chapter 7: Carbohydrates and the Glycoconjugates of Cell Surfaces

How Are Carbohydrates Named?

Carbohydrates are hydrates of carbon with the general formula (CH2O)n

• Monosaccharides (simple sugars) - cannot be broken down into simpler sugars under mild conditions

• Oligosaccharides (oligo = "a few“) - usually 2 to 10 simple sugar residues

• Polysaccharides - are polymers of the simple sugars

Carbohydrate Classification

Aldose vs. Ketose

Monosaccharides are either aldoses or ketoses
C3H6O3

ketone- middle C double O

aldone- end C double O (ALl the way to the end)

Carbons Give Naming

Numbering the Carbons

Carbonyl (C=O) gets preference as lowest number

Stereochemistry of sugars

1. Fischer Projection: linear representation

2. D=OH to the right L=OH to the left of highest # carbon

ex. D- and L-Forms of a Monosaccharide are Mirror Images

Unique D-Forms of a Monosaccharide are given different names

• All distinct monosaccharides with the same #C’s are DIASTEREOMERS

• Diastereomers that differ at only 1 chiral center are EPIMERS

Stereochemistry: Putting it together

• D,L designation refers to the configuration of the highest-numbered asymmetric center

• D,L DO NOT specify the sign of rotation of plane- polarized light

• D-sugars predominate in nature

• Stereoisomers that are mirror images of each other are enantiomers

• Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers

• Two sugars that differ in configuration at only one chiral center are epimers

Cyclic Monosaccharides

• Sugars generally cyclize for stability

Hemiacetal and Hemiketal

Formation of Pyranoses

Anomeric carbon: the chiral (formerly achiral) carbon created due sugar cyclization
Anomer: epimeric, cyclic monosaccharide

• Note α and β designations. α: points “down” WRT Haworth & trans to C6; β :
  points “up” WRT Haworth & cis to C6

Fischer vs. Haworth

Pyranose vs. Furanose

F for five

P for six?

Fru- 5 attacks 2

Pyr-5 attacks 1

Formation of Furanoses

Pyranose Structures: Boat vs. Chair

• Chair conformation: generally, more stable

Anomers: directionality matters!

A simple rule:

• For D-sugars, α has OH down, β up in the customary Haworth drawing

• For L-sugars, the reverse is true

alpha down D-sugar

Anomeric carbon: the chiral (formerly achiral) carbon created due sugar cyclization
Anomer: epimeric, cyclic monosaccharide

Monosaccharides Can Be Converted to Several Derivative Forms

Some of the most common are:
– Sugar acids
– Sugar alcohols
– Deoxy sugars
– Sugar esters
– Amino sugars
– Acetals, ketals, and glycosides

Sally Sucked D**k And Got Killed After

Sugar acids: sugars with carboxylic acid group

• at least one COOH (acid)

Sugar acids: formation via Fehling’s Test

Reducing Sugars

• Reducing sugars are generally aldoses with a free aldehyde

• In polysaccharides, must have a free anomeric carbon to be reducing sugar

Sugar alcohols: sugars with reduced R=O groups

• Linear molecules; formed via mild reductions

Sugar Alcohol Formation

Deoxy Sugars: sugars with lost ROH groups

DNA vs. RNA

Sugar Esters: sugars that have ester bonds

• most common type of sugar ester: phosphoester!

Amino Sugars: sugars with amino groups

• Amino groups most commonly found at the C2 position

Muramic and Sialic acids: highly modified sugars

Have multiple types of derivatizations; important for cell-surface recognition

Acetals, Ketals and Gylcosidic Bonds

• Glycosidic bonds are the keys to making complex di/poly-saccharides

Glycosides: sugars bound to another functional group

• the pyranose and furanose forms of monosaccharides react with alcohols in dehydration synthesis reactions to form glycosides, with retention of the α- or β-
  configuration at the C-1 carbon.

• The new bond formed is called a glycosidic bond.

• This is the process by which oligosaccharides are formed

Glycosidic Bonds

Numbering and type of linkage related to anomeric carbon:

• Retention of a/B designations

• Numbers related to monosaccharide numbering (must be specified)

• Stereochemistry of all anomeric carbons must be specified!

Disaccharides

• Disaccharides are the simplest oligosaccharides!

Reducing Sugars: Reminder

• Provides directionality of the oligosaccharide (like proteins)

Sucrose: a tricky example

When two anomeric carbons are involved, you must name both!

Trehalose: another tricky example

What is the difference between the structure and reactivity of
reducing and non-reducing sugars?

• In polysaccharides, must have a free anomeric carbon to be reducing sugar

chat:

• Reducing sugars: Have a free anomeric carbon (can open into a linear form with a reactive carbonyl group), so they can be oxidized and give positive Benedict’s or Tollens’ tests. Examples: glucose, fructose.

• Non-reducing sugars: Have their anomeric carbon(s) locked in a glycosidic bond, so they cannot open into a reactive carbonyl form and do not give positive reducing-sugar tests. Example: sucrose.