biochem

BIOCHEMISTRY

2ND SEM: PRELIMS

Biochemistry

• the application of chemistry to the study of biological processes at the cellular and  molecular level.

• It emerged as a distinct discipline around the  beginning of the 20th century when scientists  combined chemistry, physiology and biology to investigate the chemistry of living systems

• All organisms use the same type of molecules: CHO, proteins, lipids & nucleic acids.

• Instructions for growth, reproduction and developments for each organism is encoded in their DNA.

• It is a special branch of organic chemistry that deals with matter inside the living cell called Protoplasm.

Protoplasm is an enormously complex mixture of organic compounds where high levels of chemical activity occur.

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis

                                           aldehyde                     ketone

Carbohydrates  also appear in cyclic rings instead of linear molecules. This is because the carbonyl group (aldehyde and ketone) can react with a hydroxyl group (-OH group).

• Contain single (one) polyhydroxy aldehyde or ketone unit.

• Simplest carbohydrates

• Classified according to the number of carbon atoms they contain.

The presence of an aldehyde is indicated by the prefix aldo-

and a ketone by the prefix keto-.

• Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter than sucrose) 

• They are solids at room temperature. 

• They are extremely soluble in water:

  • Despite their high molecular weights (MW), the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW.

  • Glucose can dissolve in minute amounts of water to make a syrup (1 g/ 1 ml H2O).

Consists of two monosaccharide units linked together by a 

covalent bond or through a glycosidic linkage.

1. Maltose (malt sugar)

• it is found in germinating grain (such as barley), and is formed during the hydrolysis of starch to glucose during digestion

• because it has a hemiacetal group, it reduces sugar.

2. Lactose (milk sugar) 

• it constitutes 5% of cow's milk and 7% of human milk

• it is digested by the enzyme lactase

3. Sucrose (table sugar) 

• Both anomeric carbons of glucose and fructose are tied together in the glycosidic linkage; thus neither ring can open, and sucrose is not a reducing sugar. 

• Sucrose is abundant in sugar cane and sugar beets; maple syrup contains about 65% sucrose, with glucose and fructose present as well; caramel is the solid residue formed from heating sucrose. 

• A flavoring agent called invert sugar is produced by the hydrolysis of sucrose under acidic conditions, which breaks it apart into glucose and fructose; invert sugar is sweeter than sucrose because of the fructose. 

• Some of the sugar found in honey is formed in this fashion; invert sugar is also produced in jams and jellies prepared from acid-containing fruits.

Contain from three (3) to ten (10) monosaccharide units.

1. Raffinose- an oligosaccharide found in peas and beans; largely undigested until reaching the intestinal flora in the large intestine, releasing hydrogen, carbon dioxide, and methane)

2. Fructooligosaccharides- are oligosaccharides that occur naturally in plants such as onion, chicory, garlic, asparagus, banana, artichoke, among many others. 

- Oligosaccharides are often found as a component of glycoproteins or glycolipids. 

- often used as chemical markers on the outside of cells, often for cell recognition. 

- responsible for determining blood type.

• Contains hundreds or thousands of carbohydrate units.

• Polysaccharides are not reducing sugars, since the anomeric carbons are connected through glycosidic linkages. 

• Three kinds of polysaccharides, all of which are polymers of glucose: starch, glycogen, and cellulose.

1. Starch

- Starch is a polymer consisting of D-glucose units. 

- Starches (and other glucose polymers) are usually insoluble in water because of the high MW, but they can form thick colloidal suspensions with water.

2. Glycogen

- The a-1,6 glycosidic bonds in glycogen are produced by the non-regulatory branching enzyme (and broken by debranching enzyme), while a-1,4 glycosidic bonds in glycogen are produced by the regulatory glycogen synthase (and broken by glycogen phosphorylase).

3. Cellulose 

Carbohydrates are compounds of tremendous biological 

importance: 

• they provide energy for the body 

• they supply carbon for the synthesis of cell components

• they serve as a form of stored chemical energy

• they form part of the structures of some cells and tissues 

Carbohydrates, along with lipids, proteins, nucleic acids, and other compounds are known as biomolecules because they are closely associated with living organisms.

Almost 75% of dry plant material is produced by photosynthesis. 

• Most of the matter in plants, except water, are carbohydrate material. 

• Examples of carbohydrates are cellulose which are a structural component of the plants, starch the energy reservoir in plants and glycogen (animal starch) found in animal tissues and the human body in smaller quantities.

• Carbohydrates, and other organic compound, can be classified as chiral or achiral. 

• Chiral molecules are said to be optically active. Molecules which rotate the plane of polarized light are optically active. 

• A levorotatory (–) substance rotates polarized light to the left. [e.g., L-glucose; (-)-glucose 

• A dextrorotatory (+) substance rotates polarized light to the right. [E.g., d- glucose; (+)-glucose]

• Most biologically important molecules are chiral, and hence are optically active. 

• Often, living systems contain only one of all of the possible stereochemical forms of a compound. In some cases, one form of a molecule is beneficial, and the enantiomer is a poison.

- The 2n rule applies equally well to molecules with three or more stereocenters.

- Isomers are compounds with the same chemical formula but different structures.

- 2n Rule: when a molecule has more than one chiral carbon, each carbon can possibly be arranged in either the right-hand or left-hand form, thus if there are n chiral carbons, there are 2n possible stereoisomers. Maximum number of possible stereoisomers = 2n

- is a property of asymmetry. The word chirality is derived from the Greek (kheir), "hand", a familiar chiral object. 

- An object or a system is chiral if it is distinguishable from its mirror image; that is, it cannot be superimposed onto it. 

- Generally, when an object has a point of symmetry, it is ACHIRAL.

• Chiral molecules have the same relationship to each other that your left and right hands have when reflected in a mirror.

• Any carbon atom which is connected to four different groups will be chiral, and will have two nonsuperimposable mirror images; it is a chiral carbon or a center of chirality (encircled in red).

• if any of the two groups on the carbon are the same, the carbon atom cannot be chiral.

• Many organic compounds, including carbohydrates, contain more than one chiral carbon.

• Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms that are mirror images of each other:

• These forms are stereoisomers of each other.

• Glyceraldehyde is a chiral molecule. The two mirror image forms of glyceraldehyde are enantiomers of each other.

• Fischer projections are a convenient way to represent mirror Image In two dimensions 

• Place the carbonyl group at or near the top and the last achiral CH2OH at the bottom.

• When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group: if the hydroxyl group points to right when the carbonyl is "up" it is the D-isomer, and when the hydroxyl group points to the left, it is the L-isomer

• D-glucose, is the most commonly occurring isomer of glucose in nature. Common source of energy (Dextrose).

• L-glucose cannot be used as a source of energy in cellular respiration. Used as laxative in some cases.