Sugars are a subcategory of carbohydrates. They taste sweet, and consist of one or two smaller units called monosaccharides, which are the building blocks of all carbohydrates. Some sugars, and other carbohydrates, have the ability to participate in chemical reactions called redox reactions. These are reducing sugars. Sugars that can't participate in these chemical reactions are called non-reducing sugars. There is no nutritional difference whatsoever between reducing and non-reducing sugars -- the body processes them identically.
Sugars all consist of one or more monosaccharide units, and monosaccharide structure is at the heart of whether a sugar is reducing or non-reducing in nature. A monosaccharide is made up of a ring of carbon and oxygen -- in most nutritional sugars, this ring contains five or six members. Most nutritional monosaccharides have the chemical formula C6H12O6, where atoms that aren't part of the ring form arms off the ring.
Some nutritional sugars are simple monosaccharides. The common sugars glucose and fructose, for instance, are both monosaccharides. Glucose is often called blood sugar, while fructose is fruit sugar. Other nutritional sugars are disaccharides, made up of two monosaccharides chemically linked together. Sucrose, or table sugar, and lactose, or milk sugar, are the two most common disaccharides in the human diet, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
A redox reaction is an electron transfer reaction, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry." Monosaccharides can all participate in redox reactions, so they're all reducing sugars. Only certain disaccharides, however, can participate in redox reactions. All monosaccharides have a single point or atom at which the main structural ring opens. As long as this atom -- called the anomeric carbon -- doesn't form a bond to another monosaccharide or any other molecule, the sugar remains a reducing sugar.
Reducing Sugar Linkage
When two monosaccharides bond together to form a disaccharide, they can do so in several different ways. In the case of lactose, the monosaccharides glucose and galactose bond to one another such that glucose's anomeric carbon bonds to galactose, meaning it can't participate in redox reactions. Galactose's anomeric carbon, however, remains unbonded, so galactose in lactose can still participate in redox reactions. Overall, therefore, lactose can participate in redox reactions, and is a reducing sugar.
Non-Reducing Sugar Linkage
Sucrose is an example of a non-reducing sugar. Sucrose consists of a fructose bonded to a glucose. When these monosaccharides form a bond, they do so between fructose's anomeric carbon and glucose's anomeric carbon, meaning that both monosaccharides' anomeric carbons have formed bonds, and can't participate in redox reactions. This makes sucrose a non-reducing sugar.
- “Biochemistry”; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- “Biochemistry”; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
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