Exoamylases

Exoamylases are glycoside hydrolases that catalyze the hydrolysis of alpha-1,4-glycosidic bonds at the non-reducing ends of starch, dextrins, and related polysaccharides, releasing maltose or glucose units. They are distinguished from endoamylases, which cleave internal bonds within the polymer.

The principal exoamylases are beta-amylase, which liberates maltose from the non-reducing end of starch, and glucoamylase (also known as gamma-amylase or amyloglucosidase), which releases glucose by cleaving both alpha-1,4 and alpha-1,6 bonds. Some exoamylases primarily yield maltose; others yield glucose depending on substrate and enzyme characteristics.

Beta-amylases are common in plants, especially germinating seeds such as barley; glucoamylases are found in fungi, bacteria, and animals, and are widely used in industry. Structural forms range from monomeric to oligomeric enzymes, with various optimal conditions for activity.

Mechanistically, exoamylases require a non-reducing end substrate. Beta-amylase is unable to bypass branch points at alpha-1,6 linkages and tends to produce limit dextrins, whereas glucoamylase can act on alpha-1,6 bonds in many substrates, enabling more complete saccharification to glucose in practical preparations.

Applications include starch processing to generate maltose or glucose syrups, brewing and baking for fermentable sugar production, and biofuel production where starch-to-ethanol conversion is facilitated. Ongoing research seeks to improve thermostability, pH tolerance, and activity on branched substrates.