Carbohydrates - Biochemistry Notes

Module 2: Carbohydrates - Biochemistry

Objectives

Identify and classify carbohydrates based on their types, structures, physical and chemical properties, reactions, and characteristics.
Classify carbohydrates based on their chemical nature.
Correlate the physical and chemical properties of carbohydrates with their function.
Enumerate the importance and physiologic relevance of carbohydrates.
Compare the structures and reactions of disaccharides and polysaccharides.
Illustrate the structures and reactions of biochemically important monosaccharides.

What are Carbohydrates?

Carbohydrates are also called “sugars” and “starches”.
They are synthesized in green plants through photosynthesis, a process that uses energy from the sun to convert carbon dioxide and water into glucose and oxygen.
In the body, they are used for bursts of energy needed during exercise in the form of glucose.
$Cn(H2O)_n$ = “hydrates of carbon”

Photosynthesis

Energy is stored in photosynthesis.
$6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2$
Reactants: Sunlight, Carbon dioxide, Water
Products: Oxygen, Sugars

Monosaccharides, Disaccharides, Oligosaccharides, and Polysaccharides

Monosaccharide: Building blocks of carbohydrates are polyhydroxy aldehydes or ketones; that is, they are molecules with more than one hydroxyl group, and a carbonyl group either at the terminal carbon atom (aldose) or at the second carbon atom (ketose).
Disaccharide: Composed of two molecules of simple sugars. The monosaccharides within them are linked by a glycosidic bond (or glycosidic linkage).
Oligosaccharide: Any carbohydrate of from three to ten units of simple sugars. A large number of oligosaccharides have been prepared by partially breaking down more complex carbohydrates (polysaccharides).
Polysaccharide: Also called glycan, the form in which most natural carbohydrates occur. One sugar or one sugar derivative is called homopolysaccharides (homoglycans). Derivatives of more than one sugar are called heteropolysaccharides (heteroglycans).

Monosaccharides

Classified by type of carbonyl group and number of carbon chains present.

Type of Carbonyl Group

Aldose: A monosaccharide containing an aldehyde group.
Ketose: A monosaccharide containing a ketone group.

Number of Carbon Chains

[aldo or keto] + [Greek numerical prefix corresponding to number of carbons] + -ose
- Triose
- Tetrose
- Pentose
- Hexose

Chirality

A carbon atom in a molecule with four different groups bonded to it.
All carbohydrates except dihydroxyacetone have one or more chirality centers.

Fischer Projection

In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.
The configuration of the chirality center farthest from the carbonyl group determines whether a monosaccharide is D or L.
Glucose has four chirality centers.
All naturally occurring sugars are D sugars.
- OH on the right = D-sugar (naturally occurring enantiomer)
- OH on the left = L-sugar (unnatural enantiomer)

Examples of Monosaccharides

Ketopentose
- D-Ribulose
- D-Xylulose
Ketohexose
- D-Psicose
- D-Fructose
- D-Sorbose
- D-Tagatose
Aldotetrose
- D-Erythrose
- L-Erythrose
- D-Threose
- L-Threose
Aldopentose
- D-Ribose
- D-Arabinose
- D-Xylose
- D-Lyxose
Aldohexose
- D-Allose
- D-Altrose
- D-Glucose
- D-Mannose
- D-Gulose
- D-Idose
- D-Galactose
- D-Talose

Monosaccharides

D-Galactose
- “Brain sugar”
- One of the components of the disaccharide lactose.
- Patients with galactosemia lack an enzyme needed to metabolize galactose, which accumulates and causes cataracts and cirrhosis.
- Also present in the chemical markers that distinguish various blood types: A, B, AB, O.
D-Fructose
- Levulose and “fruit sugar”
- One of the components of the disaccharide sucrose.
- Sweetest-tasting of all sugars.
- Ketohexose found in honey and almost twice as sweet as table sugar with the same number of calories per gram.
D-Ribose
- A component of a variety of complex molecules, including ribonucleic acids (RNAs) and energy-rich compounds such as adenosine triphosphate (ATP).
- The compound 2 - deoxy - D- ribose is also important in nucleic acid chemistry. This monosaccharide is a component of DNA molecules.

Haworth Projection

Cyclic Forms of Monosaccharides

Tautomerism: A phenomenon where a single chemical compound tends to exist in two or more interconvertible structures that are different in terms of the relative position of one atomic nucleus which is generally the hydrogen.
Hemiacetal Formation: Aldehydes/Ketones react with alcohol to give hemiacetals (an alcohol and ether attached to the same carbon).
Intramolecular Hemiacetal Formation: If the aldehyde (or ketone) are attached in a same chain, then an intramolecular hemiacetal formation will happen. Cyclic hemiacetals form readily when hydroxyl and carbonyl groups are part of the same molecule, and their interaction can form a five- or six-membered ring.

Anomers

An anomeric carbon atom is the hemiacetal carbon atom present in a cyclic monosaccharide structure.
It is the carbon atom that is bonded to an OH group and to the oxygen atom in the heterocyclic ring.
Cyclic monosaccharide formation always produces two stereoisomers —an alpha form and a beta form. These two isomers are called anomers.
Anomers are cyclic monosaccharides that differ only in the positions of the substituents on the anomeric (hemiacetal) carbon atom. α and ß anomers.

Pyranose and Furanose

A cyclic monosaccharide containing a six-atom ring is called a pyranose.
A five-atom ring is called furanose because their ring structures resemble the ring structures in the cyclic ethers pyran and furan.
Aldohexose
- Pyranose: -OH of C5 to C1
- Furanose: -OH of C4 to C1
Ketohexose (Hemiketal)
- Pyranose: -OH of C6 to C2
- Furanose: -OH of C5 to C2

Disaccharides, Oligosaccharides, Polysaccharides

Oxidation to Produce Acidic Sugars

Monosaccharide oxidation can produce acidic sugars.
Weak oxidizing agents, such as Tollens and Benedict’s solutions, oxidize the aldehyde end of an aldose to give an aldonic acid.
Oxidation of the aldehyde end of glucose produces gluconic acid
Oxidation of the aldehyde end of galactose produces galactonic acid.
With Tollens solution, glucose reduces $Ag^+$ ion to Ag.
With Benedict’s solution, glucose reduces $Cu^{2+}$ ion to $Cu^+$ ion.
A reducing sugar is a carbohydrate that gives a positive test with Tollens and Benedict’s solutions.
Strong oxidizing agents can oxidize both ends of a monosaccharide at the same time (the carbonyl group and the terminal primary alcohol group) to produce a dicarboxylic acid.
Such polyhydroxy dicarboxylic acids are known as aldaric acids.
For glucose, this oxidation produces glucaric acid.

Reduction to Produce Sugar Alcohol

The carbonyl group present in a monosaccharide (either an aldose or a ketose) can be reduced to a hydroxyl group, using hydrogen as the reducing agent.
For aldoses and ketoses, the product of the reduction is the corresponding polyhydroxy alcohol, which is sometimes called a sugar alcohol.
The reduction of D - glucose gives D - glucitol. It has a common name of D - sorbitol.

Phosphate Ester Formation

The hydroxyl groups of a monosaccharide can react with inorganic oxyacids to form inorganic esters.
Esterification of the hemiacetal group (carbon 1) and the primary alcohol group (carbon 6) in glucose produces the compounds glucose 1- phosphate and glucose 6- phosphate, respectively.

Glycoside Formation

The general name for monosaccharide acetals is glycoside.
A glycoside is an acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon OH group with an OR group.
Glycoside produced from glucose is called a glucoside.
For galactose, it is called a galactoside.

Disaccharides

A carbohydrate in which two monosaccharides are bonded together.
In disaccharide formation, one of the monosaccharide reactants functions as a hemiacetal, and the other functions as an alcohol.

Glycosidic Linkage

The bond that links the two monosaccharides of a disaccharide (glycoside) together is called a glycosidic linkage.
A glycosidic linkage is the bond in a disaccharide resulting from the reaction between the hemiacetal carbon atom OH group of one monosaccharide and an OH group on the other monosaccharide.

Maltose

Often called malt sugar.
Structurally, maltose is made up of two D - glucose units, one of which must be an alpha- D- glucose.
The glycosidic linkage between the two glucose units is called α(1 : 4) linkage. The two OH groups that form the linkage are attached, respectively, to carbon 1 of the first glucose unit and to carbon 4 of the second.
The most important chemical reaction of maltose is that of hydrolysis. Hydrolysis of D - maltose produces two molecules of D - glucose. Acidic conditions or the enzyme maltase is needed for the hydrolysis to occur.
$D-Maltose + H_2O \rightarrow D-Glucose + D-Glucose$

Cellobiose

Produced as an intermediate in the hydrolysis of cellulose.
It contains two D - glucose monosaccharide units. It differs from maltose in that one of the D - glucose units —the one functioning as a hemiacetal —must have a beta configuration.
For the hydrolysis to occur, the enzyme cellobiase is needed to produce two units of D - Glucose.
$D-Cellobiose + H_2O \rightarrow D-Glucose + D-Glucose$

Lactose

The major sugar found in milk; common name is milk sugar.
Important ingredient in commercially produced infant formulas that are designed to simulate mother’s milk.
Lactose intolerance is the inability of the human digestive system to hydrolyze lactose.
Made up of a β- D- galactose unit and a - D- glucose unit joined by a β(1 : 4) glycosidic linkage.
Lactose can be hydrolyzed by acid or by the enzyme lactase, forming an equimolar mixture of galactose and glucose.
$D-Lactose + H_2O \rightarrow D-Galactose + D-Glucose$

Sucrose

Also known as common table sugar
Most abundant of all disaccharides.
Produced commercially from the juice of sugar cane and sugar beets.
The two monosaccharide units present in a D - sucrose molecule are alpha- D- glucose and beta - D- fructose.
The glycosidic linkage is an alpha, β(1 : 2) glycosidic linkage.
Sucrase, the enzyme needed to break the α, β(1 : 2) linkage in sucrose.
Sucrose hydrolysis (digestion) produces an equimolar mixture of glucose and fructose called invert sugar.
$D-Sucrose + H_2O \rightarrow D-Galactose + D-Fructose$

Naming Disaccharides

The first sugar should contain the non-reducing end (anomeric: acetal or ketal carbon).
Find the anomeric carbon of the first sugar then identify the configuration ( α, β)
- Check the $CH_2OH$ and anomeric OH.
  - If same side: β
  - If opposite side: α
To the first sugar, add the suffixes:
- -pyranosyl if 6-membered ring
- -furanosyl if 5-membered ring
Use the notation (N1→ N2) which indicate the location of glycosidic bond
- N1 is the carbon # of the 1st sugar attached to the glycosidic bond
- N2 is the carbon # of the 2nd sugar attached to the glycosidic bond
Identify the configuration of the anomeric carbon in the 2nd sugar
- Only optional if the anomeric carbon of the 2nd sugar is free (reducing sugars) since these sugar exist in equilibrium in different configuration
Add the suffix –pyranose or –furanose
- If the anomeric carbon is part of the glycosidic linkage, use an additional suffix –oside, hence –pyranoside or –furanoside.
For non-reducing sugars, two names can be used since both sugar have non - reducing end which mean either of the two sugar can be the first sugar or the second sugar.
Example: Sucrose can be named as
- α- D- gluco pyranosyl - (1→2)- β- D- fructo furanoside
- β- D- fructo furanosyl - (2→1)- α- D- gluco pyranoside
Example:
- 1ST Sugar: Galactose
- 6- membered ring: galactopyranosyl
- Galactose: Anomeric OH is on the same side with $CH_2OH$ , thus β
- Glycosidic bond is located on C1 of the 1st sugar and C4 of the 2nd sugar
- 2nd Sugar: Glucose
- Anomeric carbon is not involved, thus glucopyranose
- β-D-galactopyranosyl-(1→4)-glucopyranose
Example:
- 1ST Sugar: glucose
- 6- membered ring: gluco pyranosyl
- Glucose: Anomeric OH is on the opposite side with $CH_2OH$ , thus α
- Glycosidic bond is located on C 1 of the 1st sugar and C2 of the 2nd sugar
- 2nd Sugar: fructose
- Anomeric carbon is involved, and on the same side with $CH_2OH$ thus β and fructo furanoside
- α- D- gluco pyranosyl - (1→2)- β- D- fructo furanoside

Oligosaccharides

Oligosaccharides are carbohydrates that contain three to ten monosaccharide units bonded to each other via glycoside linkages.
There are two naturally occurring oligosaccharides found in onions, cabbage, broccoli, brussel sprouts, whole wheat, and all types of beans, these are the trisaccharide raffinose and the tetrasaccharide stachyose.

Raffinose

Trisaccharide composed of galactose, glucose, and fructose.

Stachyose

Tetrasaccharide composed of two galactose units, one glucose unit, and one fructose unit.

Other Oligosaccharides

One of the monosaccharides present in the oligosaccharides associated with blood type is a 6 - deoxy - L- monosaccharide.
Solanine, a compound found in the potato plant. Potato plants produce toxins as a defense against insects and predators. Solanine is the potato plant’s toxin.
The trisaccharide present in solanine’s structure involves two common monosaccharides (D - glucose and D - galactose) and a rarely encountered monosaccharide (L - rhamnose).

Polysaccharides

A polysaccharide is a polymer that contains many monosaccharide units bonded to each other by glycosidic linkages.
Polysaccharides are often also called glycans.
Homopolysaccharide is a polysaccharide in which only one type of monosaccharide monomer is present.
Heteropolysaccharides contain structures with two or more types of monosaccharide monomers.
The identity of polysaccharides depends on:
1. repeating unit(s) in the polymer
2. length of the polymer chain
3. type of glycosidic linkage between monomer units
4. degree of branching of the polymer chain

Storage Polysaccharides

Storage form for monosaccharides
Used as an energy source in cells.
The most important storage polysaccharides are starch (in plant cells) and glycogen (in animal and human cells).

Starch

A homopolysaccharide containing only glucose monosaccharide units.
Amylose and Amylopectin are the two different polyglucose polysaccharides that can be isolated from most starches.
Iodine is often used to test for the presence of starch in solution. Starch-containing solutions turn a dark blue-black when iodine is added. As starch is broken down through acid or enzymatic hydrolysis to glucose monomers, the blue-black color disappears.

Glycogen

Similar to starch, it is a polysaccharide containing only glucose units. Also known as animal starch.
Liver cells and muscle cells are the storage sites for glycogen in humans.
Glycogenesis - formation of glycogen
Glycogenolysis - decomposition of Glycogen
Consists of α (1 → 4) and α (1 → 6)

Structural Polysaccharides

A structural polysaccharide is a polysaccharide that serves as a structural element in plant cell walls and animal exoskeletons.
Two of the most important structural polysaccharides are cellulose and chitin.
Both are homopolysaccharides.

Cellulose

The structural component of plant cell walls.
Cellulose chains contain about 5000 glucose units.
Cotton is almost pure cellulose (95%)
Wood is about 50% cellulose.
Consists of β(1 → 4)

Chitin

The second most abundant naturally occurring polysaccharide.
Its function is to give rigidity to the exoskeletons of crabs, lobsters, shrimp, insects, and other arthropods.
Also found in the cell walls of fungi.

Acidic Polysaccharides

An acidic polysaccharide is a polysaccharide with a disaccharide repeating unit in which one of the disaccharide components is an amino sugar and one or both disaccharide components has a negative charge due to a sulfate group or a carboxyl group.
Acidic polysaccharides are heteropolysaccharides;
Two of the most well-known acidic polysaccharides are hyaluronic acid and heparin.

Hyaluronic Acid

Contains alternating residues of N - acetyl - beta - D- glucosamine (NAG) and D- Glucuronate.
Highly viscous hyaluronic acid solutions serve as lubricants in the fluid of joints.
They are also associated with the jelly-like consistency of the vitreous humor of the eye.

Glycolipids and Glycoproteins

Glycolipids

Has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to a lipid.
Are called cerebrosides and gangliosides in the brain tissue.

Glycoproteins

Has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to a protein.
Are called immunoglobins are key components of the body's immune system response to invading foreign materials
The lipid or protein part of the glycolipid or glycoprotein is incorporated into the cell membrane structure and the carbohydrate part functions as a marker on the outer cell membrane surface.