Part II Biochemistry The branch of chemistry that deals with the different molecules, their structure, composition, and chemical processes in the living cell IUG, Fall 2013 Dr. Tarek Zaida

Carbohydrates Carbohydrates occur in all plants and animals and are essential to life. Through photosynthesis, plants convert atmospheric carbon dioxide to carbohydrates, mainly cellulose, starch, and sugars. Cellulose is the building block of rigid cell walls and woody tissues in plants, whereas starch is the chief storage form of carbohydrates for later use as a food or energy source. Some plants (cane and sugar beets) produce sucrose, ordinary table sugar. Another sugar, glucose, is an essential component of blood. Two other sugars, ribose and 2-deoxyribose, are components of the genetic materials RNA and DNA.

Definitions and Classification The word carbohydrate arose because molecular formulas of these compounds can be expressed as hydrates of carbon. Glucose, for example, has the molecular formula C6H12O6, which might be written as C6(H2O)6. Carbohydrates are now defined as: 1. polyhydroxyaldehydes, 2. polyhydroxyketones, or 3. substances that give such compounds on hydrolysis.

Carbohydrates have two functional groups: the hydroxyl group and the carbonyl group. Carbohydrates are usually classified according to their structure as: 1. monosaccharides, 2. Oligosaccharides, 3. polysaccharides. The term saccharide comes from Latin (saccharum, sugar) and refers to the sweet taste of some simple carbohydrates. The three classes of carbohydrates are related to each other through hydrolysis

For example, hydrolysis of starch, a polysaccharide, gives first maltose and then glucose.

Monosaccharides (or simple sugars, as they are sometimes called) are carbohydrates that cannot be hydrolyzed to simpler compounds. Polysaccharides contain many monosaccharide units—sometimes hundreds or even thousands. Usually, but not always, the units are identical. Two of the most important polysaccharides, starch and cellulose, contain linked units of the same monosaccharide, glucose.

Oligosaccharides (from the Greek oligos, few) contain at least two and generally no more than a few linked monosaccharide units. They may be called disaccharides, trisaccharides, and so on, depending on the number of units, which may be the same or different. Maltose, for example, is a disaccharide made of two glucose units. Sucrose, another disaccharide, is made of two different monosaccharide units: glucose and fructose.

Monosaccharides Monosaccharides are classified according to the number of carbon atoms present (triose, tetrose, pentose, hexose, and so on) and according to whether the carbonyl group is present as an aldehyde (aldose) or as a ketone (ketose). There are only two trioses: 1. Glyceraldehyde and 2. Dihydroxyacetone Each has two hydroxyl groups, attached to different carbon atoms, and one carbonyl group.

Glyceraldehyde is the simplest aldose Dihydroxyacetone is the simplest ketose. Each is related to glycerol in that each has a carbonyl group in place of one of the hydroxyl groups. Other aldoses or ketoses can be derived from glyceraldehyde or dihydroxyacetone by adding carbon atoms, each with a hydroxyl group. In aldoses, the chain is numbered from the aldehyde carbon. In most ketoses, the carbonyl group is located at C-2.

Naming Monosaccharides • Carbohydrate nomenclature is unique we do not name monosaccharides using the IUPAC rules. • Monosaccharide names end in ―ose

Naming Monosaccharides

Naming Monosaccharides Examples:

Name each of the following monosaccharides as an aldose or ketose & according to its number of C atoms.

Origin of Carbohydrates 1. Plants catch up CO2 from air & H2O from soil 6CO2 + 6H2O sun light C6H12O6 + 6O2 The above reaction is called photosynthesis (formation of carbohydrates from CO2, H2O and sun light) enzymes, chlorophyll glucose

Plants build disaccharides from monosaccharides: 2. The carbohydrates produced during photosynthesis are monosaccharides. Plants build disaccharides from monosaccharides: C6H12O6 + C6H12O6 C12H22O11 + H2O glucose glucose Disaccharide

3. Plants as well as animals can combine many molecules of monosaccharides into large polysaccharide molecules: nC6 H12O6 polymerization (C6H10O5)n + nH2O Polysaccharides in plants are: - Cellulose in stalk and stem - Starch in roots and seeds - Mono & disaccharides are found in fruits

Oxygen – Carbon dioxide cycle 1. Plants pick up CO2 from air, water from soil to build carbohydrates (monosaccharides). 2. Animals can not do that, so they have to rely on plants to get their carbohydrates CO6H12O6 + 6O2 6CO2 + 6H2O + energy The above reaction is the reverse of photo-synthesis. Krebs cycle

During Photosynthesis: Energy from the sun needed for aerobic reactions. During catabolism of carbohydrates in animals same amounts of energy is liberated from oxidation of food. So energy from metabolism in animals comes from the sun through plants that store solar energy in carbohydrates.

Stereoisomerism Stereoisomers: Are compounds having the same molecular formula but a different structural formula. Stereoisomers have structures that are mirror image of one another If you put your left hand on top of your right hand, you will see that they are not superimposable

Both methane and mirror image are said to be superimposable (achiral) Some molecules are superimposable with their mirror images and others are not: H-C-H H-C-H Both methane and mirror image are said to be superimposable (achiral) H H H H methane Mirror image

In general any object with a plane of symmetry is achiral Any object or molecule does not have a plane of symmetry it is chiral If there are 4 different atoms attached to a central carbon atom, it is said to be chiral

Stereoisomers in Carbohydrates Carbohydrates are chiral molecules since they have carbon atoms carrying four different groups. The simplest three carbon sugar is glyceraldehyde. This sugar exists as a pair of enantiomers.

Two forms of glyceraldehyde (D and L) have the same physical properties except they behave differently in the way they rotate polarized light and the way they are affected by catalysts.

Remember: Compounds with n chiral carbon atoms has a maximum of 2n possible stereoisomers and half that many pairs of enantiomers (mirror images). •This aldotetrosose, has 2 chiral carbon atoms and a total of 22 = 4 possible stereoisomers (2 pairs of enantiomers).

The D and L Forms of Sugars: Drawing Sugar Molecules Fischer Projections (or open chain) represent three-dimensional structures of stereoisomers on a flat page.

A chiral carbon atom is represented in the Fisher projection as the intersection of two crossed lines

Bonds that point above the page are shown as horizontal lines. Bonds that curve behind and below the page are shown as vertical lines.

Fischer Projections of Sugar Molecules In a Fischer projection, the aldehyde or ketone carbonyl group of a monosaccharide is always placed toward the top of the page.

Fischer Projections of Sugar Molecules Example

Fischer projection of glucose:

Monosaccharides are divided into two families: D form and L form sugars. D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right. L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left. The D and L relate directly only to the position of –OH group on the bottom carbon in a Fischer projection.

D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right. L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left.

Chiral Drugs Many enzymes will interact with only one particular enantiomer In human body, enzymes will react with carbohydrates of D- form, but will react with proteins of L-form. Some drugs are enantiomers: 1. Ibuprofen used in pain relief 2. Indinavir used in Aids 3. Levofloxacin used as antibiotic

some important pentoses and hexoses, and their derivatives 1. D-glucose, also called dextrose or blood sugar , is the most important monosaccharide in human metabolism.

2. D-fructose, or fruit sugar, is most common natural ketose. Honey contains about is 38% fructose

In deoxy sugars a hydrogen atom replaces one or more of the -OH groups in a monosaccharide. D-ribose and its derivative D-2-deoxyribose (deoxy = minus one oxygen atom) are found in various coenzymes and in DNA.

The Cyclic Form of Monosaccharides – A monosaccharide contains both an alcohol and an aldehyde group. – It can react with itself to form a cyclic hemiacetal.