Amylose is a

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

made of α-D-

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

units, bonded to each other through α(1→4) glycosidic bonds. It is one of the two components of

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...

, making up approximately 20–25% of it. Because of its tightly packed helical structure, amylose is more resistant to digestion than other starch molecules and is therefore an important form of

resistant starch Resistant starch (RS) is starch, including its degradation products, that escapes from digestion in the small intestine of healthy individuals. Resistant starch occurs naturally in foods, but it can also be added as part of dried raw foods, or u ...

Structure

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 The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are connected to one another. The chemical bonding within the molecule is al ...

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, found both in starch granules and in hydrated amylose (when starch is cooked in water). There are also two different helical forms. Starting with purified amylose, it can bind with another amylose molecule in a

double helix In molecular biology, the term double helix refers to the structure formed by base pair, double-stranded molecules of nucleic acids such as DNA. The double Helix, helical structure of a nucleic acid complex arises as a consequence of its Nuclei ...

(A or B form), or it can bind with a guest molecule such as

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

, a

fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...

, or an

aromatic compound Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

, known as the V form. Within this form, 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 A helix (; ) is a shape like a cylindrical coil spring or the thread of a machine screw. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is for ...

structure, making it possible for hydrogen bonds to form between the oxygen atoms bound at the 2-carbon of one glucose molecule and the 3-carbon of the next glucose molecule. 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.

Physical properties

High-amylose forms of starch make it more resistant to digestion. Amylose is soluble in hot 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.

Function

Amylose is important in plant energy storage. It is less readily digested than

amylopectin Amylopectin is a water-insoluble polysaccharide and highly branched polymer of α-glucose units found in plants. It is one of the two components of starch, the other being amylose. Plants store starch within specialized organelles called amyloplas ...

. It makes up about 30% of the stored starch in plants, though the percentage varies by species and variety. The digestive enzyme α-amylase breaks down starch molecules into

maltotriose Maltotriose is a trisaccharide (three-part sugar) consisting of three glucose molecules linked with α-1,4 glycosidic bonds. It is most commonly produced by the digestive enzyme alpha-amylase (a common enzyme in human saliva) on amylose in star ...

and

maltose } Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the tw ...

, which can be used as sources of energy. Amylose is also an important thickener, water binder, emulsion stabilizer, and gelling agent in industrial and food-based contexts. Loose helical amylose chains have a

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

interior that can bind to hydrophobic molecules such as

lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...

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 firmness increases. When other things, including

, bind to amylose, the

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

can be affected, but incorporating κ-

carrageenan Carrageenans or carrageenins ( ; ) are a family of natural linear sulfation, sulfated polysaccharides. They are extracted from red algae, red edible seaweeds. Carrageenans are widely used in the food industry, for their gelling, thickening, an ...

alginate Alginic acid, also called algin, is a naturally occurring, edible polysaccharide found in brown algae. It is hydrophilic and forms a viscous gum when hydrated. When the alginic acid binds with sodium and calcium ions, the resulting salts are k ...

xanthan gum Xanthan gum () is a polysaccharide with many industrial uses, including as a common food additive. It is an effective thickening agent and stabilizer that prevents ingredients from separating. It can be produced from simple sugars by fermentat ...

, 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 White is the lightest color and is achromatic (having no chroma). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully (or almost fully) reflect and scatter all the visible wavelen ...

to thicken, but, upon cooling, the solid and the water will partly separate. Amylose is known for its good film-forming properties, useful in food packaging. Excellent film-forming behavior of amylose was studied already in 1950s. Amylose films are better for both barrier properties and mechanical properties when compared to the amylopectin films. In a laboratory setting, it can act as a marker.

Iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

molecules fit neatly inside the helical structure of amylose, binding with the starch polymer that absorbs certain known

wavelengths In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...

of light. Hence, a common test is the

iodine test Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vio ...

for starch. If starch is mixed with a small amount of yellow iodine solution, 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.

Recent studies

High-amylose varieties of

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

, the less sticky long-grain rice, have a much lower

glycemic load The glycemic load (GL) of food is a number that estimates how much the food will raise a person's blood glucose level after it is eaten. One unit of glycemic load approximates the effect of eating one gram of glucose. Glycemic load accounts for ho ...

, which could be beneficial for diabetics. Researchers have identified the Granule Bound Starch Synthase (GBSS) as the enzyme that specifically elongates amylose during starch biosynthesis in plants. The waxy locus in maize encodes for the GBSS protein. Mutants lacking the GBSS protein produce starch containing only

, such as in

waxy corn Waxy corn or glutinous corn is a type of corn characterized by its sticky texture when cooked. It has big round kernels that have endosperms that are almost universally white, though the aleurone layers can sometimes be purple or red which cause ...

. In Arabidopsis leaves, another gene, encoding the Protein Targeting to STarch (PTST) protein, is required in addition to GBSS for amylose synthesis. Mutants lacking either protein produce starch without amylose. Genetically modified potato cultivar Amflora by BASF Plant Science was developed to not produce amylose.

References

External links