Biochemistry Lecture No.13: Carbohydrates
Thursday October 4 , 2012
Four Main Classes Of Biomolecules:
-The four main building blocks of the cell are: sugars, fatty acids, amino acids, and nucleotides. Their
larger polymers are respectively: polysaccharides, fats, lipids and membranes, proteins, and nucleic
acids. The group that encompasses monosaccharides and polysaccharides is referred to as
-The general formula for all monosaccharides is (CH O) 2 Tney are always single sugar unit molecules and
range in n values from 3 to 7. Notable examples of monosaccharides include glyceraldehyde (triose),
ribose (pentose) and glucose (hexose) sugars.
-Monosaccharides are often represented by Fischer projections, whereby sugar molecules with the
same number carbons (same formula) are arranged in a different stereochemistry (as enantiomers).
They (the D and L sugars) are chiral molecules due to the different spatial arrangements of atoms. D and
L sugars can alter the direction of plane-polarized light as well.
-Most biological monosaccharides are in the D orientation at the highest numbered chiral carbon. This is
determined by starting at the end that is closest to the double bonded oxygen and numbering each
individual carbon noting any chirality. D isomers have the hydroxyl group on the right at the highest
chiral carbon (All of the sugars studied in this course will exhibit D-isomerism).
-Hexoses that contain an aldol group (are aldehydes) are known as aldoses. E.g. glucose, galactose,
mannose. Hexoses that contain a keto group (are ketones) are known as ketoses. E.g. fructose.
-Most monosaccharides form ring structures and are most prominently present in that form (not linear).
Rings in monosaccharides (particularly hexoses) result from a nucleophilic attack by the hydroxyl group
(OH) on carbon 1. This results in the joining of carbon 1 and carbon 5 by oxygen (hydroxyl becomes a
doubly-bonded oxygen and former oxygen double-bonded to carbon 1 becomes a hydroxyl group).
-Cyclized monosaccharides are represented best by Haworth projections, which depict the cyclized
monosaccharide in a chair conformation. Haworth projections are not exactly planar. Anomeric Carbon:
-A monosaccharide can form one of two different structures when it cyclizes. The anomeric carbon can
have its hydroxyl group either opposite (alpha) or adjacent (beta) to the other oxygen in the ring.
Because the ring can often open and close periodically, the two (alpha and beta) cyclic monosaccharide
conformations are freely interconverting structures. Other carbons cannot interconvert.
Cyclization Of Fructose:
-In the cyclization of fructose (a ketose), the hydroxyl group on carbon 5 performs a nucleophilic attack
carbon 2. The alpha and beta forms are dependent upon the anomeric carbon’s structural qualities. The
position of the OH is important for the interconversion between alpha and beta conformations.
Pyranoses Vs. Furanoses:
-6-membered monosaccharide rings are referred to as pyranoses, while 5-membered monosaccharide
rings are referred to as furanoses. These structures are only classified according to the number of atoms
in the ring; it is independent of the number of carbons that actually exist in the molecule.
-Monosaccharides can be joined together to form disaccharides. This occurs through a condensation
reaction whereby water helps to form O-glycosidic bonds (a β 1 4 linkage). The orientation of the