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22 • Primary Structure How important is the exact amino acid sequence? • See Human insulin. Human Cow Hog Sh eep A Chain p ositions 8-9-10 B Chain p os ition 30 -Thr-Ser-Ile-A la-Ser-Val-Thr-Ser-Ile-Ala-Gly-Val- -Thr -Ala -Ala -Ala Human insulin Minor A.A. replacements tolerated (sometimes) Otherwise allergic reactions . . . NAMING? start @ N-terminus; e.g. A.A. #1 in insulin is “Gly” © 2006 Thomson Learning, Inc. All rights reserved 22-1 22 Primary Structure (cont.) • Vasopressin and oxytocin - nonapeptides but have quite different biological functions. Cys S S Cys Pro Gly NH2 Tyr A sn Ph e Gln Vas op res sin Cys S S Cys Pro Leu NH2 Tyr A sn Ile Gln Oxytocin • Vasopressin is an antidiuretic hormone. (Alcohol inhibits) • Oxytocin ~ contractions of uterus in childbirth and the muscles of the breast that aid in the secretion of milk. Minor replacements big changes in biology. . . Primary Sequence matters © 2006 Thomson Learning, Inc. All rights reserved 22-2 22 Secondary Structure Conformations of A.A.s in localized regions of a polypeptide chain. Two Main Types: 1. a-Helix: polypeptide chain coils into a Right-handed spiral 2. b-Pleated sheet: two polypeptide chains align parallel to each other (chains may be parallel or antiparallel --------------- (minor) 3. Random coils (orange) β-Pleated sheet (blue) α-Helix (green) 4. Extended helix (collagen © 2006 Thomson Learning, Inc. All rights reserved 22-3 22 a-Helix • Figure 21.6. © 2006 Thomson Learning, Inc. All rights reserved •α-Helix The C=O group (peptide bond) is H- bonded to N-H group other peptide bond, 4 amino acid units away from it. 22-4 22 • Figure 21.6. © 2006 Thomson Learning, Inc. All rights reserved b-Pleated Sheet •C=O, N-H groups of peptide bonds from adjacent chains H-Bond (intermolecular/intramolecular H-bonds) 22-5 22 Conformational changes to B sheets (Many diseases) Mad Cow, Alzheimers etc . . Amyloid plaques © 2006 Thomson Learning, Inc. All rights reserved 22-6 22 Tertiary Structure overall conformation of an entire polypeptide chain. Stabilized in 4 ways: 1. Covalent bonds, e.g. formation of disulfide bonds b/w cysteine side chains. 2. H- bonding, e.g. b/w side chains (R groups) e.g. OH groups of serine and tyrosine. 3.Salt bridges, e.g. -NH3+ group of lysine + -COO- group of aspartic acid. 4. Hydrophobic interactions, b/w N. polar side chains of Phenylalanine and leucine. (e.g. oil/vinegar analogy) © 2006 Thomson Learning, Inc. All rights reserved 22-7 22 Tertiary Structure • See figure 21.11 NOTE: R-groups in A.A.s play major role in forming tertiary structure via 4 bonding types . . . Otherwise . . . © 2006 Thomson Learning, Inc. All rights reserved 22-8 22 Chaperone Proteins • Facilitate folding of newly synthesized polypeptide chains secondary, tertiary structures Prevent denaturing loss of biological activity © 2006 Thomson Learning, Inc. All rights reserved 22-9 22Challenge Question? • What kind of noncovalent interation occurs between the side chains (R-groups) of Arginine and glutamic acid? O + H3N CH C O O- + H3N CH CH2 CH2 CH2 CH2 CH2 C NH O- C C O- O NH2+ NH2 © 2006 Thomson Learning, Inc. All rights reserved 22-10 22 Quaternary Structures arrangement of polypeptide chains into a noncovalently bonded aggregation. -chains held together by H-bonds, salt bridges, hydrophobic interactions. • e.g. Hemoglobin • Adult hemoglobin: two alpha chains of 141 amino acids each, and two beta chains of 146 amino acids each. • Each chain surrounds an iron-containing heme unit. • Fetal hemoglobin: two alpha chains and two gamma chains; fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin. © 2006 Thomson Learning, Inc. All rights reserved 22-11 22Hemoglobin • Figure 21.13 The 4° structure of hemoglobin. © 2006 Thomson Learning, Inc. All rights reserved 22-12 22Hemoglobin • Figure 21.14 The structure of heme. © 2006 Thomson Learning, Inc. All rights reserved 22-13 22Integral Membrane proteins • Provide structure and perform services for cell • e.g. transport, © 2006 Thomson Learning, Inc. All rights reserved 22-14 22Quaternary Structures • Figure 21.15 Integral membrane protein of rhodopsin made of a-helices. © 2006 Thomson Learning, Inc. All rights reserved 22-15 22Quaternary Structure • Figure 22.18 The b-barrel of an integral membrane protein of the outer membrane of a mitochondrion is made of eight b-pleated sheets. © 2006 Thomson Learning, Inc. All rights reserved 22-16 22Glycoproteins • proteins with covalently bonded carbos. P.546 • Via O-linked sacharides • (e.g. mucins, coat membranes • Respiratory tract) • Via N-linked saccharides • (e.g. immunoglobins, IgG,IgA) © 2006 Thomson Learning, Inc. All rights reserved 22-17 22Denaturation • process of destroying the native conformation of a protein by chemical or physical means. Why Cook Food? • Some denaturations are reversible, while others permanently damage the protein. © 2006 Thomson Learning, Inc. All rights reserved 22-18