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Proteins AP Biology 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 © 2014 Pearson Education, Inc. Table 3-3 © 2014 Pearson Education, Inc. Proteins Most structurally & functionally diverse group Function: involved in almost everything AP Biology 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) 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 AP Biology Figure 3-17 Structural proteins Hair © 2014 Pearson Education, Inc. Horn Silk 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 © 2014 Pearson Education, Inc. Amino acids Structure central carbon amino group carboxyl group (acid) R group (side chain) H O H | || —C— C—OH —N— | H R variable group different for each amino acid confers unique chemical properties to each amino acid like 20 different letters of an AP Biology alphabet can make many words (proteins) Oh, I get it! amino = NH2 acid = COOH Figure 3-18 Amino acid structure variable group (R) amino group carboxylic acid group hydrogen © 2014 Pearson Education, Inc. 3 - Proteins (Honors) contain C, H, O, and N (nitrogen) amino acids (monomer) bonded together AP Biology 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 © 2014 Pearson Education, Inc. Figure 3-20 Protein synthesis amino acid amino group dehydration amino acid synthesis carboxylic amino acid group group © 2014 Pearson Education, Inc. peptide peptide bond water Building Proteins dehydration synthesis / condensation rxn: AP Biology http://www.wisc-online.com/objects/index_tj.asp?objID=AP13304 Proteins Structure H2O 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 AP Biology Rubisco growth hormones Figure 3-19 Amino acid diversity glutamic acid (glu) aspartic acid (asp) Hydrophilic functional groups phenylalanine (phe) leucine (leu) Hydrophobic functional groups © 2014 Pearson Education, Inc. cysteine (cys) Sulfur-containing functional group 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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Animation: Protein Structure Figure 3-21 The four levels of protein structure Primary structure: Secondary structure: The sequence of amino acids linked by peptide bonds Usually maintained by hydrogen bonds, which shape this helix leu val heme group lys lys gly his hydrogen ala bond lys val Quaternary structure: Tertiary structure: lys helix pro © 2014 Pearson Education, Inc. Folding of the helix results from hydrogen bonds with surrounding water molecules and disulfide bridges between cysteine amino acids Individual polypeptides are linked to one another by hydrogen bonds or disulfide bridges Figure 3-22 The pleated sheet and the structure of silk protein hydrogen bond stack of pleated sheets Pleated sheet © 2014 Pearson Education, Inc. Structure of silk disordered segment strand of silk Building proteins Peptide bonds covalent bond between NH2 (amine) of one amino acid & COOH (carboxyl) of another C–N bond H2O dehydration synthesis AP Biology peptide bond 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 AP Biology Protein structure & function Function depends on structure 3-D structure twisted, folded, coiled into unique shape pepsin hemoglobin AP Biology collagen 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! AP Biology lysozyme: enzyme in tears & mucus that kills bacteria 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 AP Biology Secondary (2°) structure AP Biology Sulfur containing amino acids Form disulfide bridges covalent cross links betweens sulfhydryls stabilizes 3-D structure H-S – S-H You wondered why perms smell like rotten eggs? AP Biology 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 AP Biology sulfurs in sulfhydryls (S–H) anchors 3-D shape Quaternary (4°) structure More than one polypeptide chain bonded together only then does polypeptide become functional protein hydrophobic interactions AP Biology = skin & tendons collagen hemoglobin Protein structure (review) R groups hydrophobic interactions disulfide bridges (H & ionic bonds) 3° multiple polypeptides hydrophobic interactions 1° amino acid sequence peptide bonds determined by DNA AP Biology 4° 2° R groups H bonds 3.5 What Are Proteins? The functions of proteins are related to their threedimensional 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 © 2014 Pearson Education, Inc. Protein denaturation Unfolding a protein In Biology, size doesn’t matter, SHAPE matters! 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 AP Biology Denatured protein AP Biology AP Biology Sickle cell anemia I’m hydrophilic! AP Biology Just 1 out of 146 amino acids! But I’m hydrophobic!