Chapter 10 Proteins Workers of the Cell
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Outline • 10.1 Amino Acids—A Second Look • 10.2 Protein Formation • 10.3 The Three-Dimensional Structure of Proteins • 10.4 Denaturation of Proteins • 10.5 Protein Functions • 10.6 Enzymes—Life’s Catalysts • 10.7 Factors That Affect Enzyme Activity
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10.1 Amino Acids—A Second Look • “Amino” indicates a protonated amine (–NH3+). • “Acid” indicates a carboxylic acid (–COO-). • These groups are bonded to a central alpha (α) carbon atom. • The α carbon is also bonded to a hydrogen atom and a side chain.
Insert colored diagram of amino acid structure from page 382.
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10.1 Amino Acids—A Second Look • In all but one amino acid (glycine), the α carbon is a chiral center. Insert L- and D- amino acids and Fischer projections from page 383
• The L-amino acids are the building blocks of proteins. Some D-amino acids do occur in nature but rarely in proteins. © 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look • The R group gives each amino acid its unique identity and characteristics. • Twenty amino acids are found in most proteins. • Nine families of organic compounds are represented: alkanes (hydrocarbon), aromatics, thioethers, alcohols, phenols, thiols, amides, carboxylic acids, and amines. • The 10 amino acids designated with an asterisk (*) in the table are called essential amino acids because they must be obtained in the diet. • A complete protein meal can be obtained by combining foods like rice and beans or peanut butter on whole-grain bread. © 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look
© 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look
© 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look
© 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look
© 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look
© 2014 Pearson Education, Inc.
10.1 Amino Acids—A Second Look Classification of Amino Acids • Amino acids are separated into nonpolar and polar. • With few exceptions, the side chains of nonpolar amino acids are composed entirely of carbon and hydrogen and are hydrophobic. • Polar amino acid side chains contain functional groups that create an uneven distribution of electrons in the side chain. • Polar acidic and polar basic amino acids have charged side chains, allowing them to form ion–dipole interactions with water.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
10.2 Protein Formation • Condensation reactions occur between amino acids, and the product formed is a dipeptide. • The carboxylate ion (–COO-) of one amino acid molecule reacts with the protonated amine (–NH3+) of a second amino acid. • A water molecule is removed, and an amide functional group is formed.
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10.2 Protein Formation • In a dipeptide, the N-terminus (or amino terminus) has an unreacted α-amino group. • The C-terminus (or carboxy terminus) has an unreacted carboxylate group. • By convention, peptides are always written from the N-terminus to the C-terminus.
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10.2 Protein Formation • Each pair of amino acids can combine to form two different dipeptides. • The two dipeptides are structural isomers, different compounds, and have different properties. • The order of the amino acids is critical to the structure and function of the compound.
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10.2 Protein Formation • Dipeptides are the smallest members of the peptide class. • Any compound containing amino acids joined by a peptide bond can be called a peptide. • A compound with three amino acids is a tripeptide, one with four amino acids is a tetrapeptide, and so on. • As the number of amino acids increases, the compound is referred to as a polypeptide. • A biologically active polypeptide containing 50 or more amino acids is a protein.
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10.3 The Three-Dimensional Structure of Proteins Primary Structure • The primary (1°) structure of a protein is the order in which the amino acids are joined together to form the protein backbone. • The side chains of the amino acids are substituents dangling from this backbone.
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10.3 The Three-Dimensional Structure of Proteins Secondary (2°) Structure • The α helix is a coiled structure stabilized by hydrogen bonds formed between the carbonyl (C=O) oxygen atom (δ-) of one amino acid and the N–H hydrogen atom (δ+) of the amino acid on the fourth amino acid away from it in the primary structure. • The positioning of the hydrogen bonds allows the helix to stretch and recoil. Multiple hydrogen-bonding interactions make the helix a strong structure. • In the α helix, the side chains of the amino acids project outward away from the axis of the helix. © 2014 Pearson Education, Inc.
10.3 The Three-Dimensional Structure of Proteins Secondary (2°) Structure • The β-pleated sheet is an extended structure in which segments of the protein chain align to form a zigzag structure like a folded paper fan. • Beta strands are held together side by side by hydrogen-bonding interactions between their backbones. • In the β-pleated sheet, the side chains of the amino acids project above and below the sheet.
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10.3 The Three-Dimensional Structure of Proteins Tertiary (3°) Structure • The α helices and β-pleated sheets of the polypeptide chain interact with each other and the environment to create the tertiary structure (3°). • Nonpolar side chains are repelled by an aqueous environment and turn toward the interior of the protein. • Polar side chains are attracted to aqueous surroundings and appear on the surface. • Tertiary structure is stabilized by attractive forces between side chains and the environment as well as by attractive forces between side chains themselves. • To satisfy all the competing interactions, the protein folds into a specific three-dimensional shape. © 2014 Pearson Education, Inc.
10.3 The Three-Dimensional Structure of Proteins
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10.3 The Three-Dimensional Structure of Proteins Tertiary (3°) Structure Interactions 1. Nonpolar interactions. Nonpolar amino acid side chains are repelled by the aqueous environment and aggregate in the interior of the protein. 2. Polar interactions. Polar amino acid side chains interact with water and each other through dipole–dipole, ion–dipole, and hydrogen-bonding interactions. 3. Salt bridges (ionic interactions). Acidic and basic amino acid side chains exist in their ionized form in an aqueous environment. The opposite charges attract, thereby forming a stabilizing ionic interaction called a salt bridge. 4. Disulfide bonds Two –SH groups (thiols) can react with each other through an oxidation reaction (losing hydrogens) to form a disulfide bond –S–S–. The disulfide bond is a covalent bond.
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10.3 The Three-Dimensional Structure of Proteins Tertiary (3°) Structure • Globular proteins fold into a compact, spherical shape with polar amino acid side chains on their surface and nonpolar amino acid side chains forming a nonpolar core. Enzymes and many cellular proteins are globular proteins. • Fibrous proteins have long, thread-like structures. Aligned helices form strong, durable structures. Fibrous proteins tend to be insoluble in water.
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10.3 The Three-Dimensional Structure of Proteins Collagen and Vitamin C •
Scurvy, a disease caused by a deficiency of vitamin C in the diet, affects collagen formation.
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Collagen contains a modified amino acid called hydroxyproline. The additional hydroxyl group on hydroxyproline allows hydrogen bonds between the chains, adding extra strength to the triple helix formed in collagen.
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Vitamin C is critical to the conversion of proline to hydroxyproline. Without hydroxyproline, collagen is weakened, resulting in spongy and bleeding gums, opening of scars, and nail loss.
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Scurvy can be reversed by a diet containing adequate vitamin C. In the United States, smokers and people who do not get enough fresh produce or take vitamin supplements are at risk for scurvy.
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10.3 The Three-Dimensional Structure of Proteins Quaternary (4°) Structure • Quaternary (4°) structure describes the interactions of two or more polypeptide chains to form a single biologically active protein. • The individual polypeptide chains or subunits are held together by the same interactions that stabilize the tertiary structure of a single protein. • Not all biologically active proteins have a quaternary (4°) structure.
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