Mannose and glucose are two types of sugars. Both are simple monosaccharides. They are both composed of six carbon molecules, 12 hydrogen molecules and 6 oxygen molecules. They are known as hexoses, in which "hex" refers to the six carbon molecules, and -ose is a sugar suffix. Despite their similarities in chemical formula, they play very different roles in the human body.
Glucose Structure
Glucose plays a key role in many biological processes. Most sugars are converted to glucose in the body, as part of the body's metabolic processes to obtain energy. Glucose is a simple, six-carbon, hexose sugar molecule with the chemical formula C6H12O6. Other names for glucose include dextrose or simply, blood sugar. The International Union of Pure and Applied Chemistry's (IUPAC) formal name for glucose is 6-(hydroxymethyl)oxane-2,3,4,5-tetrol.
The straight chain structure of glucose shows that it has an aldehyde or CHO group at one end, and is therefore called an aldohexose. The other five carbons atoms have alcohol groups.
In solution, the structure of glucose is ring. The ring structure is created by the reaction of the hydroxy group on the carbon-6 atom with the aldehyde group on the carbon-1 atom. The ring structure of glucose is called glucopyranose.
Glucose is also called D-glucose. The D stands for dextrorotatary, which is Latin for the phrase "rotating to the right" because in solution, the naturally occurring glucose isomer rotates polarized light to the right.
Mannose Structure
Mannose, similar to glucose, is also a hexose sugar monomer. This means it is a single sugar molecule made of six carbon atoms, having the similar formula of C6H12O2. For mannose, the formal International Union of Pure and Applied Chemistry name is (2S,3S,4S,5S,6R)-6-(hydroxymethyl)tetrahydro-2H-pyran-2,3,4,5-tetraol. The ring form of mannose in solution displays a pucker-like structure.
Mannose is solid at room temperature, with an off-white to yellow crystal appearance. It is odorless, and the taste has been reported to be both sweet and bitter.
Role of Glucose in the Body
Glucose plays a pivotal role in many biological processes. Glycolysis, for example, is the process by which the body breaks down glucose molecules to produce energy in the from of a molecule called ATP. Many sugars are first converted to glucose in the body for the sugar to be converted into energy. For example, lactose, the sugar found in milk, is a disaccharide made up of glucose and galactose. The bond between the two sugar molecules is broken, releasing a glucose molecule that can enter the glycolytic pathway. In the liver, galactose is converted to glucose-6-phosphate so that it too can enter the glycolytic pathway.
Glucose is the form of sugar synthesized by plants in photosynthesis reactions. Polymers of glucose make up cellulose and starch, energy stores for cells.
Role of Mannose in the Body
Unlike glucose, mannose is not well metabolized in humans. The majority of dietary mannose is excreted in urine, unconverted, within a very short time after ingestion. During that period of time, blood-glucose levels remain unchanged.
Despite its low metabolism, mannose is present in the human body as part of sugar-linked protein polymers. After some proteins are synthesized, polymers of mannose are attached at the carboxy terminal ends of the proteins. This is known as C-mannosylation and, for some proteins, it is essential for their proper structure and function.
Interestingly, when human proteins are artificially produced in yeast, for example, for vaccine formulations, the mannose molecules added are in different configurations than those produced naturally in human cells. This difference in configuration of the mannose molecules may influence the effectiveness of vaccines derived from those proteins.
Comparison of Glucose and Mannose
Although mannose and glucose have the same chemical formula, their structures are slightly different. The carbon-2 atoms, the carbons that are attached to the carbon-1 aldehyde groups, are mirror images of each other. In glucose, the hydroxyl groups on the carbon-2 atom faces the same direction as the hydroxyl groups on the carbon-4 and carbon-5 atoms. For mannose, the hydroxyl group on the carbon-2 atom faces the same direction as the hydroxyl group on the carbon-3 atom.
