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3. PROTEINS Polypeptide Chains = polymers of amino acids that are arranged in a specific linear sequence and are linked by peptide bonds Protein = one or more polypeptide chains that make up one macromolecule Make up 50% or more of cellular dry weight Functions include: o Metabolism: ENZYMES speed up chemical reactions in cells o Structural support o Transport: carriers in cell membranes and large-scale, like hemoglobin o Defense: Antibodies o Regulation: Hormones and other intercellular messengers o Motion: Contractile proteins (muscles) and intracellular transport proteins Made of only 20 amino acid monomers Amino Acids = monomer building blocks of proteins o Made of an alpha carbon, which is covalently bonded to: H atom amino group carboxyl group Variable R group (side chain) o Grouped by properties of side chains Nonpolar side groups (hydrophobic) Polar side groups (hydrophilic) May be Uncharged or charged polar Polypeptides are held together by Peptide Bonds = covalent bond formed by a dehydration reaction that links the carboxyl group of one amino acid with the amino group of another o Gives it a repeating backbone of N-C-C-N-C-C-… Protein function depends on its 3-D shape = Conformation Native Conformation = normal shape o Enables a protein to recognize and bind to another molecule 4 Levels of Protein Structure 1. Primary Structure = unique sequence of amino acids in a protein Determined by genes Can be sequenced in the lab 2. Secondary Structure = regular, repeated coiling and folding of a protein’s polypeptide backbone Stabilized by H-bonds between peptide linkages o Carbonyl of one a.a. and amino group of another a.a. Fibrous Proteins (e.g. keratin or silk) are composed of this level Two types: a) Alpha Helix = helical coil stabilized by H-bonding between every 4th peptide bond b) Beta-Pleated Sheet = sheet of antiparallel chains folded into accordion pleats Held together by either intrachain or interchain H-bonds 3. Tertiary Structure = the overall 3-D shape of a single polypeptide chain Shape is held together by bonding between and among side chains (R- groups) and by interactions between R groups and the aqueous environment Weak Interactions o H-bonds between polar side chains o Ionic bonding between charged side chains o Hydrophobic side chains orient themselves so that they are minimally exposed to water in the protein’s interior Covalent Linkage o Disulfide Bridges form between 2 cysteine amino acid monomers brought together by the folding of the protein. This is a strong bond! Globular Proteins (ex. many enzymes) stop at this level 4. Quaternary Structure = a 3-D structure that results from the interactions between and among two or more polypeptide chains that work together to act as one overall protein Ex. Hemoglobin (4 globular polypeptides) Protein Conformation Conformation is influenced by the PHYSICAL and CHEMICAL environmental conditions If the environment changes too much, the protein may become denatured and lose its native conformation Can happen if: o The protein is placed in an organic (nonpolar) solvent o Chemical agents disrupt H-bonds, ionic bonds, or difulfide bridges Ex. Change in pH o The protein is subjected to excess heat The protein may or may not return to the native conformation when the environment returns to normal, depending on: o The degree of denaturation o The need for chaperone proteins = temporarily scaffold proteins as they fold in the cell Believed that they don’t fold it really, but help correct it if it misfolds May play a role in Alzheimer’s or Cystic Fibrosis? Prion = a misfolded protein that causes other proteins of the same type to fold incorrectly as well o TSE diseases, like Mad Cow Disease, are due to these