CHAPTER 7CARBOHYDRATES

MEDICAL BIOCHEMISTRYMr. Genaro Alderite, MSERM

Carbohydrates- are the most abundant biological molecules-They are composed of C, O, H according to the formula (C•H2O)n where n ≥3- are carbon compounds that contain large quantities of hydroxyl groups.

I (CH2O)n or H - C - OH

I

Outlined Functions of Carbohydrates:

1. Structural-Main skeletal support-cellulose, chitin-Network skeletal support- alginate, pectin

2. Energy storage-starch (amylose and amylopectin)-glycogen

3. Transport function-Glycoproteins

4. Regulatory function- Some glycoproteins

5. Catalytic functions- Some glycoproteins

6. Antigen-antibody interactions- Some glycoproteins

7. Filtration barriers- Gylycoproteins and proteoglycans

8. Cell differentiation9.Cell lubrication10. Components of cellular membrane11. Components of clotting proteins

- Fibrinogen12. Components of protective cellular coats

Carbohydrates are among the most abundant compounds on earth. They are normally broken down into five major classifications of carbohydrates:

-Monosaccharides -Disaccharides -Oligosaccharides -Polysaccharides -Nucleotides

Monosaccharides -are classified by their number of carbon atoms :

Name Formula Relevant Ex:triose C3H6O3 Glyceraldehyde

tetrose C4H8O4 Erythrose

pentose C5H10O5 Ribose, Ribulose

hexose C6H12O6 Glucose, Galactose

heptose C7H14O7 Sedoheptulose

octose C8H16O8 Sialic acid

Monosaccharide•aldose:contains an aldehyde group•ketose:contains a ketone group

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

Monosaccharides- The common monosaccharides are glucose, fructose, and galactose.

1. Glucose -the main sugar metabolized by the body for energy. -The D-isomer of glucose predominates in nature and it is for this reason that the enzymes in our body have adapted to binding this form only.

-the concentration of glucose in the bloodstream usually falls within a narrow range of 70 to 115mg/100 ml of blood.

-Sources of glucose include starch, the major storage form of carbohydrate in plant.

Glucose

2. Galactose -nearly identical to glucose in structure except for one hydroxyl group on carbon atom number four of the six-sided sugar.

-Since it differs in only one position about all six asymmetric centers in

the linear form of the sugar, galactose is known as an epimer of glucose.

- Galactose is not normally found in nature in large quantities, however it combines with glucose to form lactose in milk.

3.Fructose -a structural isomer of glucose, meaning it has the same chemical formula but a completely different three-dimensional structure.

-The main difference is that fructose is a ketone in its linear form while glucose is an aldehyde.

-Sources of fructose include fruit, honey and high-fructose corn syrup.

Fructose as a straight-chain carbohydrate

Disaccharides, meaning "two sugars", are commonly found in nature as sucrose, lactose and maltose.

-formed by a condensation reaction where one molecule of water condenses or is released during the joining of two monosaccharides. The type of bond that is formed between the two sugars is called a glycosidic bond.

CONDENSATION REACTION RESULTING IN GLYCOSIDIC BONDS OF MALTOSE

Lactose is a disaccharide formed through the condensation of glucose and galactose. The bond formed between the two monosaccharides is called a beta glycosidic bond .

GLUCOSE + GALACTOSE = GLUCOSE

Lactose: is found exclusively in the milk of mammals and consists of galactose and glucose in a β–(1,4) glycosidic bond.

• Lactose or milk sugar occurs in the milk of mammals - 4-6% in cow's milk and 5-8% in human milk. It is also a by product in the the manufacture of cheese.

• Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar of milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down the lactose, milk sugar, into glucose and galactose that can then be absorbed into the bloodstream.

Sucrose is found in common table sugar and is composed of glucose and fructose linked via a 1-2 alpha glycosidic bond.

Sucrose -is an excellent preservative because it has no "reducing end" or reactive group like the other sugars. -Other natural sources of sucrose are found in plants such as sugar cane, sugar beets, and maple syrup

Maltose -is the final disaccharide and consists of two glucose molecules joined by an alpha glycosidic bond.

-is an interesting compound because of its use in alcohol production. Through a process called fermentation, glucose, maltose and other sugars are converted to ethanol by yeast cells in the absence of oxygen.

-Through an analogous process, muscle cells convert glucose into lactic acid to obtain energy while the body operates under anaerobic conditions.

-Although maltose is uncommon in nature, it can be formed through the breakdown of starch by the enzymes of the mouth.

GLUCOSE + GLUCOSE= MALTOSE

Oligosaccharides  

-Important oligosaccharides are raffinose and stachyose.

-Composed of repeating units of galactose, glucose and fructose, these oligosaccharides are of nutritional importance because they are found in beans and legumes.

-Because of their unique glycosidic bonds, raffinose and stachyose cannot be broken down into their simple sugars. -Therefore, they cannot be absorbed by the small intestine and are often metabolized by bacteria in the large intestine to form unwanted gaseous byproducts.

-Commercial enzyme preparations such as Beano can be consumed before a meal rich in beans and legumes in order to aid the small intestine in the breakdown of these oligosaccharides.

Polysaccharides- (Complex Carbohydrates)

- are usually monomers and consist of thousands of repeating glucose units. Naturally, they allow for the storage of large quantities of glucose.

Starch-the major storage form of carbohydrate in plants and has two different types: amylose (straight chain polymer) and amylopectin(highly branched).

These differences account for the fact that amylopectin can form stable starch gels which are able to retain water while amylose is unable to do so. - Amylopectin is often used by manufacturers to produce many different kinds of thick sauces and gravies. -Sources of starch include potatoes, beans, bread, pasta, rice and other bread products

Glycogen - is a highly branched polymer of glucose that is the main storage form of carbohydrate in humans.

- is stored in the liver and muscle where it is synthesized and degraded depending upon the energy requirements of the body.

Indigestible forms of polysaccharides are known as dietary fiber and come in many different forms including cellulose, hemicellulose, pectin, gum and mucilage.

-Cellulose is by far the most abundant biochemical compound on the earth because it forms part of the structure of many plants.

-Cellulose and most forms of hemicellulose are insoluble fibers.

-Pectin, gum and mucilage are all soluble fibers and readily dissolve or swell when mixed with water.

NucleotidesOther sugars of importance are found in

nucleotides such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both RNA and DNA are five sided cyclic sugars; however, RNA has one more hydroxyl group than DNA. Glucose-6-phosphate, an intermediate in the breakdown of glucose for energy, can be used for the synthesis of these compounds.

Glycoproteins- these are proteins with carbohydrates attached to their peptide portion.

- has the backbone which is essentially protein but the carbohydrate part of the molecule takes the form of oligosaccharide chains which are pendant and covalently bound to the protein chain.

Important glycoproteins in the body:

1. Glycophorin- located in the red cell membrane

- measures the quantity of sialic acid in the red blood cells as the sign of aging.

2. Follicle stimulating hormone3. Luteinizing hormone4. Thyroid stimulating hormone

5.Blood group glycoproteins

6. Antibodiesa. Interleukin 1- inflammation and

response to injuryb. Interleukin 2- mediates

proliferation of T cells.c.Interleukin 3- Effects early stage

of hematoporetic cell differentiationd. Interleukin 4- Induces proliferation of B cellse. Interleukin 5- stimulates

differentiation of B lymphocytes