39 by: Laura Medina Castellanos
Student – Laura Medina Castellanos
Enzyme – Lactase
E.C number – 3.2.1.108
The enzyme is found primarily in the brush border of the small intestine, and only expressed by mature enterocytes (Heine, et. al, 2017). Lactase is a specific beta-galactosidase that only acts on lactose (Nelson, 2021). Lactase is a homotetramer, meaning it has 4 identical subunits where its substrate, lactose can bind. Lactose is a disaccharide consisting of 2 monosaccharides, glucose and galactose held by a beta 1 → 4 bonds. When lactose is consumed, lactase hydrolyzes lactose into its two monosaccharides, glucose and galactose by breaking the glycosidic beta 1 → 4 bond (Forsgård, 2019). Following hydrolysis glucose and galactose enter the blood stream to metabolize in the liver (Kretchmer, 1972).
Some interesting facts about lactase is that lactase is not present in the intestine of the embryo or fetus until late gestation and its activity attains a maximum immediately after birth (Kretchmer, 1972). Lactose intolerance is a common occurrence in most populations, except those originating in Northern Europe and some parts of Africa (Nelson, 2021). It seems to occur after childhood, and sometimes into adulthood, when of most or all the lactase activity of the intestinal epithelial cells become inactive, inhibiting lactose to be completely digested and absorbed in the small intestine (Forsgård, 2019). The undigested lactose passes into the large intestine and here, the increased osmolarity due to dissolved lactose opposes the absorption of water from the intestine, causing watery, loose stools. Additionally, fermentation of the lactose by intestinal bacteria produces large volumes of CO2, which leads to the bloating, cramps, and gas, which is commonly associated with lactose intolerance (Nelson, 2021).
References:
BRENDA Enzyme Database. (n.d.). Enzyme entry: β-galactosidase (EC 3.2.1.108). BRENDA: The Comprehensive Enzyme Information System. Retrieved October 17, 2024, from https://www.brenda-enzymes.org/enzyme.php?ecno=3.2.1.108
Heine, R. G., AlRefaee, et. al. (2017). Lactose intolerance and gastrointestinal cow’s milk allergy in infants and children – common misconceptions revisited. World Allergy Organization Journal, 10, 41. https://doi.org/10.1186/s40413-017-0173-0
Kretchmer, N. (1972). LACTOSE AND LACTASE. Scientific American, 227(4), 70–79. http://www.jstor.org/stable/24922892
Nelson, D. L., Cox, M. M. (2021). Lehninger principles of biochemistry (7th ed.). W.H. Freeman & Company.
Richard A Forsgård, Lactose digestion in humans: intestinal lactase appears to be constitutive whereas the colonic microbiome is adaptable, The American Journal of Clinical Nutrition, Volume 110, Issue 2, 2019, Pages 273-279, ISSN 0002-9165, https://doi.org/10.1093/ajcn/nqz104.
(https://www.sciencedirect.com/science/article/pii/S0002916522011765)
Shafi, A., & Husain, Q. (2023). Structural and functional insights of β galactosidase and its potential applications. In A. Goyal & K. Sharma (Eds.), Foundations and Frontiers in Enzymology: Glycoside Hydrolases (pp. 323-347). Academic Press. https://doi.org/10.1016/B978-0-323-91805-3.00007-7
