BIOMOLECULES  1. PROTEINS:What Do Proteins Do?Key ConceptsSlide 4The Structure of Amino AcidsFunctional Groups Affect ReactivitySlide 7Slide 8Monomers and PolymersAssembling and Breaking Apart PolymersSlide 11The Peptide BondSlide 13Polypeptide CharacteristicsSlide 15What Do Proteins Look Like?Primary StructureSecondary StructureSlide 19Tertiary StructureR-group Interactions That Form Tertiary StructuresSlide 22Slide 23Slide 24Slide 25Slide 26Quaternary StructureSummary of Protein StructureSlide 29Folding and FunctionSlide 31Slide 32© 2011 Pearson Education, Inc.BIOMOLECULES  1. PROTEINS:•PROTEINS, the work horses of the cell•Building blocks of proteins are the amino acids (C, H, O, N )Examples: enzymes catalyzing biochemical reactions, antibodies protecting against diseases , Hemoglobin in red blood cells transporting oxygen etc….© 2011 Pearson Education, Inc.What Do Proteins Do? Proteins are crucial to most tasks required for cells to exist:–Catalysis – enzymes speed up chemical reactions.–Defense – antibodies and complement proteins attack pathogens.–Movement – motor and contractile proteins move the cell or molecules within the cell.–Signaling – proteins convey signals between cells.–Structure – structural proteins define cell shape and comprise body structures.–Transport – transport proteins carry materials; membrane proteins control molecular movement into and out of the cell.© 2011 Pearson Education, Inc.Key Concepts Most cell functions depend on proteins.Proteins are made of amino acids. Amino acids vary in structure and function.The structure of a protein can be analyzed at four levels:1. Amino acid sequence2. Substructures called -helices and -pleated sheets3. Interactions between amino acids that dictate a protein’s overall shape 4. Combinations of individual proteins that make up larger, multiunit molecules In cells, most proteins are enzymes that function as catalysts.© 2011 Pearson Education, Inc.AMINO ACIDS  PEPTIDES PROTEINS© 2011 Pearson Education, Inc.The Structure of Amino AcidsAll proteins are made from just 20 amino acid building blocks. •All amino acids have a central carbon atom that bonds to NH2, COOH, H, and a variable side chain (“R-group”).•In water (pH 7), the amino and carboxyl groups ionize to NH3+ and COO–, respectively—this helps amino acids stay in solution and makes them more reactive.The 20 amino acids differ only in the unique R-group attached to the central carbon. The properties of amino acids vary because their R-groups vary.© 2011 Pearson Education, Inc.Functional Groups Affect Reactivity•R-groups differ in their size, shape, reactivity, and interactions with water. 1. Nonpolar R-groups: hydrophobic; do not form hydrogen bonds; coalesce in water2. Polar R-groups: hydrophilic; form hydrogen bonds; readily dissolve in water •Amino acids with hydroxyl, amino, carboxyl, or sulfhydryl functional groups in their side chains are more chemically reactive than those with side chains composed of only carbon and hydrogen atoms.The Nature of Side Chains:© 2011 Pearson Education, Inc.The Nature of Side Chains© 2011 Pearson Education, Inc.The Nature of Side Chains© 2011 Pearson Education, Inc.Monomers and Polymers•Many mid-size molecules, such as amino acids and nucleotides, are individual units called monomers. They link together (polymerize) to form polymers, such as proteins and nucleic acids.•Macromolecules are very large polymers made up of many monomers linked together.•Thus, proteins are macromolecules consisting of linked amino acid monomers.© 2011 Pearson Education, Inc.Assembling and Breaking Apart Polymers•Polymerization requires energy and is nonspontaneous. •Monomers polymerize through condensation (dehydration) reactions, which release a water molecule. •Hydrolysis is the reverse reaction, which breaks polymers apart by adding a water molecule.•In the prebiotic soup, hydrolysis is energetically favorable and thus would predominate over condensation. However, polymers clinging to a mineral surface are protected from hydrolysis, and thus polymerization of the amino acids into proteins may have occurred spontaneously.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Peptide Bond•Condensation reactions bond the carboxyl group of one amino acid to the amino group of another to form a peptide bond.•A chain of amino acids linked by peptide bonds is called a polypeptide.–Polypeptides containing fewer than 50 amino acids are called oligopeptides (peptides).–Polypeptides containing more than 50 amino acids are called proteins.© 2011 Pearson Education, Inc.Nutrasweet  a Dipeptide© 2011 Pearson Education, Inc.Polypeptide Characteristics •Within the polypeptide, the peptide bonds form a “backbone” with three key characteristics:1. R-group orientation–Side chains can interact with each other or water.2. Directionality –Free amino group, on the left, is called the N-terminus.–Free carboxyl group, on the right, is called the C-terminus.3. Flexibility–Single bonds on either side of the peptide bond can rotate, making the entire structure flexible.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.What Do Proteins Look Like? Proteins can serve diverse functions in cells because they are diverse in size and shape as well as in the chemical properties of their amino acids.•Proteins have just four basic levels of structure: primary, secondary, tertiary, and quaternary. (Quaternary only in multi-subunit proteins).© 2011 Pearson Education, Inc.Primary Structure•A protein’s primary structure is its unique sequence of amino acids. •Because the amino acid R-groups affect a polypeptide’s properties and function, just a single amino acid change can radically alter protein function.© 2011 Pearson Education, Inc.Secondary Structure•Hydrogen bonds between the carbonyl group of one amino acid and the amino group of another form a protein’s secondary structure.–A polypeptide must bend to allow this hydrogen bonding, forming:–-helices –-pleated sheets•Secondary structure depends on the primary structure.–Some amino acids are more likely to be involved in -helices; others, in -pleated sheets. •The large number of hydrogen bonds in a protein’s secondary structure increases its stability.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Tertiary Structure•The tertiary