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Proteins

Proteins. Proteins. Learning objective To be able to describe the primary, secondary, tertiary and quarternary structure of proteins. In a protein the polypeptide chain may be hundreds of amino acids long. Amino acid polymerisation to form polypeptides is part of protein synthesis .

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Proteins

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  1. Proteins

  2. Proteins • Learning objective • To be able to describe the primary, secondary, tertiary and quarternary structure of proteins

  3. In a protein the polypeptide chain may be hundreds of amino acids long. • Amino acid polymerisation to form polypeptides is part of protein synthesis. • It takes place in ribosomes, and is special because it requires an RNA template. • The sequence of amino acids in a polypeptide chain is determined by the sequence of the genetic code in DNA.

  4. Protein Structure • Polypeptides are just a string of amino acids, • They fold up to form the complex and well-defined three-dimensional structure of working proteins. • To help to understand protein structure, it is broken down into four levels:

  5. Primary Structure • This is just the sequence of amino acids in the polypeptide chain, so is not really a structure at all. • However, the primary structure does determine the rest of the protein structure. • Finding the primary structure of a protein is called protein sequencing, and the first protein to be sequenced was the protein hormone insulin, by the Cambridge biochemist Fredrick Sanger, for which work he got the Nobel prize in 1958.

  6. Secondary Structure • Interactions between the R groups of the amino acids in the chain cause the chain to twist and fold into a three dimensional shape • Lengths of the chain may first coil into α-helices or β-pleated sheets. • These are known as the secondary structures.

  7. α-helices • Within the helix, hydrogen bonds form between the C=O of the carboxylic acid group and the NH of the amine group of the different amino acids.

  8. β-sheets • Several chains may link together with hydrogen bonds holding the parallel chains together.

  9. Tertiary structure • The polypeptide chain often folds and bends to produce a three dimensional shape • The tertiary structure is held together by bonds between the R groups of the amino acids in the protein, and so depends on what the sequence of amino acids is. There are three kinds of bonds involved: • hydrogen bonds, which are weak. • ionic bonds between R-groups with positive or negative charges, which are quite strong. • sulphur bridges - covalent S-S bonds between two cysteine amino acids, which are strong.

  10. The secondary structure is due to backbone interactions and is thus largely independent of primary sequence. • The tertiary structure is due to side chain interactions and thus depends on the amino acid sequence

  11. Quaternary Structure • This structure is found in proteins containing more than one polypeptide chain, and simply means how the different polypeptide chains are arranged together. • The individual polypeptide chains are usually globular, but can arrange themselves into a variety of quaternary shapes.

  12. Globular and fibrous proteins • Proteins can be divided into two distinct groups

  13. Globular proteins • The polypeptide chain is folded into a compact spherical shape • These proteins are soluble due to hydrophilic side chains that project from the outside of the molecules • Globular proteins are therefore important in metabolic reactions

  14. Globular proteins • Enzymes – 3D shape gives the ability to form enzyme – substrate complexes and catalyse reactions • Transport proteins – 3D shape allows them to bind with other molecules eg: proteins in cell membranes or haemoglobin in red blood cells. • Antibodies – precise shape binds to microorganisms

  15. Fibrous proteins • These do not fold into a ball shape • They remain in long chains, often with several polypeptide chains cross linked together for additional strength • They are insoluble

  16. Fibrous proteins • Keratin – in hair and skin • Collagen – in the skin, tendons, cartilage and blood vessel walls

  17. Types of proteins • Fibrous proteins • e.g. collagen • Insoluble • structural • Globular proteins • e.g.enzymes • Soluble • 3D shape

  18. Tertiary structure • Bonding between R-groups gives rise to a 3D shape • H-bonds =O HN- • Ionic bonds –NH3-COO- • Disulphide bridge --CH2S-SCH2- affected by temp & pH affected by pH affected by reducing agents

  19. Examples of Quaternary Structures • Haemoglobin, the oxygen-carrying protein in red blood cells, consists of four globular subunits arranged in a tetrahedral (pyramid) structure. • Each subunit contains one iron atom and can bind one molecule of oxygen.

  20. Examples of Quaternary Structures • Immunoglobulins, the proteins that make antibodies, comprise four polypeptide chains arranged in a Y-shape. • The chains are held together by sulphur bridges. This shape allows antibodies to link antigens together, causing them to clump.

  21. Examples of Quaternary Structures • Actin, one of the proteins found in muscles, consists of many globular subunits arranged in a double helix to form long filaments.

  22. Just to be awkward… • A few proteins have both structures: the muscle protein myosin has a long fibrous tail and a globular head, which acts as an enzyme

  23. This diagram shows a molecule of the enzyme dihydrofolate reductase, which comprises a single polypeptide chain. It has been drawn to highlight the different secondary structures. • Can you identify which is which? • β sheet • α – helix • α – helix 3 2 1

  24. Questions • Name the elements found in proteins. • What are the monomers of proteins? • Draw the structure of an amino acid (label the groups). • What is the name for the bond between two amino acids? • What bonds are found in the secondary structure of proteins? • What is the test for protein? • List 6 functions of proteins. • What is hydrolysis? • What is denaturation? • What kind of protein is an enzyme? • What is the function of a fibrous protein?

  25. H O O C R C H N H H Answers • Name the elements found in proteins. • CHONS • What are the monomers of proteins? • Amino acids • Draw the structure of an amino acid (label the groups). carboxylic acid group The group that determines what amino acid is amine group

  26. What is the name for the bond between two amino acids? • Peptide • What bonds are found in the secondary structure of proteins? • Hydrogen bonds • What is the test for protein? • Biuret test gives purple with protein • List 6 functions of proteins. • Enzymes, structural, carriers, pumps, hormones, antibodies • What is hydrolysis? • Breakdown of a substance by adding water • What is denaturation? • Altering the shape of a protein • What kind of protein is an enzyme? • Globular • What is the function of a fibrous protein? • Structural

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