Biochemistry/Carbohydrates

Introduction

Carbohydrates are one of the fundamental classes of macromolecules found in biology. Carbohydrates are commonly found in most organisms, and play important roles in organism structure, and are a primary energy source for animals and plants. Most carbohydrates are sugars or composed mainly of sugars. By far, the most common carbohydrate found in nature is glucose, which plays a major role in cellular respiration and photosynthesis. Some carbohydrates are for structural purposes, such as cellulose (which composes plants' cell walls) and chitin (a major component of insect exoskeletons). However, the majority of carbohydrates are used for energy purposes, especially in animals. Carbohydrates are made up of a 1:2:1 ratio of Carbon, Hydrogen, and Oxygen (CH₂O)_n

Simple Carbohydrates (Monosaccharides)

These are used only for energy in living organisms.

Simple carbohydrates are also known as "bad" carbs.The chemical formula for all the monosaccharides below is C₆H₁₂O₆. They are all structural or stereoisomers of each other. There are two main types of monosaccharides. The first type are aldoses, containing an aldehyde on the first carbon, and the second type are ketoses, which have a ketone on the second carbon (This carbonyl group is always located on the second carbon).

In solution, monosaccharides of 4 or more carbons form rings; a hydroxyl group from the 4th or 5th carbon will add to the carbonyl, producing a ring and turning the carbonyl into a hydroxyl group. Monosaccharides will form either 5- or 6-membered rings. Some, such as glucose, are more stable as 6-membered rings, whereas others are more stable as 5-membered rings, such as fructose. 6-membered rings are called pyranoses, based on the structural similarities to pyran, a 6-membered aromatic ring containing oxygen. 5-membered rings are called furanoses, based on furan.

In forming the ring, a new stereocenter is created at the carbon which the ether linkage is created. If the hydroxyl group is "down" relative to the ring, then it is called the α-isomer. If it is up, then it is called the β-isomer. Thus, one gets names such as α-D-glucose. In glucose, the α isomer is preferred to the β isomer at a ratio of 64:36. There are two competing effects that determine whether a monosaccharide will prefer a α- or a β-conformation. First, a group that is equatorial will be more stable, because it is less crowded. On the other hand, a group that is axial will be more stable because it can hydrogen bond with the ether linkage. It is not possible to tell which effect will be stronger just by looking at structures.

Some examples of Monosaccharides are:

Fischer projection formula of D-glucose.

piration to release energy. It is found in most foods, though most predominantly in honey and fruits such as blackberries, strawberries, raspberries, melon, oranges, lemon,kiwi.

Fructose

Fructose is a ketohexose with the molecular formula of C₆H₁₂O₆ ; which is same as the molecular formula of Glucose but different structure. The structure of fructose differs from glucose at carbon 1 and 2 by the location of the carbonyl group. Fructose is the sweetest naturally occurring sugar. Fructose is also called levulose and fruit sugar; As its name implies, fructose is also found in fruits, root vegetables(such as sweet potato and onion) and honey. It is a isomer of glucose and a ketose simple sugar. Fructose has the highest solubility among all sugars. Fructose can be converted to its isomer glucose, after it enters the blood stream.

Glucose

The hexoses glucose,galactose, and fructose are important monosaccharides.Glucose is the most prevalent monosaccharide in diet. The most common hexose, D-glucose, C₆H₁₂O₆, also known as dextrose and blood sugar, is found in fruits,vegetables, corn syrup, and honey. Glucose is a building block of the disaccharides(sucrose,maltose,lactose) and polysaccharides ( glycogen,cellulose,starch). In the body, excess glucose is converted to glycogen and then stored in the muscle and liver.

Glucose is predominantly found in its pyranose form. In the figure of the “Fischer Projection formula of D-glucose,” the pyranose has two possible conformations most commonly represented by the Haworth Projection: alpha and beta. In this form, glucose is stabilized through its hemi-acetal reaction, forming a six-member ring chair conformation (glucopyranose) and allowing allowing flexibility in the structure. To obtain the glucopyranose form, start with the Fischer Projection representation and count from carbon one to carbon five. At this point, there are currently five members in the ring. Obtaining the most stable formation requires a sixth member: the hydroxy group adjacent to carbon five. The hydroxy group of carbon six is not chosen because it would form an unstable seven-member ring. Now, the oxygen adjacent to carbon five attacks carbon one. Here, the oxygen has a choice of attacking from the bottom or from the top due to carbon one’s trigonal planarity. In the case of attacking from the bottom, the resulting hydroxy group from the anomeric carbon will be on top, forming the beta conformation of the six-member ring, and vice-versa. The question of whether the three remaining hydroxy groups and -CH₂OH substituent will be beta or alpha. Starting to the left of the oxygen in the six-member ring, the configuration of the substituent on carbon five must be determined via the Cahn-Ingold-Prelog priority rules. By placing the lowest priority group in the back (hydrogen), the configuration results in a clockwise manner, formally known as the “R” configuration. Here, the -CH₂OH is placed on top and the hydrogen on the bottom. By the same logic, carbon four also results in the “R” configuration, placing the hydroxy group on the bottom and the hydrogen on top. Carbon three is a “S” configuration, putting the hydroxy group on top and the hydrogen on bottom. Carbon two, a “R” configuration, presents the hydroxy group on bottom and the hydrogen on top. Again, because carbon one is the anomeric carbon, it would depend on either the beta or alpha conformation for the placing of the hydroxy group. In the adjacent figure, the hydroxyl group is on top due to the beta conformation.

Haworth Projection of D-Glucose.

Galactose

Galactose is a sugar component of the disaccharide lactose, as found in milk, and is the portion that contributes to lactose intolerance. It is not as sweet as glucose and is separated from the glucose in lactose via hydrolysis.In addition, galactose is an aldohexose that does not occur in the free form in nature; galactose has an important role in the cellular membrane of the brain and nervous system. D-galactose has a structure similar to D-glucose and the only difference between them is in the arrangement of the –OH group on carbon number 4. Galactose can be found in human breast milk and is incorporated into the structure of Human Milk Oligosaccharides. The backbone of Human Milk Oligosaccharides is the disaccharide lactose, which is formed by the linkage between galactose and glucose sugars. The final structure of HMO all depends on whether the backbone, lactose, is fucosylated or sialated, in either beta or alpha configurations or at a different carbon.

Galactooligosaccharide

Galactooligosaccharides are synthesized through an enzymatic conversion of lactose It is comprised of chain units of galactose group through consecutive transgalactosylation reactions, which the degree of polymerization ranging from 2 to 8 monomeric units. It is known that these new classes of prebiotics have an important role in improving gut health by sustaining beneficial and balanced gut microbiota. Today, many infant formula companies have galactooligosaccharides in their formula milk.

Compound Carbohydrates (Disaccharides)

These are used by living organisms for energy. They are composed of two monosaccharides joined together by the process known as dehydration synthesis. In this process, one molecule loses one hydrogen atom, while the other loses one hydrogen atom and one oxygen atom. The reverse of this is hydrolysis, where water is added to break down a molecule into two or more simpler molecules. Disaccharides have the chemical equation C₁₂H₂₂O₁₁. The reason it does not follow the 1:2:1 ratio is, obviously, due to the H₂O taken way from it. Some examples of disaccharides are:

Maltose

Maltose is a white crystal sugar, also known as malt sugar and a reducing disaccharide (hemiacetal) made from two glucose units. The bonding of two glucose units is called 1-4 glycosidic linkage which joins the carbon number 1 of one glucose to carbon number 4 of the second glucose. Alpha-maltose or beta-maltose is determined simply by looking at the direction of OH group attaching on the reducing end. If it is pointing upward, the sugar will be the beta-maltose. If it is pointing downward, the sugar will be the alpha-maltose. In the presence of enzyme maltase,1-4 linkage of two glucose is broken down and maltose is hydrolized into glucose

Sucrose

Composed when α-D-glucose combines with a β-D-fructose molecule and is formed by plants. Sucrose is commonly known as "table sugar." Sucrose has a sweet taste and can be broken down by hydrolysis.

Lactose

Composed when one glucose molecule joins a galactose molecule. It is also Milk sugar. The hydrolysis of Lactose gives the following monosacchride: Lactose +H₂O → glucose + glactose The bond in lactose is β-1,4-glycosidic bond because the β anomer of glactose form a bond with a -OH group on carbon number 4 of glucose. The -OH group on carbon number 1 of glucose gives both α and β lactose. Lactose can be a reducing sugar since the open chain has an aldehyde group which can be oxidized.

Complex Carbohydrates (Polysaccharides)

Some source of complex carbohydrates are pasta, brown bread, brown rice, corn, beans, potatoes, and peas. Digestion of complex carbohydrates could take more time because digestive enzyme have to work harder to break down the chain into individual sugars. Both monosaccharides and disaccharides are used only for energy.In order to produce the molecule energy (ATP) all carbohydrates should break down to glucose.

Polysaccharides differ in that aspect. While animals still use it for energy, plants use it for energy and structure. Another difference is that while monosaccharides can be used for energy immediately, and disaccharides can be used for energy relatively rapidly because they can be broken down quickly, polysaccharides release their energy slowly , therefore they provide an ongoing steady flow of energy during the day. These are the different types of Homopolysaccharides, or polysaccharides with identical monosaccharide constituents:

Cellulose (Fiber)

Cellulose is a special kind of carbohydrate. It is insoluble and most organisms can not produce enzymes to break it down. It is found only in plants, and it's found in the cell wall. It is composed of β glucose molecules, which create a more rigid structure when joined than the α links found in energy storage glycogen and starch molecules. The reason for this is that when you make 1-4 glycosidic bonds with beta glucose it creates a straight chain. The chains of cellulose lie close to each other so you get hydrogen bonding between the chains. This makes the molecule very stable and highly resistant to breakdown. You are probably wearing cellulose as you sit here and read this. Cotton used to make clothes etc. is cellulose from around the seeds of the cotton plant. Fibre helps the plant keep a strong structure. Humans can't digest fibre, but it is an important part of a healthy diet, because it helps the digestive tract by giving it more to push on. This helps the contents move through at a reasonable rate. It also helps to stop too much water being absorbed in the large intestine and so reduces constipation. It also feeds the bacteria in your large intestine. These bacteria are not bad, they are good. They can digest some of the cellulose and other molecules your enzymes can not.it gives no color with iodine.

Starch

Starch is the energy storage molecule of plants.It is formed by long chains of α glucose molecules linked together. Starch is actually a mixture of 2 types of molecules, amylose and amylopectin.

Amylose is helical shape because of the bond angle created when 2 alpha glucose molecules bond between carbon 1 and carbon 4. Amylose have unbranched chain of glucose. Amylopectin is branched with glycosidic bonds from C1-C6.

These large polysaccharides are very good for energy storage because they have lots of glucose molecules crammed into a small space which can then be easily broken off, one at a time, by hydrolysis and then used for energy. They are also good for storage because although you have hundreds of glucose molecules packed into a tight space, the starch/glycogen molecules are not soluble. If all those sugar molecules were dissolved, too much water would be attracted into the cell by osmosis.

Glycogen

Glycogen is the energy storage molecule of animals. It is formed by branched chains of alpha glucose molecules with 1-4 glycosidic bonds on the main chains and 1-6 glycosidic bonds to form the branches. Humans store small amounts of glycogen in the liver and muscles. It is created when there are high blood sugar levels. The pancreas secretes insulin, which stimulates the creation of glycogen from glucose and signals the body to use glucose as its main form of energy.