Proteins

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Proteins

  1. 1. STRUCTURE & FUNCTIONS OF PROTEINS <ul><li>Proteins are essential in biological systems as controls e.g enzymes and structural elements e.g. cytoskeleton </li></ul><ul><li>Proteins are heteropolymers as they are made up of different amino acids (20 different types) </li></ul><ul><li>The type and order of amino acids determines the structure and function of proteins allowing them to carry out many different roles </li></ul>
  2. 2. Amino Acids <ul><li>Amino acids are characterised by the amino group (NH 2 ) and the carboxylic acid (COOH) </li></ul><ul><li>These are attached to a central carbon atom which also carries a hydrogen </li></ul><ul><li>The side chains are variable, the ‘R’ group can be joined here </li></ul>
  3. 3. <ul><li>At neutral pH, amino acids exist in ionised forms. Once joined, the charges on amino acids disappear. </li></ul><ul><ul><li>R GROUP </li></ul></ul><ul><li>This gives the amino acid it’s unique chemical properties and specific shape. </li></ul><ul><li>The R group can be classified as acidic, basic, uncharged polar or non-polar </li></ul>
  4. 4. Types of Amino Acid <ul><li>ACIDIC: R group contains a CARBOXYL group COOH (exists as COO - at neutral pH) </li></ul><ul><li>e.g. Aspartic Acid [ R group - CH 2 COOH] </li></ul><ul><li>BASIC : R group contains a basic AMINO group NH (which has a positive charge at neutral pH NH 3 + ) </li></ul><ul><li>e.g. Lysine [R group - (CH 2 ) 4 NH 4 ] </li></ul><ul><li>POLAR : R group contains an ALCOHOL or a PHENOL OH group which is polar and hydrophilic] </li></ul><ul><li>e.g. Serine [R group - CH 2 OH] </li></ul><ul><li>NON-POLAR : R group contain a hydrophobic, non-polar side chain </li></ul><ul><li>e.g. Glycine [R group - H ] </li></ul>
  5. 5. The Peptide Bond <ul><li>Proteins are made by joining amino acids together by an amide linkage / peptide bond </li></ul><ul><li>A chain of amino acids is called a polypeptide </li></ul><ul><li>The peptide bond is formed by DEHYDRATION SYNTHESIS or a condensation reaction between the carboxyl group of one amino acid and the amine group of the next amino acid </li></ul><ul><li>Amino acids joined in this way are called residues </li></ul>
  6. 6. The Peptide Bond <ul><li>The Peptide bond is very strong </li></ul><ul><li>C-N bond is planar (flat) so peptide bond allows NO rotation </li></ul><ul><li>The single bonds either side DO allow rotation of the residues, so polypeptide chains are flexible </li></ul>ANIMATION
  7. 7. Protein Structure <ul><li>Chemical bonding is critical in determining a protein’s shape and the different types of bonds are important for different levels of protein structure </li></ul><ul><ul><li>PEPTIDE BOND = COVALENT BOND = VERY STRONG </li></ul></ul><ul><li>In higher order protein structures, weaker interactions are important too.These include: </li></ul><ul><ul><li>Non-covalent bonds </li></ul></ul><ul><ul><li>Hydrogen Bonds </li></ul></ul><ul><ul><li>Ionic bonds </li></ul></ul><ul><ul><li>Van der Waals interactions </li></ul></ul><ul><ul><li>Hydrophobic interactions between R groups </li></ul></ul>
  8. 9. Primary Structure (1 o ) <ul><li>Primary structure refers to the &quot;linear&quot; sequence of amino acids. </li></ul><ul><li>The amino end or N terminus is positioned to the left. The carboxyl end or C terminus is positioned to the right </li></ul>N C Generally 3 or 1 letter abbreviations are used to denote amino acids when primary structures are drawn
  9. 10. Secondary Structure (2 o ) <ul><li>Secondary structure is &quot;local&quot; ordered structure brought about via hydrogen bonding mainly within the peptide backbone </li></ul><ul><li>A single polypeptide many contain several secondary structures </li></ul><ul><li>The most common secondary structure elements in proteins are the alpha (  ) helix and the beta (  ) sheet (sometime called b pleated sheet) </li></ul>
  10. 11. <ul><li>ALPHA HELIX </li></ul><ul><li>The polypeptide chain is coiled into a right handed helix by Hydrogen bonding (stabilises the helix) between the NH group of the peptide and the C=O of the peptide bond, four residues away from it </li></ul><ul><li>BETA PLEATED SHEET </li></ul><ul><li>The polypeptide chains are linked together in a side by side configuration by hydrogen bonding. Beta sheets can be either parallel or anti-parallel depending on the orientation of the constituent parts </li></ul>
  11. 12. Tertiary Structure (3 o ) <ul><li>This describes the way in which the polypeptide folds to give the final structure of the protein. </li></ul><ul><li>The 3 o structure is determined by hydrophobic interactions which place the amino acids non-polar R groups towards the centre of the molecule </li></ul><ul><li>In many proteins an additional important type of bond is the disulphide bond. This bond forms between sulphydryl (SH) groups on cysteine residues; so may be formed between 2 different polypeptides or within the polypeptide itself. </li></ul><ul><li>Within any tertiary structure, parts of the amino acid sequence may adopt an  -helix,  -sheet or more complex  sheet arrangements e.g. myoglobin </li></ul>
  12. 13. <ul><li>The ion group is a prosthetic group – a non-protein group associated with a folded protein </li></ul><ul><li>If the attached group is : </li></ul><ul><ul><li>CARBOHYDRATE = Glycoprotein </li></ul></ul><ul><ul><li>LIPID = Lipoprotein </li></ul></ul><ul><ul><li>NUCLEIC ACID = Nucleoprotein </li></ul></ul><ul><li>As proteins have a relatively stable structure in a cellular environment, it is remarkable that the forces that hold them together can be easily disrupted if the chemical environment changes or the sequence of amino acids is changed </li></ul>These are known as conjugated proteins
  13. 14. Quaternary Structure (4 o ) <ul><li>Proteins that are composed of 2 or more polypeptide sub-units </li></ul><ul><li>Examples of proteins with quaternary structure include HAEMOGLOBIN , DNA POLYMERASE , and ION CHANNELS </li></ul>

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