Salivary amylase, carbohydrate digestion

Suppose we start with a starch-rich meal, say one containing a lot of pasta or bread. The digestion of starches begins in the mouth. Saliva contains an enzyme, salivary amylase (aka ptyalin), which catalyzes the conversion of starch to simple sugars such as glucose. This process is completed in the small intestine under the influence of other enzymes in the amylase class. This completes the first phase of carbohydrate catabolism the conversion of complex, polymeric carbohydrates (e.g., starches) to their simple monomeric units, the sugars. 

The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen. Salivary a-amylase acts on dietary starch (glycogen, amylose, amylopectin), producing oligosaccharides. Pancreatic a-amylase continues the process of starch digestion. 

The shift to a larger portion of carbohydrates in early humans required genetic adaptations to produce the enzyme salivary amylase. This enzyme begins the process of converting starches to sugar in the digestive system. 

Salivary a-amylase is a protein that contributes to the enamel pellicle (Sect. 12.1.3). More importantly, it attaches bacteria, especially streptococci, to teeth surfaces. Thus, following a meal rich in carbohydrates, amylopectin, amylase, and glycogen are digested to maltose at the surface of many oral bacteria. The maltose is taken into the cytosol by a phosphoenolpyruvate transporter homologous to the fructose transporter of S. mutans. Within these bacteria, the maltose is digested to two molecules of glucose 6-phosphate and metabolized to lactic acid. Thus, twice as much acid is produced per mole maltose than per mole sucrose and it contributes to tooth demineralization even if less sucrose is consumed. 

We typically consume in our diets a generous amount of starch and a smaller amount of glycogen. These complex carbohydrates must be converted into simpler carbohydrates for absorption by the intestine and transport in the blood. Starch and glycogen are digested primarily by the pancreatic enzyme a-amylase and to a lesser extent by salivary ot-amylase. Amylase cleaves the a-1,4 bonds of starch and glycogen, but not the ot-1,6 bonds. The products are the di- and trisaccharides maltose and maltotriose. The material not digestible because of the a -1,6 bonds is called the limit dextrin. 

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