Biochemistry/Carbohydrates

Introduction

"Carbohydrates" are chemically defined as "polyhydroxy aldehyde or polyhydroxy ketones or complex substances which on hydrolysis yield polyhydroxy aldehyde or polyhydroxy ketone." 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 "Monosaccharides".The chemical formula for all the monosaccharides is CnH2nOn. They are all structural isomers 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).

 Name            Formula            Aldoses                Ketoses 
 Trioses         C3 H6 O3           Glycerose              Dihydroxyacetone
 Tetroses        C4 H8 O4         Erythrose              Erythrulose
 Pentoses        C5 H10 O5          Ribose                 Ribulose
 Hexoses         C6 H12 O6          Glucose                Fructose
 Heptose         C7 H14 O7          Glucoheptose           Sedoheptulose

The suffix -oses is kept for the aldoses & the suffix -uloses is kept for the ketoses except fructose. Ketoses are as common as aldoses. The most abundant monosaccharide in nature is the 6-Carbon sugar -i.e.D-Glucose. All aldoses are reducing sugars. Many ketoses are also considered reducing sugars because they can isomerise to aldoses.

Monomers

There are two main forms of monomers, aldoses and ketoses, of which, all have the same backbone. The simplest form of monomers are trioses. The trioses consist of three carbons in the backbone.

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; 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.

         H-C-OH

         H-C-OH

           CH2OH

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.

CHO

                  H-C-OH

                 OH-C-H

                  H-C-OH

                  H-C-OH

                    CH2OH

Galactose

Galactose is a sugar component of the disaccharide lactose, as found in milk. 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..

Oligosachharides

Tha oligosachharides yields 2 to 10 monosachharides on hydrolysis. Disachharides are most common oligosachharide found in nature.

Disaccharides

Monosaccharides are simple forms of sugars consisting of one sugar, disaccharides on the other hand, consists of two. The two sugars are bonded together via a glycosidic bond. Most linkages between sugars occur between the first and fourth carbon on the sugars.

Polysaccharides

Sugars do not stop at disaccharides, they are much more complex than that. Multiple sugars are linked together forming many polysaccharides. Polysaccharides are made up of monosaccharides linked together.

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:

                                        Disachharides

                       Non reducing                        reducing

    C1-C1 glycosidic     C1-C2 glycosidic          C1-C4 glycosidic    C1-C6 glycosidic
       linkage              linkage                   linkage             linkage
     Trehalose            Sucrose                    Lactose             Gentiobiose
                                                     Maltose             Melibiose
                                                     Cellobiose          Isomaltose

Maltose

Chemistry-

Maltose is a white crystal sugar, also known as malt sugar and a reducing disaccharide 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.In the presence of enzyme maltase,1-4 linkage of two glucose is broken down and maltose is hydrolized into glucose

occurrence-

Maltose is present in sprouting barley. It is the major product of hydrolysis of starch.

alpha - maltose
beta - maltose

Sucrose

3Occurrence- It is the costituent of sugarcane. it is also found in Beet,carrot, etc. Nectar of flower is particularly rich in sucrose. 3Chemistry- Sucrose is white crystal sugar.When heated above its melting point it forms a brown substance known as "Caramel". it is non reducing sugar. Sucrose is formed by the C1-C2 glucosidic bond between glucose & fructose. Sucrose on hydrolysis yield equimolar mixture of glucose & fructose which is offen called as "invert sugar" by the "enzyme".

Lactose

3 Occurrence- It is found in milk of animal. It is also "Milk sugar". The percentage of lactose in human milk is 6.8 % & in cow milk is 4.8 % 3 Chemistry- Lactose is made by the C1-C4 glycosidic bond between galactose & glucose. The hydrolysis of Lactose gives the monosacchride: glucose + glactose by the enzyme lactase. Lactose can be a reducing sugar.

β-D-galactopyranosyl-(1→4)-D-glucose
The molecular structure of α-lactose, as determined by X-ray crystallography.

Complex Carbohydrates (Polysaccharides)

Polysaccharides, also called as glycanes, are high molecular weight which on hydrolysis yield monosaccharides. Some sources of complex carbohydrates are pasta, bread, rice, cereals, crackers, corn, beans, potatoes, pumpkin, 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. Complex carbohydrates contain hundreds of sugar units. Studies show that glucose levels both rise and fall more slowly after the consumption of bread compared to sugars or fruit, suggesting slower digestion.^[1] Both monosaccharides and disaccharides are used only for energy. Another difference is that while monosaccharides and disaccharides can be used for energy immediately, polysaccharides release their energy slowly. Research shows that the carbohydrate oxidation rate rises more slowly after the consumption of bread rather than sucrose following an overnight fast. The carbohydrate oxidation rate is also lower over a six-hour period following the consumption of bread in lieu of sucrose.^[2] Chemically , the polysaccharide may be distinguished into homopolysachharide & heteropolysachharide. Homopolysachharides which on hydrolysis yield only single monosachharide. Eg. starch, cellulose, etc. Hateropolysachharide which on hydrolysis yield two or more monosaccharide. Eg. heparin, chondroitin, etc.

                                     POLYSACHHARIDE
                            
                   Homopolysachharide              Heteropolysachharide
                  Starch                          Mucopolysachharids
                  Glucogen                        Chondroitin
                  Inulin                          Hyaluronic acid
                  Cellulose                       Keratosulfate
                  Pectin                          Heparin
                  Chitin                          Vegetable gum
                  Xylan                           Agar agar

polysachharide may be classified on the basis of functional aspect as Nutritive & Structural. Nutritive- starch,glycogen,inulin etc. Structural- cellulose,chitin,pectin,etc.

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 made by β-D-glucose. 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. 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.it gives no color with iodine.It is completely absent in meat, egg, fish & milk.

Starch

Starch is the energy storage molecule of plants.It is formed by long chains of α glucose molecules linked together. It is mostly found in potato. Starch is actually a mixture of 2 types of molecules, amylose and amylopectin. 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. It is hydrolysed by enzyme Amylase.It is non reducinh sugar. It gives blue, purple colour with iodine

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. It is non reducing sugar. It gives red colour with iodine.

References

↑ Wolever, Thomas M. S. (2006), The Glycaemic Index: A Physiological Classification of Dietary Carbohydrate, CABI, pg. 65, ISBN 9781845930516. “Indeed, blood glucose responses elicited by pure sugars and fruits suggest rapid absorption because the blood glucose concentration rises more quickly and falls more rapidly than after bread (Wolever et al., 1993; Lee and Wolever, 1998).”
↑ Daly ME, Vale C, Walker M, Littlefield A, Alberti KG, Mathers JC (2000). "Acute fuel selection in response to high-sucrose and high-starch meals in healthy men." Am J Clin Nutr 71, 1516-1524. See Figure 1.

n.p. Carbohydrates Notes. n.d. www.chem.ucla.edu/harding/notes/notes_14C_carbos.pdf

[1] Wolever, Thomas M. S. (2006), The Glycaemic Index: A Physiological Classification of Dietary Carbohydrate, CABI, pg. 65, ISBN 9781845930516. “Indeed, blood glucose responses elicited by pure sugars and fruits suggest rapid absorption because the blood glucose concentration rises more quickly and falls more rapidly than after bread (Wolever et al., 1993; Lee and Wolever, 1998).”

[2] Daly ME, Vale C, Walker M, Littlefield A, Alberti KG, Mathers JC (2000). "Acute fuel selection in response to high-sucrose and high-starch meals in healthy men." Am J Clin Nutr 71, 1516-1524. See Figure 1.

[1]

[2]