PROTEINS

 PROTEINS

AMINO ACIDS

·         Proteins are complex organic compounds found in all living organisms.

·         These are essential for cell structure and function.

·         Amino acids are the building blocks of proteins.

·         Amino acids have amine (-NH2) and carboxylic acid (-COOH) functional groups and have an R (a side chain hydrocarbon).

                                              

·         Change in the R group changes the amino acid.

·         There are only 20 amino acids which form billions of proteins.

                                               

·         The centre carbon atom in amino acid is called the alpha-carbon.

                                             

·         Amino acids are colourless and crystalline.

AMINO ACIDS

The list of amino acids, their 3-letter code and 1-letter code.

• Alanine - ala – A
• Arginine - arg - R
• Asparagine - asn - N
• Aspartic acid - asp - D
• Cysteine - cys - C
• Glutamine - gln - Q
• Glutamic acid - glu - E
• Glycine - gly - G
• Histidine - his - H
• Isoleucine - ile - I
• Leucine - leu - L
• Lysine - lys - K
• Methionine - met - M
• Phenylalanine - phe - F
• Proline - pro - P
• Serine - ser - S
• Threonine - thr - T
• Tryptophan - trp - W
• Tyrosine - tyr - Y
• Valine - val - V

ZWITTER ION

-          Amino acids have amine (-NH2) and carboxylic acid (-COOH) functional groups and have an R (a side chain hydrocarbon).

-          In an amino acid, when the carboxylic acid group loses H+ ion and amine group gains H+ ion, a zwitterion is formed. Zwitterion contains COO- croup and NH3+ group.

                             

-          Zwitterion have both positive and negative charges, and hence neutral.

-          Zwitterions are sometimes are also called ‘inner salts’.

PEPTIDES

·         Two or more amino acids combine together to form a peptide.

                                                 

·         A peptide is formed when two amino acids are joined by a peptide bond.

·         Peptide bond is formed by removal of a water molecule between the carboxyl group of one amino acid and amine group of another amino acid.

·         Peptide bond is backbone of protein chain.

·         Hence each polypeptide has a free carboxyl terminal (C-) and an amine terminal (N-).

·         Classification of amino acids based upon their length:-

Ø  Dipeptide: - made up of 2 amino acids.

Ø  Tripeptide: - made up of 3 amino acids.

Ø  Oligopeptide: - made up of 2-10 amino acids.

Ø  Polypeptide: - made up of 10 or more amino acids.

Ø  Macropeptide

CHEMICAL BONDS IN PROTEIN STRUCTURES

·         Primary bond: -

Ø  Covalent peptide bond.

 

·         Secondary bonds: -

Ø  Disulphide bond: - formed by oxidation of thiol (-SH) group. Found in insulin.

Ø  Hydrogen bond: - found in secondary structure of proteins.

Ø  Non-polar

PROTEIN CONFIGURATION

·         Proteins can be divided into four categories based upon their structure.

·         Primary structure: -

Ø  Primary structure refers to the linear sequence of amino acids from the N-terminal to the C-terminal.

·         Secondary structure: -

Ø  In this a polypeptide chain is folded about itself to form alpha-helices and beta-sheets.

Ø  These structures are stabilizes by hydrogen bonds.

Ø  Alpha-helices: -

      These are spiral structures.

      These have right hands or left handed orientation.

      Stabilized by hydrogen bond between amine and carboxyl group.

      These fibres are insoluble.

      Keratin is an example of alpha-helices.

      These form hair, nails, hooves and horns.

Ø  Beta-sheets: -

      These are sheet like structures where two or more sections of polypeptides come together.

      Formed by parallel or anti-parallel arrangement of polypeptides in a plane.

      When all N-terminals are on one edge they are called parallel beta-sheets.

      When alternative chains of N-terminal and C-terminal lie on one edge they are called anti-parallel beta-sheets.

      Silk is an example of beta-sheets.

·         Tertiary structure: -

Ø  This involves folding of a single polypeptide into a globular structure called its tertiary structure.

Ø  Myoglobin and ribonuclease are proteins that have a tertiary structure.

·         Quaternary structure(protein-protein interaction): -

Ø  Association of number of peptide chains into a complex and large sized molecule is called quaternary structure.

Ø  Haemoglobin is the best example of protein with quaternary structure.

PRIONS

·         An infectious protein is called a prion

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·         These are often misfolded proteins.

·         Cellular prion proteins (PrPc) may act as acetyl choline receptors and help in nerve transmission, these are normally found in cell membranes

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·         The infectious form is called prion Proteins scrapie (PrPSc).

                                             

·         When PrPSc are injected into a body they convert PrPc into PrPSc.

·         PrPSc cause neurodegenerative diseases by aggregating in the central nervous system and disrupt the healthy tissue.

·         The best known disease caused by prions is Bovine spongiform encephalopathy (BSE) or commonly called the mad cow disease.