The wonders of salivary amylase

Artwork of salivary amylase by Minty Lumsden

Mon, 10/03/2022 - 09:27

Sweet taste

Did you ever wonder why rice, corn, and potatoes have a slightly sweet taste when you chew them? This is because they contain large amounts of starch, a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. Starch itself is mostly tasteless, but when it is degraded the glucose molecules reach your taste buds and the magic happens. The enzyme responsible for starch degradation is called amylase, and is found in saliva, among other places. Whatever your diet, the carbohydrates in food provide fuel for your body in the form of glucose. However, finding free glucose is relatively rare in our typical diets, and it is the work of enzymes like amylase to break down complex carbohydrates or starch, into smaller, simpler sugars such as glucose.

Starch is manufactured in the green leaves of plants from excess glucose produced during photosynthesis and is used by the plant as a reserve food supply. Starch is stored in chloroplasts in the form of granules in the seeds of corn, wheat, and rice, or in the tuber of the potato.

Amylose chain — Figure 1. Starch is a polysaccharide comprising glucose monomers joined in α-1,4 linkages. Its basic chemical formula is (C6H10O5)n. The simplest form of starch is the linear polymer amylose, as shown in the picture above.

Salivary amylase, encoded by the gene AMY1, is a major component of human saliva that initiates carbohydrate digestion in the mouth. This process continues in the small intestine, where amylase produced by the pancreas performs the final steps of carbohydrate digestion. In addition to amylase, our saliva also contains many other enzymes including lipases, peptidases, and hydrolases, each breaking down different nutrients into smaller units for digestion.

Ever since the agricultural revolution, starch has become a central component of the human diet. Its role in other mammals’ diet has also increased due to the domestication of animals and agriculture. Therefore, many mammals have seen great expansions in the copy number of the amylase gene. These duplications allow for the pancreatic amylase, encoded by the gene AMY2, to re-target to the salivary glands, allowing animals to detect starch by taste and to digest starch more efficiently and in higher quantities. There is a direct correlation between the starch consumption and the number of AMY1 gene copies in populations. Recent studies have also shown that the number of copies of the AMY1 gene is associated with obesity. Interestingly, close evolutionary relatives of the humans such as chimpanzees and bonobos, possess either one or no copies of the gene responsible for producing salivary amylase.

The ABC of amylases

There are several different types of amylase proteins, and each of them are designated by different Greek letters. They all have in common that they are glycoside hydrolases and act on α-1,4-glycosidic bonds.

The α-amylases are calcium metalloenzymes, and the major digestive enzymes in animals. In human physiology, both the salivary and pancreatic amylases are α-amylases. They act at random locations along the starch chain, breaking it down into di- and tri-saccharides (maltose and maltotriose), which will be converted by other enzymes to glucose to supply the body with energy. Because they can act anywhere on the substrate, α-amylases tend to be faster-acting than the other amylases. The α-amylase form is also found in plants, fungi, and bacteria.

The β-amylases are synthesized by bacteria, fungi, and plants, but absent from animals (although it may be present in microorganisms contained within the digestive tract). β-amylases catalyse 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 flavour of ripe fruit.

The γ-amylases (also known as glucoamylases) are present in animals and microbes but absent from plants. They are able to cleave α(1–6) glycosidic linkages, as well as the last α-1,4 glycosidic bond at the non-reducing end of amylose, yielding glucose. The γ-amylase has the most acidic optimum pH of all amylases because it is most active around pH 3, coinciding with the acidity of the stomach.

The three types of amylases, alpha, beta and gamma — *Figure 2. Left: structure of human alpha-amylase (PDB 1SMD) (for colours legend, see Fig. 3). Middle: structure of barley beta-amylase (PDB 2XFR). Right: Structure of Penicillium oxalicum’s gamma-amylase (PDB 6FHV*).

Alpha-amylase from human saliva

Salivary alpha-amylase is a major component of human saliva. It not only plays a role in the initial digestion of starch but is also involved in the colonization of bacteria involved in early dental plaque formation. Salivary amylase is a monomeric calcium-binding protein with a single polypeptide chain. Its 3D-structure consists of three domains, A, B and C. Domain A has a (β/α)8 barrel structure, domain B has no definite topology, and domain C has a Greek-key barrel structure. The calcium ion is bound to Asn100, Arg158, Asp167, His201 and three water molecules. The chloride ion is bound to Arg195, Asn298 and Arg337 and one water molecule. A highly mobile glycine-rich loop 304-310 acts as a gateway for substrate binding and is involved in a 'trap-release' mechanism in the hydrolysis of substrates. The active site is located at the C-terminal end of the central β-barrel, and the residue Trp58 has been shown to be critical for enzyme activity. The C domain is loosely linked to the rest of the molecule.

Structure of the human salivary amylase PDB 1SMD — *Figure 3. Human salivary amylase (PDB 1SMD). Colours are as follows: domain A (yellow), domain B (orange), domain C (red), calcium ion (green), chloride ion (magenta), mobile glycine-rich loop (dark blue).*

More than just sweet: the other flavours of amylase

Salivary amylase is a multifunctional enzyme, and beyond its hydrolytic activity, is also involved in other biological functions.

One of its roles is in the oral microbial ecology, where it binds with high affinity to Streptococcus viridans, a group of streptococci that is part of the normal flora of the mouth, and usually responsible for dental caries. This helps the clearance and/or adherence of these bacteria in the oral cavity. When salivary amylase is bound to the enamel of the tooth, it plays a role in dental plaque formation and in the subsequent process of dental caries formation and progression.

Everyday life uses of amylases

Alpha- and beta-amylases are widely used in fermentation, for brewing beer and liquor made from sugars derived from starch, that yeast consumes to produce ethanol. By selecting different mixes of starch sources (e.g., barley, potatoes), grain-to-water ratio, and temperature, one can optimize the activity of alpha- and beta-amylases, resulting in different mixtures of sugars, changing the flavour and alcohol content of the resulting beverage. In some historic methods of producing alcoholic beverages, the brewer actually chews grain in order to mix it with saliva and start the conversion of starch to simple sugars.

Amylases are also used in breadmaking, again to help yeast feed on simple sugars and convert them into ethanol and carbon dioxide, causing the bread to rise and giving it its flavour. While amylases are found naturally in yeast cells, adding extra amylases allows the dough preparation to happen faster and is more practical for commercial use. Alpha-amylase is therefore often listed as an ingredient on commercially packaged flours. When used as a food additive, amylase has E number E1100, and may be derived from pig pancreas or mould fungi.

Bacterial amylases are also used in clothing and dishwasher detergents to dissolve starches from fabrics and dishes. While biological detergents are excellent stain removers and allow to use lower temperatures when washing, non-biological detergents (that do not contain enzymes such as amylases) are kinder to sensitive skins, and therefore recommended for people suffering from asthma or sensitivity to amylase.

Deborah Harrus