Lactase is a digestive enzyme responsible for breaking down lactose, often called milk sugar, which is a disaccharide. Its purpose is to cleave this large sugar molecule into smaller components that the body can easily absorb and use for energy. This process is fundamental to the ability of mammals, especially infants, to utilize the primary carbohydrate source in milk.
The Molecular Makeup
Lactase is fundamentally a protein, which means it is a large, complex molecule constructed from smaller building blocks called amino acids. These amino acids are linked together in a long, linear chain known as a polypeptide, which is determined by the instructions encoded in the LCT gene located on human chromosome 2. The initial product of this gene, a precursor molecule, contains a long chain of approximately 1,927 amino acids.
This long chain undergoes processing to remove segments and fold into a specific three-dimensional shape, which is necessary for the enzyme’s function. The mature lactase molecule is a single polypeptide chain, though it is often studied as part of a larger complex known as lactase-phlorizin hydrolase. The final folded structure creates a precise active site, a specialized pocket where the chemical reaction with lactose takes place. This unique conformation allows lactase to selectively recognize and bind to the lactose molecule.
The Role of Lactase
The primary job of the lactase enzyme is to facilitate the breakdown of lactose through a chemical reaction called hydrolysis. Lactose itself is a disaccharide, meaning it is made up of two simpler sugar molecules chemically bonded together. The two constituent simple sugars are glucose and galactose, which are joined by a \(\beta\)-glycosidic bond.
During hydrolysis, the lactase enzyme uses a molecule of water to break this specific bond connecting the two simple sugars. This process splits the lactose molecule into one molecule of glucose and one molecule of galactose. The breakdown is necessary because the cells lining the small intestine cannot absorb the large, complex lactose molecule directly.
Once glucose and galactose are separated, they are small enough to be transported across the intestinal wall and into the bloodstream. This allows the body to access the energy stored in the milk sugar.
Production and Deficiency
In humans, lactase is produced exclusively by the enterocytes, which are the cells lining the small intestine. Specifically, the enzyme is anchored to the brush border, a dense layer of microvilli that increases the surface area for absorption. Lactase activity is at its highest during infancy when milk is the sole source of nutrition.
For most of the world’s population, the production of lactase naturally decreases significantly after the weaning period, a phenomenon referred to as lactase non-persistence. This reduction in enzyme activity is the default human condition and often begins to manifest around age four or later. A genetic mutation, however, allows some individuals to maintain high lactase production into adulthood, a trait known as lactase persistence, which is common in populations with a long history of dairy farming.
Insufficient lactase production, or hypolactasia, means that undigested lactose continues its journey to the large intestine. There, colonic bacteria ferment the sugar, which generates gases and causes fluid to be drawn into the gut. This incomplete digestion is known as lactose intolerance, leading to symptoms like bloating, gas, and abdominal discomfort. Deficiency can also be temporary, known as secondary lactase deficiency, often caused by damage to the small intestine lining.