Therefore, the best way to describe its chemistry is to define the distribution of the molecular masses, and the average frequency with which branches occurs and their average length. Finally, it should be emphasized that glycogen is not a static entity but constantly vary over the course of its existence. As glucose is a chiral molecule, it exists as a pair of enantiomers, indicated according to the Fischer convention as D-glucose, the most widespread in nature and the monomeric unit of glycogen and starch, and L-glucose.
The folding into three-dimensional structures of macromolecules such as proteins, nucleic acids and polysaccharides is governed by the same principles: the monomeric units, namely, amino acids, nucleotides, and monosaccharides, with their more-or-less rigid structure, are joined by covalent bonds to form one dimensional polymers that spontaneously fold into three-dimensional structures stabilized by noncovalent interactions such as:. These interactions can occur within macromolecules or between macromolecules, as in supramolecular complexes such as cellulose or multienzyme complexes.
Hence, some conformations will be more stable than others. For amylose and glycogen, the most stable 3D structure is a tightly coiled helix stabilized by interchain hydrogen bonds. Due to the action of glycogenin, and then of glycogen synthase and branching enzyme EC 2.
Moreover, considering that each tier has a thickness of 3. This does not mean that these molecules are all accessible to glycogen phosphorylase because the enzyme stalls four residues from the branch point. The intervention of the debranching enzyme, whose activity is slower than glycogen phosphorylase activity, removes the branch and allows glycogenolysis to proceed.
Why is 13 th tier not possible? The 13 th tier seems to be not possible because of the steric hindrance due to high density of glucose units on the molecule surface. Such an high density of glucose residues would lead to insufficient space for the interaction between the catalytic region of glycogen metabolism enzymes, and then of glycogen synthase, too, and the growing chains.
The structure of the glycogen molecule includes the protein glycogenin , which is covalently bonded to the polysaccharide chain. Glycogenin initiates the synthesis of glycogen by autoglycosylation, catalyzing the addition of glucose units to a specific tyrosine residue. This primer chain then acts as substrate for glycogen synthase. In addition, binding to actin filaments, glycogenin anchors the oligosaccharide primer chain to the cytoskeleton.
In addition to glycogenin, glycogen molecule covalently binds phosphate groups. For many years they were considered a contaminant and their amounts were inversely correlated with purity of the sample. Only in the early s they were recognized as an integral part of the polysaccharide, where they seems to be linked to C-2 e C-3 as monoester, probably as a result of a side reaction during the activity of glycogen synthase.
Many studies have suggested that their presence plays a role in regulating glycogen metabolism, similarly to what happens for starch metabolism in plants. Evidence supporting this hypothesis are the identification of laforin , a glycogen phosphatases, and that its mutation is a key factor in Lafora disease, a form of epilepsy characterized, among other things, by an excessive phosphorylation of glycogen. But how would they act?
Several hypotheses have been proposed, and two are reported below. Individual glycogen molecules are too small to be detected by light microscopy. Conversely, electron microscopy allowed to identify three types of structures: beta granules, gamma particles, and alpha granules.
They are considered a rapid energy source. These proteins also bind to each other, to cytoskeleton or to membranes, and are all involved in the metabolism of the polysaccharide. Some of these are:. Unlike the pyruvate dehydrogenase complex or ribosomes, the stoichiometry and the composition of the beta granules is not constant, but rather dynamic, as proteins associate or dissociate from the granule depending on cellular conditions.
In addition, differences are observed not only between different cell types but also within the same cell type, for example in skeletal muscle cells depending on different subcellular localizations. In the liver , beta granules are organized to form structures called alpha granules. Alpha granules are considered a slower energy source than beta granules. To date, the mechanism underlying their formation is not yet clear, although it seems that beta granules are linked through a protein skeleton rich in disulfide bonds.
Therefore, skeletal muscle has a limited capacity to store glycogen than liver; however, as its mass is greater than that of the liver, the muscle content of glycogen is about double than that of the liver. BBA Clin. Liver glucose metabolism in humans. Liver glycogen in type 2 diabetic mice is randomly branched as enlarged aggregates with blunted glucose release.
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Characterization of branched polysaccharides using multiple-detection size separation techniques. Molecular structure of glycogen in Escherichia coli. Molecular structural features of glycogen in the kidneys of diabetic rats. Effects of fasting on liver glycogen structure in rats with type 2 diabetes. All the glucose molecules in cellulose have the beta-configuration at the C1 atom, so all the glycosidic bonds that join the glucose molecules together are also of the beta type.
This means that the cellulose molecule is straight, and many such molecules can lay side by side in a parallel series of rows. Tiny forces called hydrogen bonds hold the glucose molecules together, and the chains in close proximity. Although each hydrogen bond is very, very weak, when thousands or millions of them form between two cellulose molecules the result is a very stable, very strong complex that has enormous strength.
Starch , a word that comes from old English and means to stiffen , is also a polysaccharide made in plants. It is primarily an energy storage molecule, or fuel, for the plant and for its seeds.
If the starch molecules are gently broken down by acid hydrolysis, the disaccharide maltose is produced, indicating that the glucose molecules in starch are also joined together by linking the C1 carbon of one sugar to the C4 carbon of the next sugar in the sequence. However, in this case, the glucose molecules are joined using alpha-glycosidic bonds. However, these molecules are not straight or totally linear.
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