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7.5 Proteins

7.5 Proteins. (plus some 3.2.2 and 3.2.5 on amino acids). Proteins. Proteins have many structures , resulting in a wide range of functions Proteins do most of the work in cells Proteins are made of monomers called amino acids. 3.2.2 General structure of an amino acid. R. O. H. N. C.

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7.5 Proteins

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  1. 7.5 Proteins (plus some 3.2.2 and 3.2.5 on amino acids)

  2. Proteins • Proteins have many structures, resulting in a wide range of functions • Proteins do most of the work in cells • Proteins are made of monomers called amino acids

  3. 3.2.2 General structure of an amino acid R O H N C C H OH H R group carboxyl group amino group (acidic) (basic)

  4. CH3 CH2 O O H H N C C N C C H OH H OH H H phenylalanine (aromatic) alanine Different amino acids have different R groups Their different properties depend on their R groups Hydrophobic (‘Water-hating’) R groups

  5. SH OH CH2 CH2 O O H H N N C C C C cysteine serine H H OH OH H H Hydrophillic (‘Water-loving’) R groups

  6. Some amino acids are basic or acidic, which makes them polar and hydrophillic. In life, they are usually ionized. OH NH2 O O C C CH2 CH2 O O H H asparagine (basic) N N C C C C H H OH OH H H First isolated from asparagus juice! aspartic acid (acidic)

  7. Alanine Arginine Asparagine Aspartic acid Cysteine Glutamic acid Glutamine Glycine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine There are 20 amino acids naturally incorporated into proteins You do NOT need to know their names!

  8. 3.2.5 Formation of a dipeptide via a condensation reaction: carboxyl group of one amino acid reacts with amino group of second amino acid R1 R2 O O H H N N C C C C H H OH OH H H water molecule formed H2O R1 O R2 H O H N C C N C C a dipeptide H OH H H peptide linkage

  9. R2 R1 O O H H N N C C C C H H OH OH H H R1 O H R2 O H N C C N C C H OH H H Formation of a dipeptide produces a molecule of water, therefore… H2O …this is a condensation reaction 2 amino acids dipeptide + water

  10. R2 R1 O O H H N N C C C C H H OH OH H H R1 O H R2 O H N C C N C C H OH H H 3.2.5 Splitting a dipeptide to form two amino acids consumes one molecule of water, therefore… H2O …this is a hydrolysis reaction dipeptide + water  2 amino acids

  11. 7.5.3 Explain the significance of polar and non-polar amino acids • The shape of a protein is determined by the lowest energy (most stable) conformation the polypeptide chain can fold into. • Polar (hydrophillic) amino acids tend to be on the outside of the folded protein (near water) • Non-polar (hydrophobic) amino acids tend to be on the inside of the folded protein (away from water) • Because the shape of a protein determines its function, and the sequence of polar vs. non-polar amino acids determines the shape, polarity is VERY important!

  12. Tripeptide: three amino acids linked together Oligopeptide: a short chain of a few amino acids Polypeptide: a chain of many amino acids A protein may consist of one or more polypeptide chains Structure of haemoglobin – a protein consisting of four polypeptides

  13. Protein Conformation and Function • A protein’s specific conformation (shape) determines how it functions

  14. Amino acid subunits +H3NAmino end Pro Thr Gly Gly Thr Gly Glu Seu Lys Cys Pro Leu Met Val Lys Val Leu Asp Ala Arg Val Gly Ser Pro Ala Glu Lle Asp Thr Lys Ser Tyr Trp Lys Ala Leu Gly lle Ser Pro Phe His Glu His Ala Glu Val Thr Phe Val Ala Asn lle Thr Asp Ala Tyr Arg Ser Ala Arg Pro Gly Leu Leu Ser Pro Tyr Ser Tyr Ser Thr Thr Ala o Val c Val Glu – Lys o Thr Pro Asn Carboxyl end Figure 5.20 7.5.1 Explain the four levels of protein structure, indicating the significance of each level. • Primary structure • Is the unique sequence of amino acids in a polypeptide

  15. H H H H H H O O O O O O O H H H H H H R R R R R R R C C C C C C C C C C C C C N N N N N N N N N N N N N C C C C C C C C C C C C C C R R R R R R H H H H H H H O O O O O O O H H H H H H H  pleated sheet H O H H Amino acidsubunits C C N N N C C C R H O H H H H H H N N N N N N  helix C C O C H H H C C C R R R R R H H C C C C C C O O O O H C R O C C O H C O N N H C C H R H R Figure 5.20 • Secondary structure • Is the folding or coiling of the polypeptide into a repeating configuration • Includes the  helix and the  pleated sheet

  16. Hydrophobic interactions and van der Waalsinteractions CH CH2 CH2 H3C CH3 OH Polypeptidebackbone H3C CH3 Hyrdogenbond CH O HO C CH2 CH2 S S CH2 Disulfide bridge O -O C CH2 CH2 NH3+ Ionic bond • Tertiary structure • Is the overall three-dimensional shape of a polypeptide • Results from interactions between amino acids and R groups

  17. Quaternary structure • Is the overall protein structure that results from the aggregation of two or more polypeptide subunits

  18. +H3N Amino end Amino acid subunits helix 7.5.1-Review of Protein Structure

  19. Sickle-Cell Disease: A Simple Change in Primary Structure • Sickle-cell disease • Results from a single amino acid substitution in the alpha polypeptide chain of the protein hemoglobin • The substitution exchanges a polar amino acid for a non-polar amino acid (glutamic acid for valine) • Because there is now a non-polar amino acid on the outside of the protein (near water) this is unstable • As a result of this instability, the hemoglobin molecules clump together and cause the cell to sickle, inhibiting oxygen transport and causing sickle cell anemia

  20. Normal hemoglobin Sickle-cell hemoglobin Primary structure Primary structure . . . . . . Exposed hydrophobic region Val His Leu Thr Pro Glul Glu Val His Leu Pro Glu Thr Val 5 6 7 3 4 5 6 7 1 2 1 2 3 4 Secondaryand tertiarystructures Secondaryand tertiarystructures  subunit  subunit     Quaternary structure Hemoglobin A Quaternary structure Hemoglobin S     Molecules interact with one another tocrystallize into a fiber, capacity to carry oxygen is greatly reduced. Function Molecules donot associatewith oneanother, eachcarries oxygen. Function 10 m 10 m Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen Red bloodcell shape Red bloodcell shape Figure 5.21 Fibers of abnormalhemoglobin deform cell into sickle shape.

  21. Denaturation Normal protein Denatured protein Renaturation Figure 5.22 Denaturation is when a protein unravels and loses its native conformation (shape)*changes in pH or temperature can cause denaturation*a denatured protein will no longer work because its shape has been changed

  22. 7.5.4 State four functions of proteins with named examples Structure- Function as structural components of cells, collagen gives structure to animal tissues by inhabiting the space between cells Enzymes- Function as catalysts, the enzyme peroxidase catalyzes the reaction that breaks down hydrogen peroxide Transport- Function as carrier molecules, hemoglobin carries oxygen and carbon dioxide Hormones-Function as chemical messengers Contractile- Cause the movement of cells by contracting and relaxing, actin is an example in human muscle cells Defense- Function as part of the immune response, immunoglobulins are an example in humans.

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