The Three Main Classes of Molecules Metabolized by Our Bodies

Notes Carbohydrates

Introduction:

Carbohydrates are normally the first classification of compounds to be discussed which fall within the realm of Biochemistry. All biological systems are run on chemical reactions: some are easy to see, such as the iron ion needed to hold oxygen in red blood cells. The positively charged iron attracts the lone pairs of electrons on the oxygen molecule. HCl in the stomach breaks down many large molecules allowing the small intestines digest more manageable pieces.

The three main classes of molecules metabolized by our bodies:

Carbohydrates (sugars)
Lipids (fats)
Proteins (amino acids)

Carbohydrates are defined as sugars and their derivatives. Animals (such as humans) break down carbohydrates during the process of metabolism to release energy. For example, the chemical metabolism of the sugar glucose is shown below:

glucose + oxygen ® carbon dioxide + water + energy

C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O + Energy

Animals obtain carbohydrates by eating foods that contain them, for example potatoes, rice, breads, etc. These carbohydrates are manufactured by plants during the process of photosynthesis. Plants harvest energy from sunlight to run the reaction described above in reverse:

6 CO2 + 6 H2O + energy (from sunlight) ® C6H12O6 + 6 O2

A potato, for example, is primarily a chemical storage system containing glucose molecules manufactured during photosynthesis. In a potato, however, those glucose molecules are bound together in a long chain. As it turns out, there are two types of carbohydrates, the simple sugars and those carbohydrates that are made of long chains of sugars - the complex carbohydrates.

The simplest carbohydrates are the monosaccharide, a single unit simple sugar.

The most common monosaccharide is glucose, and this is the most important one for living organisms.

Metabolism:

Processes require energy. The term metabolism is associated with energy. This is just one aspect of metabolism.

Metabolism more specifically refers to a sequence of chemical reactions used to produce one or more products or accomplished one or more processes.

Returning to energy, per gram fats provide the most energy, carbohydrates provide the next most and proteins provide the least energy. The energy of carbohydrates is the most quickly utilized. Think about a 4 year old after sneaking into their Halloween candy bag. They are full of energy!

Structure of Carbohydrates:

Lets break down the word carbohydrate. Carbo = carbon and hydrate = water leading one to believe carbohydrates are hydrates of carbon. Remember a hydrate is a compound which has water loosely attached. An example would be FeCl3 · 6 H2O. This is iron(III) chloride hexahydrate. Each FeCl3 salt molecule has absorbed 6 water molecules. These are not chemically bound and can be removed by heating leaving FeCl3 and H2O. Since the chemical formulas are unchanged there has been no chemical reaction, it has undergone a physical process.

From the above carbohydrate ribose, it should be easy to see why the products of heating carbohydrates is water and a black soot, carbon. When heated the OH groups combine with their associated H and form water, leaving elemental carbon, a black soot. But these are clearly chemical bonds and not hydrates of water. By the way, the R in RNA is ribose.

If you look at the structure of the saccharides you will find they are either an aldehyde or a ketone. Carbohydrates are either polyhydroxy aldehydes or polyhydroxy ketones. Remember, poly means many, hydroxy refers to –OH groups and that the carbonyl carbon is either the terminal carbon, therefore an aldehyde, or it is not a terminal carbon, therefore a ketone. The aldehyde saccharides are called aldose and the ketone saccharides are called ketose.

The ribose above is an aldehyde. The carbonyl is the terminal carbon.

examples:

The structure of saccharides shows them to contain stereocenters. Even the most simple saccharides, glyceraldehydes have a stereocenters on its central carbon. The following section is a review of stereo chemistry.

Isomers: Stereoisomers

The stereoisomer is a special type geometric isomer. The connectivity is the same between two stereoisomers, the difference is their arrangement in 3-dimensional space. They are mirror images of each other. They are not super imposable. Your hands are mirror images of each other, they are the same, but opposites. Another term used is chiral, this would be used to describe a carbon that is a stereocenter, the chiral carbon, or a molecule with a stereocenter is a chiral molecule.

This phenomenon only occurs around carbon atoms that have 4 different connections. The example below shows 4 different atoms, but chains of atoms, for instance a methyl group, could be substituted for these individual atoms. See the second image down.

Example:

One of the following three carbons is chiral, which is it and why are the other two not chiral?

Example A is the chiral carbon. Both B and C do not have 4 different connections. Example B has 2 hydrogen atoms attached to the carbon and example C has 2 methyl groups attached to the carbon.

When this occurs the two mirror images are called enantiomers. The two molecules will have the same name except for a prefix(s). These prefixes originate in one of the properties of compounds with stereocenters, rotation of plane polarized light or in the convention of drawing the structures.

This may not seem like an important point, but as it turns out, even though these two molecules’ structures are almost exactly the same, they do react differently. Sometimes with terrible consequences.

To determine the number of isomers a compound will form:

Count the number of stereocenters in the molecule
Take 2 to that power

· The ribose molecule to the right has 3 stereocenters.

· 23 = 8

· Ribose will have 8 different isomers.

Carbohydrate Polymer:

Carbohydrates are also referred to as saccharides. Saccharides can be found in several forms.

single / monosaccharide
pair / disaccharide
many / polysaccharide

monosaccharide

· basic unit of metabolism

· normally 3, 4, 5, 6 or 7 carbons in length

· classified as aldose or ketose

· classified as D or L isomers based on the stereochemistry

disaccharide

· use to transport monosaccharides

· water soluble – as they are short hydrocarbon chains

· are sweet to taste

· sucrose, galactose and lactose

polysaccharides

· structure of plants – cellulose

· storage of monosaccharides

The D and L prefixes are generated by the convention of drawing the structures. If the -OH group on the carbon before the terminal carbon is on the left it is designated with an L; if the -OH group on the carbon before the terminal carbon is on the right it is designated with an D. You may ask why the -OH group on the carbon before the terminal carbon, why not use the terminal carbon? Well the terminal carbon will not be a stereo center, there will normally be 2 hydrogens on that terminal carbon.

So, if you draw these monosaccharides vertical with the carbonyl carbon on the top or as close to the top as possible it will make identifying them easier.

These structures are normally drawn in a more simplified manner, because as we know chemists are lazy. Below you will find a drawing of a Fischer projection along side its equivalent structures. The Fischer projection is named after Emil Hermann Fischer, winner of the 1902 Nobel Prize in Chemistry.

This structure is an L enantiomer. The –OH on the carbon before the terminal carbon is on the left.

What is the name of this structure? We already have the first piece, L. The carbon chain is 5 carbons long, therefore it is a pent. Lastly, the carbonyl carbon is an aldehyde.

L-aldopentose

Draw the Fischer Projections for lactic acid given the following structure:

Example:

Importance of Carbohydrates:

· Very effective energy yield

o contains carbon

o has a reactive bond – carbonyl carbon and is a polar area

o does not have 4 bonds to oxygen – which means the carbon is organic carbon, remember that organic carbon is carbon with an low oxidation number, once the oxidation number becomes + 4 it can no longer be oxidized

· Effective building material

o strong not brittle – will bend and not break

· H2O soluble

o easily transported thru the blood stream

o easily passes thru cell walls

· Sugars are carbohydrates.

· Sucrose was used as the standard, all other sugars sweetness is based on sucrose.

carbohydrate / relative
sweetness / class / common
name
sucrose / 1.00 / disaccharide / table sugar
lactose / 0.16 / disaccharide / milk sugar
maltose / 0.32 / disaccharide / malt sugar
glucose / 0.74 / monosaccharide / blood sugar
galactose / 0.22 / monosaccharide / -
fructose / 1.74 / monosaccharide / fruit sugar

Saccharide Monomers: (important)

Glucose

· classified as an aldohexose – as it is an aldehyde and a 6-carbon compound

· Most carbohydrates are converted to glucose to be metabolized for energy.

· dextrose and blood sugar are both common names for glucose

· one of the monomers found in the disaccharide found in sucrose, maltose and lactose

· a monomer of starch, cellulose and glycogen

· 25% less sweet than table sugar, sucrose

· no digestion needed can be given intravenously

· found in the urine of diabetics

· 70-150mg per dl of blood

Galactose

· classified as an aldohexose – as it is an aldehyde and a 6-carbon compound

· found in pectin and gum

· combined with glucose to form the disaccharide lactose

· 80% less sweet than table sugar, sucrose

· Galactosemia

o genetic disease – inability of body to metabolize galactose

o elevated levels of galactose in blood and urine

o vomiting, diarrhea, liver enlargement

o can cause death in days

o lactose must be removed from their diet

o http://www.galactosemia.org/galactosemia.htm

· isomer of glucose the #5 carbon has the hydroxyl and hydrogen switched

Fructose

· classified as a ketohexose as this molecule is a ketone and is a 6-carbon chain

· found in fruit juice and honey

· combined with glucose to form the disaccharide sucrose

· 175% sweeter than table sugar, sucrose

· this country’s most common sweetener

o high fructose corn syrup

o can be metabolized to glucose in the liver

Cyclic Saccharides:

The straight form of saccharides is very reactive. For the saccharide to be stable enough to transport, it forms a cyclic structure. Below are drawings for the formation of straight-chained glucose to become cyclic. The reaction breaks the double bond of the carbonyl group and shifts hydrogen of the hydroxyl group on the number 5 carbon to the carbonyl group’s oxygen.

Glucose is classified as an aldose, this ring structure will also form with the ketose saccharides like fructose.

The stereochemistry of these molecules can become overwhelming. As such other prefixes must be introduced to describe the stereochemistry, alpha, a and beta, b. Below is the cyclic structure of glucose. The carbon to the far right on each ring shows the hydroxyl group in different location. The alpha structure has the hydroxyl group down and the beta group has the hydroxyl group up.

Rings of different numbers of sides are given different names. A five-sided ring is called a furanose and a six-sided ring is called a puranose.

These are the straight and cyclic structures for fructose, a five carbon ketose.

Below are the two furanose rings of fructose. The alpha is on the left, hydroxyl on the down. The beta on the right, hydroxyl on the top.

These are the straight and cyclic structures for ribose, a six carbon aldose.

Below are the two furanose rings of ribose. The alpha is on the left, hydroxyl on the down. The beta on the right, hydroxyl on the top.

As stated in the beginning of these notes, the R in RNA is from ribonucleic acid. The D in DNA is a molecule whose structure is very close to that of ribose; the molecule is deoxyribonucleic acid. Lets break down that word. The prefix “de“ means loss, “oxy” means oxygen and “ribo” refers to ribose. So, what you have is a ribose that has lost an oxygen.

This may not seem like much of a change, but this demonstrates the specificity of chemistry. One oxygen can change the function of a molecule from making proteins, RNA, and storing the organism genetic information, DNA. Both of the above molecules are furanose, 5-member rings and are in the beta form, the hydroxyl is up.

Disaccharides:

· The three most important disaccharides are sucrose, lactose and maltose.

· The monomers are very specific. Meaning you must have stereochemistry exact, the bond will require an alpha or beta and always the D form.

· Disaccharides are formed thru a dehydration reaction.

· This reaction releases a water molecule.

· To break this bond, named a glycosidic bond, you add water, this reaction is named hydrolysis.

· Where in your body will this digestion occur?

Sucrose:

· most common disaccharide, table sugar

· 20% of sugar cane is sucrose