Proteins

Protein - a macromolecule is explained. The general characteristics, its chemical and structural components are described. Protein sources, nutritive value also dealt in it. As a major portion classification of proteins are given. Along with it properties, both physical and chemical properties and the various functions of proteins are also given

1. PROTEINS Dr.N.C.J.Packia Lekshmi Allied Health Sciences Noorul Islam Centre for Higher Education

2. INTRODUCTION Protein is a macromolecule composed of one or more polypeptide chains It’s a polymer of L α-amino acids. The term protein is derived from Greek: Proteuo means primary or holding first place. The term protein was first proposed by Berzelius Most abundant organic molecules of the living system. Its fundamental basis of structures and function of life. Proteins make up 12% of the protoplasm - 50 % of dry weight of every cell. They are body builders They contain carbon, hydrogen, oxygen, nitrogen and sometimes sulphur They are constructed largely of aminoacids 300 different amino acids occur in nature – only 20 as standard amino acids. 21st amino acid added - Selenocysteine

3. Sources of Protein Proteins are obtained both from animal and plant souces . The animal sources of proteins include milk, egg, meat, fish, liver etc… Plant sources of proteins are pulses, nuts and cereals

4. Nutritive value Nutritive value of a protein is based on two factors – amino acid composition and digestability Different foods contain different amounts and different combinations of amino acids (the building blocks of proteins). Protein from animal sources (e.g. meat, fish, eggs and dairy products) contains the full range of essential amino acids needed by the body – the nutritive value is high called as first class proteins However, vegans and vegetarians can get all the amino acids they need by combining different plant sources of protein, e.g. pulses and cereals – nutritive value is low is called as second class proteins

5. Elemental Composition of Protein All proteins contain C,H,O,N and sometimes S. Many proteins contain P also. Elements such as I,Fe,Cu and Zn are also occassionally present. Elements % C 50 O 23 N 16 H 07 S 0-3 P 0-3

6. What made Proteins Proteins are made of polymers of aminoacids . The aminoacids are the building blocks or monomers of proteins. An amino acid consists of five parts: An amino group (NH2) A carboxyl group (COOH) A hydrogen atom An R group or a side chain or alkyl A carbon atom

7. There are more than 100 amino acids. All proteins of the biological system from bacteria to man are constructed out of 20 amino acids only. The 20 amino acids make up thousands of proteins. For eg ., bacterial cell contains 1000 to 2000 proteins and the human cell contains as many as 100000 protein molecules.

8. How amino acids are linked to form Protein? In proteins, amino acids are linked together by a bond called peptide bond A peptide bond is a chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, releasing a molecule of water (H2O). This is a dehydration synthesis reaction (also known as a condensation reaction), and usually occurs between amino acids. The resulting CO-NH bond is called a peptide bond, and the resulting molecule is an amide. The four-atom functional group -C(=O)NH- is called an amide group or (in the context of proteins) a peptide group.

9. Formation of Peptide bond

10. The peptide bond is present in all proteins that bind the amino acid in the chain together. Monopeptide : having one amino acid Dipeptide : having two amino acids Tripeptide : having three amino acids Tetrapeptide : having four amino acids Pentapeptide : having five amino acids Hexapeptide : having six amino acids Heptapeptide : having seven amino acids Octapeptide : having eight amino acids Oligopeptide : having less than 10 amino acids Polypeptide: having more than 10 amino acids

11. How does a polypeptide chain becomes a Protein? A peptide is two or more amino acids joined together by peptide bonds; a polypeptide is a chain of many amino acids; and a protein contains one or more polypeptides Many protein such as myoglobin consist of a single polypeptide chain. Others contain two or more chains, which may be either identical or different. Eg ., haemoglobin is formed of 4 polypeptide chains of which two chains are of one kind and the other two are of another kind. The polypeptide chains are linked by disulphide bonds. Eg ., insulin has two chains joined by two disulphide bonds

12. What Is the Difference Between a Peptide and a Protein? The basic distinguishing factors are size and structure. Peptides are smaller than proteins. Traditionally, peptides are defined as molecules that consist of between 2 and 50 amino acids, whereas proteins are made up of 50 or more amino acids. In addition, peptides tend to be less well defined in structure than proteins, which can adopt complex conformations known as secondary, tertiary, and quaternary structures.

13. Occurrence of amino acids in various Proteins The amino acid compositions for a large variety of proteins of microbial, plant and animal origin have reliably been established. The sequence of amino acids in a protein is closely related to the genetic code. Alanine , glycine , leucine are most commonly found in proteins. But each protein has its own amino acid composition. Protamines , simple protein found in fish sperm, contain as much as 85% arginine , but lack threonine and lysine as well as cyclic, acidic and sulphur containing amino acids. Fibroin, protein of silk contains 50% of glycine . Collagen, contains hydroxylysine and hydroxyproline which are absent in other proteins.

14. Classification of Proteins They are classified into two ways On the basis of their solubility or shape On the basis of increasing complexity of structure

15. Classification of Protein on the basis on solubility or shape Proteins are classified into two groups on the basis of their solubility or shape. Globular proteins fibrous proteins

16. Globular proteins Spherical in shape Soluble in water Highly branched Polypeptide chains are linked by usual peptide bonds Tightly folded into spherical or globular shapes Eg ., enzymes, protein hormones, antibodies, haemoglobin , myoglobin

17. Fibrous Protein Insoluble in water In the form of fibres Highly resistant to digestion by proteolytic enzymes Unbranched – linear molecules Long linear protein chains are held together by intermolecular hydrogen bonds Not folded in to globular molecules Serve as structural proteins Eg ., collagen of tendons, elastin of connective tissue, fibroin of silk, keratin of silk, actin and myosin

18. Classsification of protein On the basis of increasing complexity of structure On the basis of increasing complexity of structure, proteins are classified into three groups Simple proteins Conjugated proteins Derived proteins

19. Simple Proteins They are composed of amino acids only. Some examples are; Protamine : They are positively charged (basic) proteins mostly present in animals and fishes (sperm) Protamines binds with DNA in embryonic stage and later replaced by histone It is soluble in water and ammonium hydroxide solution It is not coagulated by heat It precipitate out in aqueous solution of alcohol Protamine are rich in arginine and lysine whereas devoid of sulfur containing and aromatic amino acids.

20. Histone : They are basic protein but weak base in comparison to protamine . Histone is low molecular weight protein and are water soluble. Histones are rich in basic amino acids like histidine and arginine , but deficient in tryptophan and contain little cystine or methionine . It is not coagulated by heat. Histone is present in nucleic acids as nucleohistone binding with DNA.

21. Albumin: It is the most abundant protein in nature It is most commonly found in seeds in plants and in blood and muscles in animals. Molecular weight of albumin is 65000 KD It is water soluble and can be coagulated by heat Plant albumins; Leucosine , Legumelins etc Animal albumins; serum albumin, myosin, lactalbumin , ova-albumin etc

22. Globulin: Pseudoglobulin (water soluble) and Euglobulin (water insoluble) They are coagulated by heat. They are precipitated by lower concentrations of salts such as ammonium sulphate or sodium sulphate Eg ., plasma globulin, serum globulin, ovaglobulin in egg white, myosin in muscles and edestin in hemp seed. Glutelins : Water insoluble. Eg . Glttenin (wheat), glutelin (corn), oryzenin (rice) They are coagulated by heat. They are rich in arginine , proline and glutamic acid Eg ., Glutenin in wheat oryzenin in rice.

23. Prolamine : They are storage protein found in seeds. They are water insoluble. But soluble in dilute acid or detergents and 60-80% alcohol. They are coagulated by heat Prolamine is rich in proline and glutamine Examples; Gliadin (wheat), zein (corn), Hordein (barley), Avenin (oats)

24. Conjugated Proteins These proteins in which protein are always linked by non-protein moiety to become functional. So, they are composed of both protein and non- protein components. The non-protein component is known as prosthetic group. On the basis of prosthetic group, they are classified as follows; Metalloprotein : They have metal prosthetic group. Some metals such as Hg, Ag, CU, Zn etc, strongly binds with proteins such as collagen, albumin, casein by –SH group of side chain of amino acids. Eg . Ceruloplasmin ; contains copper as prosthetic group Some other metals such as Calcium weakly binds with protein. Eg . Calsequestrin , calmodulin Some metals such as Na, K etc do not binds with protein but associate with nucleic acids protein.

25. Chromoprotein : They have colored prosthetic group. These are simple proteins linked to a metallic prosthetic group which gives the colour to the protein Some examples are; Haemoprotein : Haemoglobin , myoglobin , chlorophyll, cytochrome , peroxidase , haemocyanin Flavoprotein : Riboflavin ( Vit B2) give yellow/orange color to FAD requiring enzymes Glycoprotein/ Mucoprotein : They have carbohydrate as prosthetic group On hydrolysis they yield amino sugars Eg . Antibody, complement proteins, Heparin, Hyaluronic acid

26. Phosphoprotein : They have phosphate group as prosthetic group. The phosphoric acid is attached to the hydroxyl group of protein by an ester linkage Eg . Caesein (milk protein binds with calcium ion to form calcium salt of caseinate ) Ovovitellin ; present in egg yolk Calcineurin Lipoprotein: They have lipid as prosthetic group. Eg . Lipovitelline , chylomicrons

27. Derived Protein These protein are the derivatives of either simple or complex protein resulting from the action of heat, enzymes and chemicals. Some artificially produced protein are included in this group. They are classified as primary derived protein and secondary derived protein. Primary derived protein: The derived protein in which the size of protein molecules are not altered materially but only the arrangement is changed. Some examples are; Proteans : Obtained as a first product after the action of acid or enzymes or water on protein. They are denatured protein They are insoluble in water. Eg . Edestan , myosin

28. Metaprotein : They are produced by further action of acid or alkali on protein at 30-60°C. They are water insoluble but soluble in dil acid or alkali. Also known as Infraprotein . Eg . Curd Coagulated protein: They are produced by the action of heat or alcohol on protein. They are insoluble in water. Eg . Coagulated egg white

29. Secondary derived protein: The derived protein in which size of original protein are altered. Hydrolysis has occurred due to which size of protein molecule are smaller than original one. Examples; a) Proteoses : They are produced by the action of dilute acid or digestive enzymes when the hydrolysis proceeds beyond the level of metaprotein . They are soluble in water They are not coagulated by heat. • Eg . Albumose , Globulose etc. Peptones They are soluble in water They are not coagulated by heat They are precipitated by saturating their solutions with ammonium sulphate Polypeptides They are derivatives of proteins containing many amino acid units

30. Properties of Proteins Physical Properties Chemical Properties

31. Physical Properties of Proteins Colour and Taste Proteins are colourless except chromoproteins and usually tasteless. These are homogeneous and crystalline. Shape and Size The proteins range in shape from simple crystalloid spherical structures to long fibrillar structures. Two distinct patterns of shapehave been recognized : A . Globular proteins- These are spherical in shape and occur mainly in plants , esp., in seeds and in leaf cells. These are bundles formed by folding and crumpling of protein chains. e.g., pepsin, edestin , insulin, ribonuclease etc. B . Fibrillar proteins- These are thread-like or ellipsoidal in shape and occur generally in animal muscles. Most of the studies regarding protein structure have been conducted using these proteins. e.g., fibrinogen, myosin etc .

32. Molecular Weight The proteins generally have large molecular weights ranging between 5 × 103 and 1 × 106. It might be noted that the values of molecular weights of many proteins lie close to or multiples of 35,000 and 70,000. Colloidal Nature Because of their giant size, the proteins exhibit many colloidal properties, such as; Their diffusion rates are extremely slow and they may produce considerable light-scattering in solution, thus resulting in visible turbidity (Tyndall effect). Denaturation Denaturation refers to the changes in the properties of a protein. In other words, it is the loss of biologic activity. In many instances the process of denaturation is followed by coagulation— a process where denatured protein molecules tend to form large aggregates and to precipitate from solution.

33. Amphoteric Nature Like amino acids, the proteins are amphoteric , i.e., they act as acids and alkalies both. These migrate in an electric field and the direction of migration depends upon the net charge possessed by the molecule. The net charge is influenced by the pH value. Each protein has a fixed value of isoelectric point (pl) at which it will move in an electric field. Ion Binding Capacity The proteins can form salts with both cations and anions based on their net charge.

34. Solubility The solubility of proteins is influenced by pH. Solubility is lowest at isoelectric point and increases with increasing acidity or alkalinity. This is because when the protein molecules exist as either cations or anions, repulsive forces between ions are high, since all the molecules possess excess charges of the same sign. Thus, they will be more soluble than in the isoelectric state. Optical Activity All protein solutions rotate the plane of polarized light to the left, i.e., these are levoratotory .

35. Chemical Properties of Proteins Hydrolysis Proteins are hydrolyzed by a variety of hydrolytic agents. A. By acidic agents: Proteins, upon hydrolysis with conc. HCl (6–12N) at 100–110°C for 6 to 20 hrs, yield amino acids in the form of their hydrochlorides. B. By alkaline agents: Proteins may also be hydrolyzed with 2N NaOH . Reactions involving SH Group A. Nitroprusside test: Red colour develops with sodium nitroprusside in dilute NH4.OH. The test is specific for cysteine . B. Sullivan test: Cysteine develops red colour in the presence of sodium 1, 2-naphthoquinone- 4-sulfonate and sodium hydrosulfite.

36. Reactions involving COOH Group A . Reaction with alkalies (Salt formation) B. Reaction with alcohols ( Esterification ) C. Reaction with amines Reactions involving NH2 Group A. Reaction with mineral acids (Salt formation): When either free amino acids or proteins are treated with mineral acids like HCl , the acid salts are formed. B. Reaction with formaldehyde: With formaldehyde, the hydroxy -methyl derivatives are formed. C. Reaction with benzaldehyde : Schiff ‘s bases are formed D. Reaction with nitrous acid (Van Slyke reaction): The amino acids react with HNO2 to liberate N2 gas and to produce the corresponding α- hydroxy acids. E. Reaction with acylating agents ( Acylation ) F. Reaction with FDNB or Sanger’s reagent G. Reaction with dansyl chloride

37. Reactions involving both COOH AND NH2 Group A. Reaction with triketohydrindene hydrate ( Ninhydrin reaction) B. Reaction with phenyl isocyanate : With phenyl isocyanate , hydantoic acid is formed which in turn can be converted to hydantoin . C. Reaction with phenyl isothiocyanate or Edman reagent D. Reaction with phosgene: With phosgene, N- carboxyanhydride is formed E. Reaction with carbon disulfide: With carbon disulfide, 2-thio-5-thiozolidone is produced

38. Reactions involving R Group or Side Chain A. Biuret test B. Xanthoproteic test C. Millon’s test D. Folin’s test E. Sakaguchi test F. Pauly test G. Ehrlich test

39. Functions of Proteins Enzyme Catalyst Almost all chemcial reactions in the biological system are catalyzed by enzymes. They increase reaction rates atleast a million fold. Transport system Proteins transport ions and small molecules Haemoglobin – conjugated protein of blood, transports oxygen Myoglobin – a muscle protein, transports oxygen in the muscles Transferrin – carries iron in the plasma of blood The membrane proteins – transport glucose, aminoacids and other nutrients across the membrane of the cell Storage Certain protein function as a storage molecules Ferritin – a protein stores iron in the liver Seeds – stores nutrient proteins eg ., Wheat, corn, rice etc…

40. Nutrients Certain proteins function as a storage molecule The egg contains ovalbumin Milk contains casein Contraction and Movement The contraction of muscle is brought about by two fibrous proteins called actin and myosin The microtubules of flagella and cilia are built on tubulin , a protein Mechanical Support Many proteins serve as supporting filaments, cables or sheets to give biological structures, strength, support and protection Collagen – fibrous protein – major component of tendons, cartilage and leather Ligaments contain elastin , a structural protein Keratin – an insoluble protein, is the main component of hair, finger nails and feathers Fibroin – major component of silk fibres and spider web

41. Immune Protection Many proteins defend against invading organisms Antibodies are protein immunoglobulins Blood clotting Blood clotting factors such as fibrinogen and thrombin are proteins Transmission of Nerve Impulse The nerve impulse is transmitted through synapse with the help of receptor proteins Gene Expression The inactivation of genes is brought by repressor proteins Hormonal Action Insulin, growth hormone, parathyroid hormone etc.. are proteins Thermoregulation The blood plasma of some antarctic fish contains antifreeze proteins, which protect the blood from freezing

42. Sometimes we’re tested not to show our weaknesses, but to discover our strengths THANK YOU