Introduction of proteins
- They are high weight polymers.
- They are polyamides that contain C, H, N, O and S.
- Proteins are derived from alpha amino carboxylic acid monomers.
- A simple protein may contain hundred even thousands of amino acid units.
- In living organisms twenty alpha amino acids occur which combine to form different protein molecules.
- A simple amino acid can be represented as R-CH(NH)2COOH (carboxy group and amino group is present in it).
Alpha amino acids
- The acid in which NH2 group is present at carbon atom adjacent to the COOH group are called alpha amino acids.
- Alpha amino acids are the building blocks of proteins the alpha carbon of all amino acids is chiral, hence all amino acids exhibit stereoisomerism that is existence of D and L types of structures.
- All the naturally occurring amino acids, belong to L form category.
- In L amino acid NH2 group lies left to the chiral carbon as shown :
H2N-CH2R (C0OH) L –amino acid
- Due to transfer of proton from carboxy to amino group, alpha amino acid exists as dipolar ion or called as Zwitter ion.
- Due to the zwitter ion structure, alpha amino acids are high meting crystalline solids and moderately soluble in water.
- In acidic medium caboxylate ion group act as a base and accepts a proton. Thus alpha amino acids exist as cations (I) under the influence of electric field.
(zwitter ion )
- In alkaline medium, NH3+groups act as an acid and thus loses a proton.
- Due to this, alpha amino acids exist as anions (III) and thus migrate towards the anode in the electric field.
- However, at some intermediate value of ph, the concentration of cationic form (I) and the anionic form (III) become equal.
- Hence, there is no migration in electric field. This pH is known as isoelectric point.
Types of amino acids
The amino acids are of two types:
Essential amino acids
- The amino acids that can’t be made in our body and must be supplied from outside.
- The lack of these amino acids in diet can cause lot of diseases like kwashiorkor (the disease in which water balance in body is disturbed).
- The essential amino acids are 10 in number out of all.
Non essential amino acids
- They are those that can be synthesized in our body.
- Out of total 20 amino acids the 10 can be synthesized in the body.
List of different types of amino acids:
- The interaction between amino group and carboxyl of amino group give compounds called peptides.
- The amide group –CONH in each such compound is called peptide linkage.
- Depending upon the number of amino acid, residues per molecule they are called as dipeptide, tripeptide etc.
- Peptides of molecular weight up to 10,000 are known as polypeptides and those with higher than 10,000 are called as proteins.
- Each polypeptide chain has a free amino group at one end and a free carboxyl group at other end.
- They are collectively called as end groups.
- The amino acid unit having –NH2group is called N-terminal end and the amino acid unit having free –COOH group is called C-terminal end.
- In this NH-CH-CO is the repeating unit in polypeptide.
Structure of proteins
Proteins have three structures:
- Primary structure
- Secondary structure
- Tertiary structure
Primary structure: The sequence in which amino acids are arranged in protein is called primary structure. The sequence determines the function of a protein.
Secondary structure: The fixed configuration of polypeptide skeleton is known as secondary structure.
- There are two types of secondary structure :
- Alpha helix
- Beta pleated sheet
Alpha helix: If the size of group R is large, intermolecular hydrogen bonds are formed between CO of one amino acid residue and NH of the fourth amino acid residue in polypeptide chain which gives right handed alpha helix structure to the protein molecule.
Example: Alpha keratin in hair etc it is also elastic.
Beta pleated sheet: If the size of group R is small, intermolecular hydrogen bonds are formed between CO of one polypeptide chain with NH of the other chain. Thus the chains are bonded together forming a sheet which can slide over each other to form a three dimensional structure called beta pleated sheet.
Tertiary structure: It implies the three dimensional structure of proteins.
There are two types tertiary structure:
- Fibrous and globular.
- Proteins contain one or more polypeptide chains.
- A protein having one polypeptide chain is known as monomeric while that having more than one polypeptide chains is called oligomeric.
- The constituent peptide chains of an oligomeric protein are called protomers which are held together by weak forces.
- Native state: At normal pH and temperature each protein takes a shape which is energetically most stable.
- In amino acid the shape is specific and is known as native state.
- Globular proteins are tightly folded and give rise to spherical form.
Forces that stabilize protein structures
The forces that are present are as follows:
- Hydrogen bonding
- Anionic bonding
- Hydrophobic bonding
- Covalent bonding
Hydrogen bonding: These forces operate between a partially positive hydrogen and partially negative atom like O or N on the same or on another molecule.
Anionic bonding: A bonding between cation and anion of side chains resulting in side linkage.
Hydrophobic bonding: Some side chains in same amino acid are hydrophobic. In aqueous solutions proteins fold in such a way that these chains get clustered inside the folds .The polar side chains which are hydrophilic lie on the outside or surface of proteins.
Covalent bonding: The bond occurs between S atoms of two residues between two adjacent chains.
- Insulin which contains 51 amino acids is arranged in two polypeptide chains containing 21 and 30 amino acid residues connected by S-S cross links.
Denaturation of proteins
- The globular proteins, which are soluble in water on heating or on treatment of mineral acids or bases undergo coagulation or precipitation to give fibrous proteins which are insoluble in water.
- After coagulation, proteins lose their biological activity this is called denaturation.
- It can be reversible or irreversible.
- Coagulation of lactalbumin to form cheese and coagulation of albumins are examples of denaturation.
Classification of proteins
On the basis of composition, proteins are of following types :
- Simple proteins
- Conjugated proteins
- Derived proteins
- Fibrous proteins
- Globular proteins
Simple proteins: On hydrolysis they give only amino acids.
Example: Globulins and albumin
Conjugated proteins: They contain non protein group attached to the protein part. These non protein groups are called prosthetic groups.
Example: Nucleo-protein contains nucleic acid, phosphor-protein contains phosphoric acid contains phosphoric acid, glycol-proteins contains carbohydrates etc.
Derived proteins: These are the degradation products obtained by the hydrolysis of simple and conjugated proteins.
Example: Peptides, peptones etc
Fibrous proteins: They are long and thread like and tend to lie side by side to form fibers .In some cases, they are held together by hydrogen bonds at many points .these proteins serves as a chief structural material of animal tissues .
Globular proteins: The molecules of these proteins are folded into compact units and form spheroid shapes .Intermolecular forces are weak. These proteins are soluble in water or aqueous solution of acids, bases or salts .Globular proteins make up all enzymes, hormones ,fibrinogen etc.
Role of proteins
- They act as enzymes and transport agents.
- They are structural materials for nails, hair etc.
- Antibodies formed in body are globular proteins.
- They are metabolic regulators like insulin etc.
Hydrolysis of proteins
- Proteins are hydrolyses when boiled with acids or alkalis or when treated with enzymes .the hydrolysis takes place as:
Proteins àproteasesàpeptones àpolypeptides àsimple peptides àamino acids
- Every protein has an isoelectric point at which their ionization is minimum. Proteins have charged groups i.e. NH3 and COO- at the ends of peptide chain.
- They are amphoteric in nature.
- Protein accepts a proton in strong basic solution.
- The pH at which the protein molecule has no net charge is known as isoelectric point.