Amilosa

Unidade estrutural da amilosa.

Amylose is a linear polymer made up of D-glucose units.

This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure. The other component is amylopectin, which makes up 7-80% of the structure.

Because of its tightly packed structure, amylose is more resistant to digestion than other starch molecules and is therefore an important form of resistant starch, which has been found to be an effective prebiotic.

Estrutura

Amylose is made up of α(1→4) bound glucose molecules. The carbon atoms on glucose are numbered, starting at the aldehyde (C=O) carbon, so, in amylose, the 1-carbon on one glucose molecule is linked to the 4-carbon on the next glucose molecule (α(1→4) bonds). The structural formula of amylose is pictured at right. The number of repeated glucose subunits (n) is usually in the range of 300 to 3000, but can be many thousands.

There are three main forms of amylose chains can take. It can exist in a disordered amorphous conformation or two different helical forms. It can bind with itself in a double helix (A or B form), or it can bind with another hydrophobic guest molecule such as iodine, a fatty acid, or an aromatic compound. This is known as the V form and is how amylopectin binds to amylose to form starch. Within this group, there are many different variations. Each is notated with V and then a subscript indicating the number of glucose units per turn. The most common is the V6 form, which has six glucose units a turn. V8 and possibly V7 forms exist as well. These provide an even larger space for the guest molecule to bind.

This linear structure can have some rotation around the phi and psi angles, but, for the most part, bound glucose ring oxygens lie on one side of the structure. The α(1→4) structure promotes the formation of a helix structure, making it possible for hydrogen bonds form between the oxygen atoms bound at 2-carbon of one glucose molecule and the 3-carbon of the next glucose molecule.

Propiedades físicas

Unlike amylopectin, amylose is insoluble in water. It also reduces the crystallinity of amylopectin and how easily water can infiltrate the starch. The higher the amylose content, the less expansion potential and the lower the gel strength for the same starch concentration. This can be countered partially by increasing the granule size.

Fiber X-ray diffraction analysis coupled with computer-based structure refinement has found A-, B-, and C- polymorphs of amylose. Each form corresponds to either the A-, the B-, or the C- starch forms. A- and B- structures have different helical crystal structures and water contents, whereas the C- structure is a mixture of A- and B- unit cells, resulting in an intermediate packing density between the two forms.

Funcións

Amylose is important in plant energy storage. It is less readily digested than amylopectin; however, because it is more linear than amylopectin, it takes up less space. As a result, it is the preferred starch for storage in plants. It makes up about 30% of the stored starch in plants, though the specific percentage varies by species. The digestive enzyme α-amylase is responsible for the breakdown of the starch molecule into maltotriose and maltose, which can be used as sources of energy.

Amylose is also an important thickener, water binder, emulsion stabilizer, and gelling agent in both industrial and food-based contexts. Loose helical amylose chains have a hydrophobic interior that can bind to hydrophobic molecules such as lipids and aromatic compounds. The one problem with this is that, when it crystallizes or associates, it can lose some stability, often releasing water in the process (syneresis). When amylose concentration is increased, gel stickiness decreases but gel firmness increases. When other things including amylopectin bind to amylose, the viscosity can be affected, but incorporating κ-carrageenan, alginate, xanthan gum, or low-molecular-weight sugars can reduce the loss in stability. The ability to bind water can add substance to food, possibly serving as a fat replacement. For example, amylose is responsible for causing white sauce to thicken, but, upon cooling, some separation between the solid and the water will occur.

In a laboratory setting, it can act as a marker. Iodine molecules fit neatly inside the helical structure of amylose, binding with the starch polymer that absorbs certain known wavelengths of light. Hence, a common test is the iodine test for starch. Mix starch with a small amount of yellow iodine solution. In the presence of amylose, a blue-black color will be observed. The intensity of the color can be tested with a colorimeter, using a red filter to discern the concentration of starch present in the solution. It is also possible to use starch as an indicator in titrations involving iodine reduction. It is also used in amylose magnetic beads and resin to separate maltose-binding protein

Investigacións recentes

High-amylose varieties of rice, less sticky long-grain rice, have a much lower glycemic load, which could be beneficial for diabetics.

Researchers have identified the gene granular binding starch synthase, or GBSS, in potatoes. It is responsible for encoding for the enzyme that directs amylase starch production. If it is inhibited, amylose production will also be interrupted.

Véxase tamén

Outros artigos

• amilopectina
• amidón
• amilomillo amidón de millo rico en amilosa
• Zhong, F., Yokoyama, W.H., Wang, Q., Shoemaker, C.F. 2006. Rice Starch, Amylopectin, and Amylose: Molecular Weight and Solubility in Dimethyl Sulfoxide-Based Solvents. Journal of Agricultural and Food Chemistry. 10.1021:A-G.

Ligazóns externas

• glycemic load
• http://www.scientificpsychic.com/fitness/carbohydrates1.html
• http://www.gmo-compass.org/eng/glossary/