Structural Biochemistry/Nucleic Acid/Sugars
Carbohydrates are comprised of monosaccharide units which create sugars ranging from simplest of sugars such as glucose (chemical formula: C6H12O6) to the more complex polysaccharides such as starch. Single nucleotide monomeric units consist of one sugar molecule connected to 1) a heterocyclic nitrogen containing organic base, and 2) a Phosphate group that connects the sugar component of different nucleotides together. The organic base is usually attached to Carbon 1' of the sugar, while the Phosphate group is connected to Carbon 5' of the sugar. When strung together, the phosphate of the neighboring nucleotide attaches to Carbon 3' of the sugar.
Monosaccharides consist of aldehyde or ketone groups with hydroxyl groups as substituents. Sugars that contain an aldehyde group are called aldoses, and the sugars that contain a ketone group are called ketoses.
Sugars that are non-super imposable mirror images of each other are called enantiomers. Sugars that are stereoisomers but mirror images of each other are called diastereoisomers. If sugars that are stereoisomers but differ in configuration at a single chiral center are called epimers.
Sugars can be open-chain form or ring form. To form a six-membered hemiacetal ring, the carbon in the aldehyde group (C-1) attaches to the oxygen atom in the C-5 hydroxyl group. The six membered cyclic hemiacetal is called pyranose because it is similar to the structure of a pyran. To form a five-membered ring, the C-2 of ketone group attackes the oxygen atom of the hydroxyl group on C-6. The five membered cyclic hemiacetal is called furanose because it is similar to the structure of a furan. When a furanose or pyranose ring is formed, a new stereocenter is formed, and this new chiral carbon is called the anomeric carbon. This carbon can have one of two configurations, it is either in the S conformation (the hydroxyl group is pointing up), and it is referred to as the alpha carbon, or it is in the R conformation (the hydroxyl group is pointing down) and it is referred to as the B configuration. These two conformations are diastereomers, not enantiomers, and the α and β forms are called anomers.
A reducing sugar is one that can react because they have a relatively reactive hemiacetal group at C-1 position. Examples include: glucose, fructose, lactose, and maltose. The anomeric carbon in all of these molecules is free to react.
A non-reducing sugar is one that does not react, such as sucrose. The acetal group at the C-1 position makes the sugar non-reactive. Their structures are modified, so that they do not have free aldehyde or ketone groups to react. In sucrose, neither of the monosaccharides in the disaccharide can easily change into an aldehyde or ketone, making it nonreactive, this non-reducing. The glycosidic bond in the disaccharide hinders the molecule from being reactive. The anomeric carbon is not free to react. In order to determine whether or not a sugar is reducing, a Fehling's or Tollen's test is performed. In the Fehling's test a brick red precipitate is the positive result, and in the Tollen's test a silver mirror is the positive result.
In contrast, when a sugar is oxidized, the aldehyde or ketone carbonyl becomes a carboxyl group.
It is called an O-glycosic bond if the anomeric carbon is attached to an oxygen atom of a hydroxyl group. It is called an N-glycosidic bond if the anomeric bond is attached to a nitrogen atom of a amine group.
Glycosidic bonds are also what form the bridges between monosaccharides. If monosaccharides are joined by O-glycosidic bonds, they are called oligosaccharides.
The difference in having an -OH group attached to Carbon 2' of the sugar is the difference between DNA and RNA. In RNA, the carbon 2' contains an -OH group, whereas in the carbon 2' of DNA, there is just a hydrogen attached. The sugar in RNA, or "ribonucleic acid" is "ribose" while the sugar for DNA or "DEOXIribonucleic acid" is "deoxiribose." DEOXI- is used to represent the lack of oxygen from the -OH group on Carbon 2' of ribose. |||
Importance of sugar in glycoproteins
Sugar attached proteins called glycoproteins is another important component of the cell. Sugar components are oriented toward the watery cell exterior of glycoproteins. These sugar components serve as an identifier like cellular address labels. When signaling molecules pass through bodily fluids they encounter certain patterns of sugars, which either gives them access or dismissal. Therefore, the glyoproteins act as a regulator or gatekeeper in cells. In addition they help direct the formation of organs and tissue by forming correct cells together. Sugar coatings also help cells move through blood vessels by providing traction by latching on cell surface receptors.
Davis, Alison. "The Chemistry of Health." 'NIGMS August 2006: 36-42. http://publications.nigms.nih.gov/chemhealth/coh.pdf
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