Proteins Proteins have many structures, resulting in a wide range of functions Proteins do most of the work in cells and act as enzymes 2. Proteins are made of monomers called amino acids

An overview of protein functions Table 5.1

Enzymes 3. Are a type of protein that acts as a catalyst, speeding up chemical reactions Substrate (sucrose) Enzyme (sucrase) Glucose OH H O H2O Fructose 3 Substrate is converted to products. 1 Active site is available for a molecule of substrate, the reactant on which the enzyme acts. Substrate binds to enzyme. 2 4 Products are released. Figure 5.16

Polypeptides 4. Polypeptides are polymers (chains) of amino acids A protein consists of multiple amino acids connected by peptide bonds

Amino acids Are organic molecules possessing both carboxyl and amino groups Differ in their properties due to differing side chains, called R groups

Twenty Amino Acids 20 different amino acids make up proteins H H3N+ C CH3 CH CH2 NH H2C H2N Nonpolar Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile) Methionine (Met) Phenylalanine (Phe) Tryptophan (Trp) Proline (Pro) H3C Figure 5.17 S 20 different amino acids make up proteins

Polar Electrically charged OH CH2 C H H3N+ O CH3 CH SH NH2 Polar Electrically charged –O NH3+ NH2+ NH+ NH Serine (Ser) Threonine (Thr) Cysteine (Cys) Tyrosine (Tyr) Asparagine (Asn) Glutamine (Gln) Acidic Basic Aspartic acid (Asp) Glutamic acid (Glu) Lysine (Lys) Arginine (Arg) Histidine (His)

Amino Acid Polymers 5. Amino acids are linked by peptide bonds

Protein Conformation and Function 6. A protein’s specific conformation (shape) determines how it functions

Four Levels of Protein Structure Primary structure Is the unique sequence of amino acids in a polypeptide Figure 5.20 – Amino acid subunits +H3N Amino end o Carboxyl end c Gly Pro Thr Glu Seu Lys Cys Leu Met Val Asp Ala Arg Ser lle Phe His Asn Tyr Trp Lle

Secondary structure Is the folding or coiling of the polypeptide into a repeating configuration Includes the  helix and the  pleated sheet O C  helix  pleated sheet Amino acid subunits N H R H Figure 5.20

Is the overall three-dimensional shape of a polypeptide Tertiary structure Is the overall three-dimensional shape of a polypeptide Results from interactions between amino acids and R groups CH2 CH O H O C HO NH3+ -O S CH3 H3C Hydrophobic interactions and van der Waals interactions Polypeptide backbone Hyrdogen bond Ionic bond Disulfide bridge

Quaternary structure overall protein structure that results from the combination of two or more polypeptide subunits Polypeptide chain Collagen  Chains  Chains Hemoglobin Iron Heme

Review of Protein Structure +H3N Amino end Amino acid subunits helix

Sickle-Cell Disease: A Simple Change in Primary Structure Results from a single amino acid substitution in the protein hemoglobin

Sickle-cell hemoglobin Primary structure Secondary and tertiary structures Quaternary structure Function Red blood cell shape Hemoglobin A Molecules do not associate with one another, each carries oxygen. Normal cells are full of individual hemoglobin molecules, each carrying oxygen   10 m Hemoglobin S Molecules interact with one another to crystallize into a fiber, capacity to carry oxygen is greatly reduced.  subunit 1 2 3 4 5 6 7 Normal hemoglobin Sickle-cell hemoglobin . . . Figure 5.21 Exposed hydrophobic region Val Thr His Leu Pro Glul Glu Fibers of abnormal hemoglobin deform cell into sickle shape.

What Determines Protein Conformation? Protein conformation Depends on the physical and chemical conditions of the protein’s environment Temperature, pH, salinity, etc. affect protein structure

Denaturation is when a protein unravels and loses its native conformation (shape) Renaturation Denatured protein Normal protein Figure 5.22

The Protein-Folding Problem Most proteins Probably go through several intermediate states on their way to a stable conformation Denaturated proteins no longer work in their unfolded condition Proteins may be denaturated by extreme changes in pH or temperature

Chaperonins Are protein molecules that assist in the proper folding of other proteins Hollow cylinder Cap Chaperonin (fully assembled) Steps of Chaperonin Action: An unfolded poly- peptide enters the cylinder from one end. The cap attaches, causing the cylinder to change shape in such a way that it creates a hydrophilic environment for the folding of the polypeptide. The cap comes off, and the properly folded protein is released. Correctly folded protein Polypeptide 2 1 3 Figure 5.23