Biomolecules Introduction These substances formed the basis of life and are responsible for growth and maintenance of all living organisms. These substances are biomolecules Biomolecules Biomolecules Biomolecules build up the living system. are carbohydrates, proteins, nucleic acids, vitamins etc. carbohydrates These are hydrates of carbon, I’e.,those have C,H and O. General formula is Cx(H2O)y. The structures that are known to satisfy this formula are: For example:- Glucose/fructose Sucrose – C6H12O6 or C6(H2O)6 – C12H22O11 or C12(H2O)11 As, there are many molecules that, satisfy similar criteria but are not carbohydrates. For example:- Acetic acid CH3COOH But C2H4O2 C2(H2O)2 this is not a carbohydrate. There are a lot of compounds that do not satisfy the formula but are carbohydrates. For example :- Rhamnose C6H12O5 it does not satisfy the formula, but it is a carbohydrates. Carbohydrates Carbohydrates are optically active poly hydroxyl aldehyde of ketones or substances which produce these on hydrolysis and have at least one chiral carbon atom. For example :- glucose, fructose, lactose, sucrose, maltose etc. Classification of Carbohydrates (1) On the basis of hydrolysis product Monosaccharides Oligosaccharides Polysaccharides (2) On the basis of reducing properties Reducing sugar Non – reducing sugar (3) On the basis of taste Sugars Non - sugars Monosaccharides Monosaccharides It is the simplest carbohydrate, which cannot be hydrolysed further to give simpler molecules of poly hydroxyl aldehyde and ketone. The general formula is CnH2nOn. For example :- glucose, fructose, ribose etc. Classification of Monosaccharides On the basis of functional group present (1) Aldose having –CHO group (2) ketose having =C=O group If a monosaccharides have an aldehyde group. It is known as an aldose and if have ketone group, it is known as ketose. No. of carbon atom General term Aldehyde Ketones 3 Triose Aldotriose Ketotriose 4 Tetrose aldotetrose Ketotetrose 5 Pentose Aldopentose Ketopentose 6 Hexose Aldohexose Ketohexose 7 Heptose Aldoheptose Ketoheptose Glucose It is present in honey and sweet fruits etc. It It is an aldohexose. is also known as dextrose as it is dextrorotatory. Glucose is also called blood sugar as it circulates in the blood at a concentration of 65 – 100 mg/ml of blood. It is synthesised by chlorophyll in plants by using water and carbon dioxide from the air and sunlight as an energy source. Glucose made in the leaves is then converted into starch for storage. Preparation of Glucose It is prepared from sucrose and starch From On Sucrose boiling sucrose with dilute HCl or H2SO4 in alcoholic solution, equal amounts of glucose and fructose are obtained. From When Starch starch is boiled with dilute H2SO4 at 393 K temperature and 2 – 3 atm pressure, glucose is obtained. Structure of Glucose The open chain structure of glucose is given on the basis of various evidences. Evidences in support of open structure of glucose (1)Glucose on reduction with HI and red P at 373 K, gives a mixture of n-Hexane and 2-Iodohexane suggesting that all the 6 carbons are linked in a straight chain. (2) glucose forms an oxime with hydroxylamine (NH2OH). (3) On adding a molecule of hydrogen cyanide, glucose forms cyanohydrin. (4) On oxidation with mild oxidising agent bromine water, glucose gives gluconic acid. It confirms the presence of aldehyde group. (5) Glucose on acetylation with acetic anhydride forms a penta acetate. It confirms the presence of 5 -OH group. (6) On oxidation with nitric acid, both glucose and gluconic acid form the same product. It confirms the presence of primary alcoholic (-OH) group in glucose. Fischer after studying many properties of glucose gave the exact spatial arrangement of –OH groups. D represent the configuration of the –OH group at the last carbon. (+) stands for optical rotation, I’e., dextrorotatory. ‘D’ and ‘L’ notations are not related to the optical activity of the compounds. Enantiomers Molecules that are non-superimposable mirror images are called enantiomers. Notations d, l or (+), (-) are used to show optical rotations. Dextrorotatory (‘+’ or ‘d’):- enantiomer which rotates the plane of polarised light towards right. Laevorotatory (‘-’ or ‘l’):- enantiomer which rotates the plane of polarised light towards left. D and L -Configuration Compound chemically correlated to (+) or ‘d’ isomer of glyceraldehyde is assigned Dconfiguration. Compound chemically correlated to (-) or ‘l’ isomer of glyceraldehyde is assigned Lconfiguration. The –OH group on the lowest asymmetric carbon is on the right side. Reactions which do not support proposed structure of glucose The open chain structure of glucose explains most of its reactions, yet it fails to explain the following facts. Despite having the aldehyde group, glucose does not give 2,4-DNP test. Despite having the aldehyde group, glucose does not give Schiff’s test. Glucose, on reaction with sodium bisulphite, does not form the addition product. Penta acetate of glucose does not react with hydroxylamine. These reactions indicate the absence of free – CHO group in glucose. Cyclic structure of glucose Fischer Projection The α-glucose and β-glucose, differ only in the orientation of the hydroxyl group at C-1 atom. Such pairs of the optical isomers that differ only in the orientation of H and OH group at C1 atom are called anomers. The C-1 atom is called anomeric carbon atom (or glycosidic carbon). It was found that the –OH group at C-5 in glucose combines with the –CHO group and forms cyclic hemiacetal structure. Haworth Projection Haworth proposed a 6-membered cyclic structure for glucose, based on the structure of a hetrocyclic compound pyran. Such a structure is known as the pyranose structure. Haworth projection of α-glucose and βglucose are as follows; Fructose It is ketohexose It have keto group Natural monosaccharide, found in fruits and honey. It melts with slight decomposition at 102 degree celsius. It is laevorotatory; as it rotates plane polarised light to the left. Cyclic structure of fructose Haworth proposed a 5-membered cyclic structure for fructose, based on the structure of a hetrocyclic compound furan. Such a structure is known as the furanose structure. Protein Proteins are found in almost all the living cell of the plants and animals. Proteins constitute about 20% of the living cell. They are essential for the growth and maintenance of life. Proteins are highly complex nitrogenous organic compounds of high molecular mass. They They are polyamides. are condensation polymer of α-amino acids. Successive Amino hydrolysis of proteins; acids:- they are basic building units of proteins. The number of amino acids in proteins may vary. Amino acids The compounds which have carboxylic acid group and amino group are called amino acids. They They are obtained on hydrolysis of protein. are differ from one another in the nature of side chain. Nearly, all naturally occurring amino acids are α-amino acids. Those amino acids in which NH2 group and COOH group attached to the same carbon are called α-amino acids. Classification of α-Amino Acids On the basis of the relative number of amino and carboxylic groups. Acidic Amino Acids When the number of amino group is less than the number of carboxylic acid group in the molecule of amino acid, it is called acidic amino acid. Basic Amino Acids When the number of amino group is more than the number of carboxylic acid group in the molecule of amino acid, it is called basic amino acid. Neutral Amino acids When the number of amino and carboxylic group is equal in the molecule of amino acid, it is called neutral amino acid. Essential and Non-essential Amino Acids Out of the 20 α-amino acids required for protein synthesis, only 10 can be synthesised in human body. On this basis, α-amino acids are classified as essential and non-essential amino acids. Essential Amino acids Those amino acids which are not synthesised by our body are called essential amino acids. Their deficiency can be cause diseases like Kwashiorkor. Non-essential Amino Acids Those amino acids which are synthesised by our body are called non-essential amino acids. Properties of amino acids Amino acids are usually colourless, crystalline solid. These are water soluble with high melting point. They behave like salt due to presence of basic amino group and acidic due to carboxylic group. In aqueous solution, the carboxylic group can lose a proton and amino group can accept a proton to give a dipolar ion which is known as ‘Zwitter ion’. Zwitter ion Amino acids show amphoteric nature. They react with both acids and bases. In acidic solution, Amino acids group accepts a proton. In basic solution, carboxylic group loses a proton. Classification of proteins On the basis of molecular shape, proteins are classified into two categories. Proteins Fibrous proteins Globular proteins Fibrous proteins In fibrous proteins, polypeptide chains are held together by hydrogen bonds and disulphide bonds and form fibre-like structure. Generally, For insoluble in water. example:- keratin in skin, hair, feather etc. Globular proteins In globular proteins, the chains of polypeptides are folded together into compact units. They are soluble in water. They act as enzymes to catalyse the biological reactions. Some Sone For of them regulate metabolic reactions. act as antibodies. example:- albumin and insulin. Structure of proteins (Peptide Bond) Peptide link is repeated many times to produce a polypeptide. A polypeptide with more tan 100 amino acid is called protein. Proteins are condensation polymers, also categorised as polyamides due to the presence of –CONH group. Structure of Proteins Primary structure of proteins Proteins have one or more polypeptide chain. Each polypeptide in a protein has amino acids linked with each other in a specific sequence of amino acids in a protein is called primary structure of proteins. Secondary structure of Proteins The arrangement of peptide chain into 3-D structures is called secondary structure of proteins. This arrangement is due to the folding of chain. The folding is due to hydrogen bond and peptide bond between carbonyl and –NH- groups of different peptide bonds. It gives the shape of the protein molecule. The folding gives two types of structures; (1) α-helix (2) β-pleated sheets α-Helix It is formed when the chain of α-amino acids coils as a right hand screw. It is due to the formation of hydrogen bonds between the amide groups of the same peptide chain. β-Pleated sheet It is formed when the chain of the α-amino acids are arranged side by side. The chains are held together by a very large number of H-bonds. β -pleated sheet is not a very stable structure because of van der waal repulsion between the substituents on the –carbon atom of one side chain and the corresponding substituents on the neighbouring chain. β -pleated sheets are arranged in two ways; (1) parallel β–sheet (2) Anti-parallel β-sheet Parallel βsheet:-Amino acid chains run in the same direction. Antiparallel β-sheet:Amino acid chains run in opposite direction. Tertiary structure of proteins It arises due to the further folding, coiling and bending of the secondary structure. It give the overall shape of proteins. The attractive forces between the amino acid side chain make the proteins result in a compact and complex structure. Quaternary structure of proteins Quaternary structure determines the number of sub-units and their arrangement in a protein molecule. Sub-units are joined to form proteins with the molecular mass greater than 50000 amu. Denaturation of proteins It is the change in the physical and biological properties without affecting its chemical composition. Causes; Change in pH Change in temperature Presence of some salts or chemicals During denaturation, the protein molecule uncoils from an ordered and specific conformation into a more random conformation. Nucleic acid Nucleic acid are biomolecules which are found in the nuclei of all living organism or cells in the form of nucleoprotein. They are biopolymers in which the repeating structural unit or monomeric unit is a nucleotide, so that nucleic acid are also called polynucleotides. Each nucleotides have three components. (1) pentose sugar ribose, deoxyribose (2) nitrogenous base these are of two types, (a) purines adenine, guanine (b) pyrimidines cytosine, thymine, uracil (3) phosphoric acid Nucleosides A base joined to a sugar molecule is called nucleosides. Depending upon the sugar present, there are two types of nucleotides; ribonucleoside and deoxyribonucleoside Nucleotides A nucleotide have all the three components. I’e., pentose sugar, nitrogenous base and phosphoric acid. The two nucleotides are connected by a phosphodiester linkage Glycosidic linkage The bond formed between C1 atom of sugar and nitrogen atom of the nitrogenous base. Nucleotide chain in a nucleic acid DNA (deoxyribonucleic acid) It is genetic material responsible for the hereditary character of the cell Present in the nucleus of the cell Composition; (1) A pentose sugar deoxyribose (2) Four nitrogenous bases (a) Purine; adenine, guanine (b) Pyrimidine; thymine, cytosine (3) phosphate group Structure of DNA The double-helix structure of DNA was established by James Watson and Francis Crick. The two nucleic acid chains are bonded to each other and held together by the hydrogen bonds. Hydrogen bonds are formed between complementary base pairs. A to T and G to C RNA (Ribonucleic acid) It is genetic material in some plant and animal viruses. It found in cytoplasm and on the membrane in ribosome. Composition; (1) A pentose sugar ribose (2) four nitrogenous bases (a) Purine; adenine, guanine (b) Pyrimidine; uracil, cytosine (3) phosphate group Structure of RNA is similar to that of DNA but it is single stranded. Types of RNA It is of three types (1) Messenger RNA:- m-RNA (2) Ribosomal RNA:- r-RNA (3) Transfer RNA:- t-RNA
