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Carbohydrate Structure and Function
Biochemistry/Lab (CHEM 3650)
Nova Southeastern University
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Carbohydrate Structure and Function Lecture notes (Can be used for MCAT review too)
Carbohydrate Classification
Nomenclature
Most basic structural unit of a carbohydrate is a monosaccharide o Simplest monosaccharides contain three carbon atoms and are called trioses. o 4 Carbons – tetroses; 5 Carbons – pentoses; 6 Carbons - hexoses Aldoses: Carbohydrates that contain an aldehyde group as their most oxidized functional group Ketoses: carbohydrates that contain a ketone as their most oxidized function group E. – a six carbon sugar with an aldehyde group would be called aldohexose or a five- carbon sugar with a ketone group would be called ketopentose Carbon atoms in a monosaccharide are numbered as described in organic chem. o In an aldose, the aldehyde carbon will always have the lowest carbon number This carbon can participate in glycosidic linkages Glyceraldehyde: simplest aldose sugar. An example of the above rules Dihydroxyacetone: simplest ketone sugar o Carbonyl carbon is the most oxidized, but the lowest possible number for it is only 2. For most ketoses, the carbonyl carbon is C-2. In monosaccharides, every carbon other than the carbonyl carbon will carry a hydroxyl group Common Names that should be remembered
Stereochemistry
Optical Isomers: compounds that have the same chemical formula, but differ from one another only in terms of the spatial arrangement of their component atoms o Enantiomers: stereoisomers that are non-identical, non-superimposable mirror images of each other. Chiral carbon atom is one that has four different groups attached to it Any molecule that contains chiral carbons and no internal planes of symmetry has an enantiomer.
Absolute Configuration: three-dimensional arrangement of the groups attached to the chiral carbon o (R) & (S) system is the newer one. L & D is the older system As the number of chiral carbons increases, the number of possible stereoisomers also increases. (n is the number of chiral carbons):
Optical Rotation of Sugars: Is denoted by a “+” or “-” sign for positive and negative rotations o These rotations must be determined experimentally o D-glyceraldehyde has a positive rotation while L-glyceraldehyde has a negative Fischer Projection: allows scientists to easily identify different enantiomers o Vertical lines represent going into the page (dashed lines) o Horizontal lines represent going out of the page (wedges) o All D-sugars have the hydroxide on the right, and L-sugars have the hydroxide on the right
Important Concepts
Enantiomers: same sugar but in different optical families Diastereomers: Two sugars that are in the same family (same number of carbons, both are either ketoses or aldoses), but are not identical and not mirror images of each other. Epimers: Subtype of diastereomers where the only change in configuration between two sugars is at exactly one chiral center.
Cyclic Sugar Molecules
Monosaccharides contain both a hydroxyl group (nucleophile) and a carbonyl group (most common electrophile) o Can undergo intramolecular reaction to form cyclic hemiacetals (from aldoses) and cyclic hemiketals (from ketoses). Only five-membered (furanose) and six-membered (pyranose) rings are stable in solution since other configurations induce too much ring strain. o Hydroxyl group acts as the nucleophile during ring formation, so oxygen becomes a member of the ring structure o Carbonyl carbon becomes chiral in this process and is known as the anomeric carbon
Monosaccharides
Contains alcohols and either aldehydes or ketones
Oxidation and Reduction
One of the most important reactions since the oxidation of carbohydrates yields energy As a monosaccharide switches between its anomeric configurations, the hemiacetal rings spend a short period of time in the open chain aldehyde form. o This aldehyde form can be oxidized to carboxylic acid o Aldonic acids: oxidized aldoses (reducing agents) Any monosaccharide with a hemiacetal ring is considered a reducing sugar If the aldose is in ring form when it is oxidized, it yields lactone o These play an important role in our body (e. – Vitamin C) Can use two standard reagents to detect the presence of reducing sugars: o Tollen’s Reagent: When reduced, produces a silvery mirror if aldehydes are present Silver oxide dissolved in ammonia, must be freshly prepared o Benedict’s Reagent: aldehyde group is readily oxidized which can be indicated by a red precipitate Ketose sugars can also be considered reducing sugars even though ketones cannot be oxidized directly to carboxylic acids o Instead the ketones tautomerize to form aldoses under basic conditions o Tautomerization is the rearrangement of bonds in a compound. Usually done by moving a hydrogen and forming a double bond In this case, ketone group picks up a hydrogen while a double bond is moved between two adjacent carbons, which results in an enol (compound with a double bond and an alcohol group) Alditol: When aldehyde group of an aldose is reduced to an alcohol Deoxy Sugar: contains a hydrogen group that replaces a hydroxyl group on the sugar
Esterification
Carbohydrates can participate in reactions with carboxylic acids and its derivatives since it has a hydroxyl group. Esterification in the body is very similar to the phosphorylation of glucose (chapter 9)
Glycoside Formation
Hemiacetals react with alcohols to form acetals Anomeric hydroxyl group is transformed into an alkoxy group which yields a mixture of alpha and beta acetals o Resulting C-O bonds are called glycosidic bonds and the acetals formed are glycosides
Can also be formed when hemiketals form ketals
Disaccharides and polysaccharides are formed as a result of glycosidic bonds between monosaccharides. Glycosides from furanose rings are called furanosides, while those derived from pyranose rings are called pyranosides
Complex Carbohydrates
Disaccharides
Glycosidic bonds forming between two monosaccharides Nonspecific linkage since the anomeric carbon of a cyclic sugar can react with any hydroxyl group of any other sugar molecule Configurations are named after the carbon numbers that are participating in the linkage Anomer configuration should also be specified if two anomers are different. E - - 1,6 glycosidic bond formation between two D-glucose molecules, the alpha anomeric carbon of the first glucose attaches to C-6 of the second glucose Important Disaccharides Sucrose, lactose and maltose are commonly produced in the cell by enzymatic activity
Polysaccharides
Long chains of monosaccharides that are linked by glycosidic bonds Three most important are cellulose, starch and glycogen
Carbohydrate Structure and Function
Course: Biochemistry/Lab (CHEM 3650)
University: Nova Southeastern University
