Proteins Proteins Multipurpose molecules 2008-2009 1
Proteins Most structurally & functionally diverse group Function: involved in almost everything u enzymes (pepsin, DNA polymerase) u structure (keratin, collagen) u carriers & transport (hemoglobin, aquaporin) u cell communication signals (insulin & other hormones) receptors u defense (antibodies) u movement (actin & myosin) u storage (bean seed proteins) Proteins Structure u monomer = amino acids 20 different amino acids u polymer = polypeptide protein can be one or more polypeptide chains folded & bonded together large & complex molecules complex 3-D shape hemoglobin H 2 O Rubisco growth hormones 2
Amino acids Structure u central carbon u amino group u carboxyl group (acid) u R group (side chain) variable group different for each amino acid confers unique chemical properties to each amino acid w like 20 different letters of an alphabet w can make many words (proteins) H O H N C C OH H R Oh, I get it! amino = NH 2 acid = COOH Effect of different R groups: Nonpolar amino acids nonpolar & hydrophobic Why are these nonpolar & hydrophobic? 3
Effect of different R groups: Polar amino acids polar or charged & hydrophilic Why are these polar & hydrophillic? Ionizing in cellular waters H+ donors 4
Ionizing in cellular waters H+ acceptors Sulfur containing amino acids Form disulfide bridges u covalent cross links betweens sulfhydryls u stabilizes 3-D structure H-S S-H You wondered why perms smell like rotten eggs? 5
Building proteins Peptide bonds u covalent bond between NH 2 (amine) of one amino acid & COOH (carboxyl) of another u C N bond dehydration synthesis H 2 O peptide bond Building proteins Polypeptide chains have direction u N-terminus = NH 2 end u C-terminus = COOH end u repeated sequence (N-C-C) is the polypeptide backbone can only grow in one direction 6
Protein structure & function Function depends on structure u 3-D structure twisted, folded, coiled into unique shape pepsin hemoglobin collagen Primary (1 ) structure Order of amino acids in chain u amino acid sequence determined by gene (DNA) u 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 7
Sickle cell anemia Just 1 out of 146 amino acids! I m hydrophilic! But I m hydrophobic! Secondary (2 ) structure Local folding u folding along short sections of polypeptide u interactions between adjacent amino acids H bonds w weak bonds between R groups u forms sections of 3-D structure α-helix β-pleated sheet 8
Secondary (2 ) structure Tertiary (3 ) structure Whole molecule folding u interactions between distant amino acids hydrophobic interactions w cytoplasm is water-based w nonpolar amino acids cluster away from water H bonds & ionic bonds disulfide bridges w covalent bonds between sulfurs in sulfhydryls (S H) w anchors 3-D shape 9
Quaternary (4 ) structure More than one polypeptide chain bonded together u only then does polypeptide become functional protein hydrophobic interactions collagen = skin & tendons hemoglobin Protein structure (review) 1 amino acid sequence peptide bonds determined by DNA 2 R groups H bonds R groups hydrophobic interactions disulfide bridges (H & ionic bonds) 3 multiple polypeptides hydrophobic interactions 4 10
Protein denaturation Unfolding a protein u conditions that disrupt H bonds, ionic bonds, disulfide bridges temperature ph salinity u alter 2 & 3 structure alter 3-D shape u destroys functionality In Biology, size doesn t matter, SHAPE matters! some proteins can return to their functional shape after denaturation, many cannot EAT X Proteins! Let s build some 2008-2009 11
Ghosts of Lectures Past (storage) 2007-2008 Chaperonin proteins Guide protein folding u provide shelter for folding polypeptides u keep the new protein segregated from cytoplasmic influences 12
Protein models Protein structure visualized by u X-ray crystallography u extrapolating from amino acid sequence u computer modelling lysozyme 13