Lecture 14 - Proteins

Lecture 14 The Chemical Basis of Life-5 ORGANIC MOLECULES: 2. Proteins Life Chap. 3 Describe the 4 levels of structure in a protein 1  Figure 3.3 Substances Found in Livi...

Lecture 14 The Chemical Basis of Life-5 ORGANIC MOLECULES: 2. Proteins Life Chap. 3 Describe the 4 levels of structure in a protein 1  Figure 3.3 Substances Found in Living Tissues Water (70%) 2  Lesson objectives Describe how amino acids differ in their side chains Describe how amino acids are joined to form a peptide Distinguish protein structures 3  Proteins Large, complex molecules Made up of many smaller units called amino acids, which are attached to one another in long chains. They are the basic building blocks of proteins Required for the structure, function, and regulation of the body’s tissues and organs. 4  Proteins have many roles Category Function Enzymes Catalyze biochemical reactions Structural proteins Provide physical stability & movement Signaling proteins Control physiological processes (e.g. hormones) Receptor proteins Receive & respond to chemical signals Membrane transporters Regulate passage of substances across cellular membranes Storage proteins Store amino acids for later use Transport proteins Bind & carry substances within the organism Gene regulatory proteins Determine the rate of expression of a gene 7 Keratin: structural protein of hair 8 ATP stores & transports energy in cells 9 Hemoglobin transports oxygen around the body 10 Carriers (membrane channels) 11  Protein Structure: Proteins are made up of one or more polypeptide molecules. Polypeptides are chains of amino acids  proteins can have 1 or more polypeptide chains folded & bonded together Have a complex 3-D shape 12 Polypeptides 13  Amino acids are the building blocks of proteins Proteins are polymers of 20 amino acids Arranged in a specific order 14 Amino acid structure 5 components  central carbon O H H  Hydrogen || |  amino group —C— C—OH —N—  carboxyl group (acid)  R group (side chain) |  variable group from 1 atom H R to 20.  confers unique chemical properties of the amino acid 15 Amino acid structure 16  Proteins are polymers of amino acids In addition to its R group, each amino acid,  when ionized, has a positive amino (NH3+) group at one end  and a negative carboxyl (COO–) group at the other end 17  The amino and carboxyl groups on a pair of amino acids can undergo a condensation reaction losing a molecule of water & forming a covalent bond A covalent bond that links two amino acids is called a peptide bond 18 19 20  More on building proteins Peptide bonds: dehydration synthesis linking NH2 of 1 amino acid to COOH of another C–N bond 21  More examples… Polypeptide chains N-terminal = NH2 end C-terminal = COOH end repeated sequence (N-C-C) is the polypeptide backbone grow in one direction Example: glycine & Phenylalanine 22  The two amino acids linked are not free to rotate around the N—C linkage…. because the peptide bond has a partial double-bond character…. unlike the N—C and C—C bonds to the central carbon of the amino acid 23  The stiffness of the peptide bond makes it possible for: chains of amino acids to form coils and other regular shapes 24 25  More on building proteins 2 amino acids linked to form a Dipeptide 3 amino acids linked to form a Tripeptide 4-10 amino acids linked by a peptide bond to form an Oligopeptide more than 10 amino acids linked to form a Polypeptide Proteins in the body and diet are long polypeptides (100s of amino acids) 26  Protein function depends on the shape of the molecule The shape of a protein determines its function 27 What Are the Chemical Structures of Proteins? The primary structure of a protein is its amino acid sequence. amino acid sequence is determined by DNA. slight change in amino acid sequence can affect protein’s structure & function. the sequence determines the secondary and tertiary structure i.e. how the protein is folded. Amino acid sequence 29  Levels of protein structure Primary structure  Specific amino acid sequence  determined by DNA  a protein can consist of any sequence of amino acids 30 31  Secondary structure Amino acid side groups are not the only parts of proteins that form hydrogen bonds The —COOH and —NH2 groups of the main chain also form hydrogen bonds 32  The polar groups of the main chain form hydrogen bonds with each other So what??? Two patterns of H bonding occur 33  In one pattern, hydrogen bonds form along a single chain,  linking one amino acid to another farther down the chain This tends to pull the chain into a coil called an alpha (α) helix 34 Note the hydrogen bonds!!! 35  In the other pattern: hydrogen bonds occur across two chains, Linking the amino acids in one chain to those in the other many parallel chains are linked forming a pleated, sheet like structure called a β-pleated sheet 36 A pleat sheet-like structure? 37 Note the hydrogen bonds!!! 38  The folding of the amino acid chain by hydrogen bonding into these characteristic coils.. and pleats is called a protein’s secondary structure 39 40  Tertiary structure The final folded shape of a globular protein Folds nonpolar side groups into the interior Protein is driven into its tertiary structure by hydrophobic interactions with water 41  The final folding of a protein is determined by its primary structure  By the nature of its side groups (hydrophobic/hydrophilic)  Many proteins can be fully unfolded (“denatured”)  and will spontaneously refold back into their characteristic shape 42 Folding determined by the nature of side groups 43  Quaternary structure Two or more polypeptide chains associating to form a functional protein  the individual chains are referred to as subunits of the protein  The subunits need not be the same  E.g. Hemoglobin is a protein composed of two α - chain subunits and two β-chain subunits 44  A protein’s subunit arrangement is called its quaternary structure 45  In summary: Protein Structure… Primary Assembly STRUCTURE PROCESS Secondary Folding Tertiary Packing Quaternary Interaction 46  Thank you.. Questions Comments 47