Amylase

A diagram of an amylase molecule from human saliva. Calcium ion visible in pale khaki; chloride ion in green. From PDB 1SMD.

Human pancreatic amylase. Calcium ion visible in pale khaki; chloride ion in green. From PDB 1HNY.

Amylase is an enzyme that breaks starch down into sugar. Amylase is present in human saliva, where it begins the chemical process of digestion. Foods that contain much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns some of their starch into sugar in the mouth. The pancreas also makes amylase (alpha amylase) to hydrolyse dietary starch into disaccharides and trisaccharides which are converted by other enzymes to glucose to supply the body with energy. Plants and some bacteria also produce amylase. As diastase, amylase was the first enzyme to be discovered and isolated (by Anselme Payen in 1833).^[1] Specific amylase proteins are designated by different Greek letters. All amylases are glycoside hydrolases and act on α-1,4-glycosidic bonds.

Classification

α-Amylase

(EC 3.2.1.1 ) (CAS# 9014-71-5) (alternate names: 1,4-α-D-glucan glucanohydrolase; glycogenase) The α-amylases are calcium metalloenzymes, completely unable to function in the absence of calcium. By acting at random locations along the starch chain, α-amylase breaks down long-chain carbohydrates, ultimately yielding maltotriose and maltose from amylose, or maltose, glucose and "limit dextrin" from amylopectin. Because it can act anywhere on the substrate, α-amylase tends to be faster-acting than β-amylase. In animals, it is a major digestive enzyme and its optimum pH is 6.7-7.0. ^[2]

In human physiology, both the salivary and pancreatic amylases are α-Amylases. They are discussed in much more detail at alpha-Amylase.

Also found in plants (adequately), fungi (ascomycetes and basidiomycetes) and bacteria (Bacillus).

β-Amylase

(EC 3.2.1.2 ) (alternate names: 1,4-α-D-glucan maltohydrolase; glycogenase; saccharogen amylase) Another form of amylase, β-amylase is also synthesized by bacteria, fungi, and plants. Working from the non-reducing end, β-amylase catalyzes the hydrolysis of the second α-1,4 glycosidic bond, cleaving off two glucose units (maltose) at a time. During the ripening of fruit, β-amylase breaks starch into maltose, resulting in the sweet flavor of ripe fruit.

Both α-amylase and β-amylase are present in seeds; β-amylase is present in an inactive form prior to germination, whereas α-amylase and proteases appear once germination has begun. Cereal grain amylase is key to the production of malt. Many microbes also produce amylase to degrade extracellular starches. Animal tissues do not contain β-amylase, although it may be present in microrganisms contained within the digestive tract.

γ-Amylase

(EC 3.2.1.3 ) (alternative names: Glucan 1,4-α-glucosidase; amyloglucosidase; Exo-1,4-α-glucosidase; glucoamylase; lysosomal α-glucosidase; 1,4-α-D-glucan glucohydrolase) In addition to cleaving the last α(1-4)glycosidic linkages at the nonreducing end of amylose and amylopectin, yielding glucose, γ-amylase will cleave α(1-6) glycosidic linkages. Unlike the other forms of amylase, γ-amylase is most efficient in acidic environments and has an optimum pH of 3.

Uses

Amylase enzymes finds use in bread making and to break down complex sugars such as starch (found in flour) into simple sugars. Yeast then feeds on these simple sugars and converts it into the waste products of alcohol and CO₂. This imparts flavour and causes the bread to rise. While Amylase enzymes are found naturally in yeast cells, it takes time for the yeast to produce enough of these enzymes to break down significant quantities of starch in the bread. This is the reason for long fermented doughs such as sour dough. Modern bread making techniques have included amylase enzymes (often in the form of malted barley) into bread improver thereby making the bread making process faster and more practical for commercial use.^[3]

When used as a food additive Amylase has E number E1100, and may be derived from swine pancreas or mould mushroom.

Bacilliary amylase is also used in clothing and dishwasher detergents to dissolve starches from fabrics and dishes.

Workers in factories that work with amylase for any of the above uses are at increased risk of occupational asthma. 5-9% of bakers have a positive skin test, and a fourth to a third of bakers with breathing problems are hypersensitive to amylase. ^[4]

An inhibitor of alpha-amylase called phaseolamin has been tested as a potential diet aid. ^[5]

Blood serum amylase may be measured for purposes of medical diagnosis. A normal concentration is in the range 21-101 U/L. A higher than normal concentration may reflect one of several medical conditions, including acute inflammation of the pancreas (concurrently with the more specific lipase)^[6], but also perforated peptic ulcer, torsion of an ovarian cyst, strangulation ileus, macroamylasemia and mumps. Amylase may be measured in other body fluids, including urine and peritoneal fluid.

History

In 1831 Erhard Friedrich Leuchs (1800-1837) described the hydrolysis of starch by saliva, due to the presence of an enzyme in saliva, "ptyalin", an amylase.^[7][8] The modern history of enzymes began in 1833 when French chemists Anselme Payen and Jean-François Persoz isolated an amylase complex from germinating barley and named it "diastase".^[9][10] In 1862 Alexander Jakulowitsch Danilewsky (1838-1923) separated pancreatic amylase from trypsin.^[11][12]

Human evolution

Carbohydrates are an energy rich food source. Amylase is thought to have played a key role in human evolution in allowing humans an alternative to fruit and protein. A duplication of the pancreatic amylase gene developed independently in humans and rodents, further suggesting its importance. The salivary amylase levels found in the human lineage are six to eight times higher in humans than in chimpanzees, which are mostly fruit eaters and ingest little starch relative to humans. ^[13]

References

External links

• Molecule of the month February 2006 at the Protein Data Bank.
• Nutrition Sciences 101 at University of Arizona.