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Proteins

Proteins. 3.5 What Are Proteins?. Proteins are molecules composed of chains of amino acids Proteins have a variety of functions Enzymes are proteins that promote specific chemical reactions Structural proteins (e.g., elastin) provide support. Table 3-3. Proteins.

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Proteins

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  1. Proteins

  2. 3.5 What Are Proteins? • Proteins are molecules composed of chains of amino acids • Proteins have a variety of functions • Enzymes are proteins that promote specific chemical reactions • Structural proteins (e.g., elastin) provide support

  3. Table 3-3

  4. Proteins • Most structurally & functionally diverse group • Function: involved in almost everything • enzymes (pepsin, DNA polymerase) • structure (keratin, collagen) • carriers & transport (hemoglobin, aquaporin) • cell communication • signals(insulin & other hormones) • receptors • defense (antibodies) • movement (actin & myosin) • storage (bean seed proteins)

  5. play a role in cell membrane function • receptor proteins: recognize and bind to substances wanting to enter membrane • transport proteins: help transport substances across cell membrane • build muscles

  6. Figure 3-17 Structural proteins Horn Hair Silk

  7. 3.5 What Are Proteins? • Proteins are molecules composed of chains of amino acids (continued) • Proteins are polymers of amino acids joined by peptide bonds • All amino acids have a similar structure • All contain amino and carboxyl groups • All have a variable “R” group • Some R groups are hydrophobic • Some are hydrophilic • Cysteine R groups can form disulfide bonds

  8. H O | —C— | H || C—OH —N— H Amino acids • Structure • central carbon • amino group • carboxyl group (acid) • R group (side chain) • variable group • different for each amino acid • confers unique chemical properties to each amino acid • like 20 different letters of an alphabet • can make many words (proteins) R Oh, I get it! amino = NH2 acid = COOH

  9. Figure 3-18 Amino acid structure variable group (R) amino group carboxylic acid group hydrogen

  10. 3 - Proteins (Honors) • contain C, H, O, and N (nitrogen) • amino acids (monomer) bonded together

  11. 3.5 What Are Proteins? • Amino acids are joined by dehydration synthesis • An amino group reacts with a carboxyl group, and water is lost • The covalent bond resulting after the water is lost is a peptide bond, and the resulting chain of two amino acids is called a peptide • Long chains of amino acids are known as polypeptides, or just proteins

  12. Figure 3-20 Protein synthesis dehydration synthesis amino acid amino acid water peptide amino group carboxylic acid group peptide bond amino group

  13. Building Proteins dehydration synthesis / condensation rxn: http://www.wisc-online.com/objects/index_tj.asp?objID=AP13304

  14. H2O Proteins • Structure • monomer =amino acids • 20 different amino acids • polymer =polypeptide • protein can be one or more polypeptide chains folded & bonded together • large & complex molecules • complex 3-D shape hemoglobin growthhormones Rubisco

  15. Figure 3-19 Amino acid diversity aspartic acid (asp) glutamic acid (glu) Hydrophilic functional groups phenylalanine (phe) leucine (leu) cysteine (cys) Hydrophobic functional groups Sulfur-containing functional group

  16. 3.5 What Are Proteins? • A protein can have as many as four levels of structure • Primary structure is the sequence of amino acids linked together in a protein • Secondary structure is a helix, or a pleated sheet • Tertiary structure refers to complex foldings of the protein chain held together by disulfide bridges, hydrophobic/hydrophilic interactions, and other bonds • Quaternary structure occurs where multiple protein chains are linked together

  17. Animation: Protein Structure

  18. Figure 3-21 The four levels of protein structure Primary structure: The sequence of amino acids linked by peptide bonds Secondary structure: Usually maintained by hydrogen bonds, which shape this helix leu val heme group lys lys gly his hydrogen bond ala lys val Quaternary structure: Individual polypeptides are linked to one another by hydrogen bonds or disulfide bridges Tertiary structure: Folding of the helix results from hydrogen bonds with surrounding water molecules and disulfide bridges between cysteine amino acids lys helix pro

  19. Figure 3-22 The pleated sheet and the structure of silk protein hydrogen bond stack of pleated sheets strand of silk disordered segment Pleated sheet Structure of silk

  20. H2O peptidebond Building proteins • Peptide bonds • covalent bond between NH2 (amine) of one amino acid & COOH (carboxyl) of another • C–N bond dehydration synthesis

  21. Building proteins • Polypeptide chains have direction • N-terminus = NH2 end • C-terminus = COOH end • repeated sequence (N-C-C) is the polypeptide backbone • can only grow in one direction

  22. hemoglobin collagen Protein structure & function • Function depends on structure • 3-D structure • twisted, folded, coiled into unique shape pepsin

  23. Primary (1°) structure • Order of amino acids in chain • amino acid sequence determined by gene (DNA) • slight change in amino acid sequence can affect protein’s structure & its function • even just one amino acid change can make all the difference! lysozyme: enzyme in tears & mucus that kills bacteria

  24. Secondary (2°) structure • “Local folding” • folding along short sections of polypeptide • interactions between adjacent amino acids • H bonds • weak bonds between R groups • forms sections of 3-D structure • -helix • -pleated sheet

  25. Secondary (2°) structure

  26. Sulfur containing amino acids • Formdisulfide bridges • covalent cross links betweens sulfhydryls • stabilizes 3-D structure H-S – S-H You wonderedwhy permssmell like rotten eggs?

  27. Tertiary (3°) structure • “Whole molecule folding” • interactions between distant amino acids • hydrophobic interactions • cytoplasm is water-based • nonpolar amino acids cluster away from water • H bonds & ionic bonds • disulfide bridges • covalent bonds between sulfurs in sulfhydryls (S–H) • anchors 3-D shape

  28. Quaternary (4°) structure • More than one polypeptide chain bonded together • only then does polypeptide become functional protein • hydrophobic interactions hemoglobin collagen = skin & tendons

  29. Protein structure (review) R groups hydrophobic interactions disulfide bridges (H & ionic bonds) 3° multiple polypeptides hydrophobic interactions 1° amino acid sequence peptide bonds 4° 2° determinedby DNA R groups H bonds

  30. 3.5 What Are Proteins? • The functions of proteins are related to their three-dimensional structures • Precise positioning of amino acid R groups leads to bonds that determine secondary and tertiary structure • Disruption of secondary and tertiary bonds leads to denatured proteins and loss of function

  31. In Biology,size doesn’t matter, SHAPE matters! Protein denaturation • Unfolding a protein • conditions that disrupt H bonds, ionic bonds, disulfide bridges • temperature • pH • salinity • alter 2° & 3° structure • alter 3-D shape • destroys functionality • some proteins can return to their functional shape after denaturation, many cannot

  32. Denatured protein

  33. Just 1out of 146amino acids! Sickle cell anemia I’mhydrophilic! But I’mhydrophobic!

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