Proteins (b) (a) Structural formula (a) and space-filling model (b) of a short segment of a protein molecule. In the structural formula, hydrocarbon side chain (green), an acidic side chain (red), a basic side chain (blue), and a sulphur-containing side chain (amber) are highlighted with different colours. The broken lines in (a) indicate where two amino acid units join.

Some amino acids COOH C H2N H COOH C H2N H CH2COOH COOH C H2N H CH3 Aminoethanoic acid (Glycine) COOH C H2N H CH2COOH Aminobutanoic acid (Aspartic acid) COOH C H2N H CH3 2-Aminopropanoic acid (Alanine)

Proteins as Macromolecules made up of Amino Acids The 20 α-amino acids from which our protein molecules are composed.

Amino Acid: Carboxyl Group Amino Group Actually

As a zwitterion contains regions of positive and negative charge, amino acids are solids at room temperature, due to the ionic bonds that exist between the zwitterions. Any acidity of amino acids in water is due to the –NH3+ group; whereas any basic properties are as a result of the –COO– group. OH- H+ H3N – CH – COO- | R + OH- H+ H3N – CH – COOH | R + H2N – CH – COO- | R

Peptide Linkages: Peptides and Proteins A protein:

Segments of this polypeptide backbone are often coiled into a regular orientation.

Hydrolysis of proteins Breaks the peptide linkages in a protein molecule The composition of the protein molecule may be deduced by using paper chromatography

Hydrolysis of proteins CH C O R’ acid H N R CH C O OH R’

Carbohydrates Function – provide energy Formula – Cx H2yOy Monosaccharides C6H12O6 (glucose, fructose) Disaccharide C12H22O11 (sucrose, maltose) Polysaccharides (C6H10O5)n (starch, cellulose)

Stereoisomers of C6H12O6 H O C C* OH 4 chiral carbons

Glucose Open chain (acyclic form) 2 Ring forms (cyclic forms) M.p. 146oC , 150oC Optical rotations +112o , +19o  +52.7 Mutarotation

Fructose 6 1 5 2 1 2 6 5 6 5 2 1

Reducing sugars Tollen’s or Fehling’s reagents

Reducing sugars

Disaccharides

Disaccharides

Polysaccharides Carbohydrate polymers Storage polysaccharides Energy storage – starch and glycogen Structural polysaccharides Use to provide protective walls to cells - cellulose

Glycosidic Linkage in Carbohydrates Maltose Glucose Glucose Glycosidic linkage H H Glucose Fructose Surcose

Starch

Cellulose

Hydrolysis of sucrose Sucrose, like all disaccharides, is hydrolysed by dilute mineral acids to two monosaccharides, glucose and fructose C12H22O11 + H2O  C6H12O6 + C6H12O6 The reaction can be effected by enzyme H+ glucose fructose

Hydrolysis of starch A solution of starch can be hydrolysed in the presence of an enzyme to a disaccharide, maltose. 2(C6H10O5)n +nH2O  nC12H22O11(maltose) It starch is boiled with dilute sulphuric acid, it is hydrolyzed to a monosaccharide, glucose. (C6H10O5)n +nH2O  nC6H12O6

DNA as Nucleic Acids Two kinds of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) The monomers of nucleic acids, called nucleotides, are formed from the following units: A phosphate unit A five carbon sugar A nitrogen – containing organic base.

The sugar component of RNA is ribose, whereas that in DNA is deoxyribose.

The following nitrogen bases are found in DNA and RNA: (in RNA)

Structure of a nucleotide.

The backbone of a DNA molecule The backbone of a DNA molecule. The n indicates that the unit is repeated many times. Each repeating unit is composed of a base, phosphate unit, and sugar. The base is one of four nitrogen bases A, G, C or T.

Two views of DNA A computer – generated model of a DNA double helix. The dark blue and light blue atoms represent the sugar – phosphate chains that wrap around the outside. Inside the chains are the bases, shown in red and yellow – green. A schematic illustration of the double helix showing the hydrogen – bond interactions between complementary base pairs.

Hydrogen bonding between complementary base pairs Hydrogen bonding between complementary base pairs. The hydrogen bonds are responsible for formation of the double stranded helical structure of DNA, shown previously in Fig. 11.10 (b).

DNA replication. The original DNA double helix partially unwinds, and new nucleotides line up on each strand in a complementary manner. Hydrogen bonds help align the new nucleotides with the original DNA chain. When the new nucleotides are joined by condensation reactions, two identical double helix DNA molecules result.