Human Salivary Amylase

Introduction

Salivary amylase is an enzyme that hydrolyzes starch in the oral cavity of humans. This enzyme is produced by salivary glands, although the majority of amylases in humans are produced in the pancreas. It consists of a single polypeptide chain of 496 amino acid residues that weighs 56 kDa. The starches are hydrolyzed to yield maltose which can then be used to produce glucose. Studies have postulated that salivary amylase stimulates physiological responses that prepare the digestive system to metabolize and absorb nutrients ^[1]. Additionally, salivary amylase is shown to inhibit the biofilm formation of several bacterial species, indicating a possible role in preventing oral bacterial infections ^[2]. Interestingly enough, this enzyme not only exists in humans but in other omnivores and some herbivores. However, obligate carnivores, such as house cats, lack this oral enzyme ^[1].

Structure

Human salivary amylase is a protein consisting of a single chain made up of three domains, A, B, and C. The overall structure of this protein is barrel shaped, much like the three individual domains of the protein. This multi domain structure is stabilized by several disulfide bridges. These disulfide bridges are seen between the cysteine residues at positions 28 and 86, 70 and 115, 141 and 160, 378 and 384, and positions 450 and 462.

Active Site

The active site forms a large, deep cleft where larger starches bind and are hydrolyzed into smaller ones. This active site is at the aspartate residue at position 197 and the glutamate residue at position 233. This active site plays a role in the enzyme’s function as a processive enzyme. This means that salivary amylase does not immediately detach from its substrates and can carry out several rounds of hydrolysis before detaching. The mechanism for hydrolyzing starches involves a proton donor group, Glu 233 cleaving the glycosidic bond. Then, a nucleophile, Asp 197, forms a covalent intermediate between the enzyme and substrate. This intermediate is then attacked by a hydroxyl ion, formed by Glu 233, resulting in an unaltered enzyme and the products of hydrolysis.

Ligands

Human salivary amylase has two ligands, calcium and chlorine. There are three metal binding sites per domain that bind calcium ^[3]. Studies that have investigated the function of these calcium ions have suggested that the presence of these ions greatly influences the thermostability of the enzyme ^[4]. This may be a result of the fact that the enzyme moves from the cooler oral cavity to warmer regions of the gastrointestinal tract; however, the lower pH of these regions may inactivate salivary amylase. These metal binding sites occur at the asparagine residue at position 100, the arginine residue at position 158, the glutamate residue at position 167, and the histidine residue at position 201. Aside from calcium, chloride ions can bind to salivary amylase and there are three binding sites for this ligand ^[3]. These chloride binding sites are found at the arginine residue at position 195, the asparagine residue at position 298, and the arginine residue at position 337. It is suggested that the negative charge associated with these chloride ions is essential for the maximal catalytic activity of the enzyme. Other ions, such as nitrate, can bind to these chloride binding sites, however, their ability to allosterically activate salivary amylase is much weaker.

Evolutionary Relationship

There are many uncertainties that surround the evolutionary advantage of having an amylase produced and secreted in the oral cavity. The majority of amylases in humans are manufactured in the pancreas and sent to aid in starch digestion in the duodenum of the small intestines ^[5]. Although human amylases share sequence homology, they are coded by different genes. Studies have suggested that humans, along with some other animals, acquired the gene for a separate salivary amylase when foreign insertions caused the gene for pancreatic amylases to split ^[5]. Additionally, it is hypothesized that salivary amylase helps make starches more palatable by quickly decreasing the length of the polymer chains and therefore lowering the viscosity ^[1]. Furthermore, the early breakdown of these starches may release products that are detected in the oral cavity. The detection of these products activates physiological processes that prepare the digestive system to break down the incoming starches ^[1].