextrins. These are acted upon by other intestinal enzymes: α-glucosidase removes individual glucoses from oligosaccharides, and a-dextrinase, also known as debranching enzyme, can break α(1-6) bonds as well as the α(1-4) bonds.
Glycogen breakdown is different since most glycogen breakdown is occurring internal to the cells of an organism rather than in the digestive tract. The primary enzyme is phosphorylase (also known as glycogen phosphorylase), which breaks the bond of a terminal glucose to its neighbor by substituting a phosphate group. This generates glucose-1-phosphate, which can be converted to glucose-6-phosphate by phosphoglucomutase. The G6P, of course, can enter the glycolytic pathway. A glycogen debranching enzyme is also important, as the phosphorylase is unable to work closer than five glucose residues to a branch site.
Figure 13. Glycogen is a storage form of glucose that is broken down by glycogen phosphorylase (linear a(1-4) bonds) and debranching enzyme (branchpoint α(1-6) bonds).
Phosphorylase is a homodimer that is allosterically controlled by glucose, G6P, and ATP negatively, and by AMP positively. In addition to allosteric binding sites for these molecules and a substrate binding site, phosphorylase also binds pyridoxal-5-phosphate as an essential cofactor. P5P is derived from pyridoxine, or vitamin B6.
Much like the phosphoglycerate mutase in step 8 of glycolysis, phosphoglucomutase is a phosphorylated enzyme that temporarily transfers is phosphate group to the substrate to form a glucose-1,6-bisphosphate intermediate.
Debranching enzyme actually has two functions: it transfers a trisaccharide from a 4-sugar branch on the “1” side of an α(1-6) branching linkage to the end of a branch connected to the “6” side of the branchpoint. It then hydrolyzes the α(1-6) connecting the nal glucose of the branch, leaving an unbranched chain of glucose for phosphorylase to attack.
The use of glycogen presents an interesting question: why use it as an energy storage molecule when fats are more abundant in most animals, and more efficient at packaging potential energy? As described in the next section, fatty acids can only be metabolized aerobically, so they cannot serve as a backup fuel source in anaerobic conditions. Furthermore, even in aerobic conditions, fatty acid catabolism cannot generate glucose, which is not only needed for cellular fuel, but in the bloodstream for feedback control mechanisms regulating organismic metabolism.