Some people suffer from lactose intolerance they lack the lactase enzyme, hence they cannot digest lactose, or dairy products containing lactose. Because of their ability to metabolize lactose, they have a competitive advantage over many pathogenic and spoilage organisms. Lactic acid bacteria produce lactic acid from lactose, which is the beginning of many fermented dairy products. One of its most important functions is its utilization as a fermentation substrate. When lactose is hydrolyzed by ß -D-galactosidase (lactase), an enzyme that splits these monosaccharides, the result is increased sweetness, and depressed freezing point. It comprises 4.8 to 5.2% of milk, 52% of milk SNF, and 70% of whey solids. Structure showing the glucose and galactose of lactose Because of the anomeric carbon on the right side of the structure below, lactose can exist as two isomers, alpha, as shown, or beta, in which the hydroxyl on the anomeric carbon would point up on the ring structure shown below. By convention the ring oxygen is placed to the right and to the rear of the structure (top right of the drawing).Lactose is a disaccharide (2 sugars) made up of glucose and galactose (which are both monosaccharides). Pyranose rings are even capable of undergoing a ring flip to change between chair conformations. Pyranose rings are often drawn in a chair conformation like cyclohexane rings ( Section 4-6) with substituents being either an axial or equatorial position. Notice that for glucose and other aldohexoses the hydroxyl that forms the cyclic hemiacetal is also the one that determines the D/L designation of a sugar. The cyclic form of glucose is called glucopyranose. The pyranose ring is formed by attack of the hydroxyl on carbon 5 of glucose to the aldehyde carbon (carbon #1, also called the anomeric carbon in carbohydrate terminology). At equilibrium less than 1% of glucose is in an open chain form with the rest being almost exclusively in its cyclic pyranose form. The size of the cyclic hemiacetal ring adopted by a given sugar is not constant, but may vary with substituents and other structural features.Īldohexoses usually form pyranose rings and their pentose homologs tend to prefer the furanose form, but there are many counter examples. For most five- and six-carbon sugars, the cyclic forms predominate in equilibrium since they are more stable. Sugars are often shown in their open-chain form, however, in aqueous solution, glucose, fructose, and other sugars of five or six carbons rapidly interconvert between straight-chain and cyclic forms. Unlike most of the biochemical reactions you will see in this text, sugar cyclization reactions are not catalyzed by enzymes: they occur spontaneously and reversibly in aqueous solution. Cyclic structures of this kind are termed furanose (five-membered) or pyranose (six-membered), reflecting the ring size relationship to the common heterocyclic compounds furan and pyran shown below.įuran (5-membered ring) and pyran (6-membered ring) structures Five and six-membered rings are favored over other ring sizes because of their low angle and eclipsing strain. Molecules which have both an alcohol and a carbonyl can undergo an intramolecular version of the same reaction forming a cyclic hemiacetal.īecause sugars often contain alcohol and carbonyl functional groups, intramolecular hemiacetal formation is common in carbohydrate chemistry. In Section 19-10 it was discussed that the reaction of one equivalent of an alcohol, in the presence of an acid catalyst, adds reversibly to aldehydes and ketones to form a hydroxy ether called a hemiacetal (R 2COHOR') (h emi, Greek, half).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |