Proteins. Amino Acids (APK) Peptides (APK) 5/23/2012. Peptide bond. Acid. Amino

Transcription

1 Proteins Amino Acids (APK) Acid Amino Image courtesy of Biotech (biotech.chem.indiana.edu/pages/ protein_intro.html) Peptides (APK) Peptide bond 1

2 Proteins (polypeptides) Segment of a protein Peptide bonds Amino acids in foods Aliphatic amino acids gly, ala, leu Hydroxyl-containing amino acids ser, thr Sulfur-containing amino acids cys, met Acidic amino acids asp, glu Amino acids in foods Basic amino acids lys, his Aromatic amino acids phe, tyr Imino acids pro 2

3 Levels of protein structure (APK) Primary Secondary Tertiary Quaternary Primary (APK) Aa1-aa2-aa3-aa4-aa5-aa6-aa7-.aan A list of the amino acids from one end of the protein to the other. Secondary (APK) Alpha helix Beta pleated sheet 3

4 Alpha helix chirality (APK) Alpha helix Examples Myosin - a muscle protein Epidermin - skin protein Fibrinogen - blood clotting protein Sheep s wool - most alpha helix These proteins are very flexible and extensible but not too strong Kinemage Beta pleated sheet (APK) 4

5 Beta pleated sheet Examples Bird feathers Silk These proteins are very strong but not very extensible Kinemage Tertiary (APK) Tertiary (hexokinase) Image courtesy of MIT Biology Hypertextbook (esg- edu:8001/esgbio/7001main.html 5

6 Quaternary structure Active Inactive Quaternary structure (hemoglobin) Four subunits Heme group Image courtesy of MIT Biology Hypertextbook (esg- edu:8001/esgbio/7001main.html Stabilizing forces in protein structure Hydrogen bond Dipole interaction Hydrophobic interaction Disulfide linkage Ionic interaction 6

7 Hydrogen bond C O H N Dipole interaction OH HCH HCH OH Hydrophobic interaction Two interacting aromatic phenyl (benzene) rings 7

8 Disulfide linkage RSH HSR Disulfide linkage RS SR Ionic interaction H NH H OOC 8

9 Hydrophobic interaction and protein folding water Conjugated proteins Glycoproteins--contain CHO s Ovomucoid in egg white Lipoproteins--contain fatty acids Good emulsifiers Provide mechanism for lipid transport Occur in membranes Metalloproteins Hemoglobin Myoglobin Conjugated proteins Phosphoproteins Casein Pepsin Protein + prosthetic group = holoenzyme 9

10 Functions of proteins Surface active agents (surfactants) Good as emulsifiers High water binding capacity Gelatin Coagulation Milk into cheese Enzymatic activity Many examples Proteins in dispersion Forms a sol Generally increases dispersion viscosity This may be due to denaturation of the secondary and tertiary structures of the protein Protein sol viscosity Assuming similar molecular weights, it depends on the tertiary structure (molecular shape) Fibrous higher viscosity Globular lower viscosity 10

11 Denaturation Denaturing agent Native state Denatured The protein depicted here is crambin, a plant seed protein. Denaturation Denaturing agents Heat Cooking (sol to a gel) Change in ph Add acid (sol to a gel) Enzymes Rennin (sol to a gel) 11

12 Denaturing agents Mechanical shearing Beating egg whites or whipping cream (sol to a foam) Change in ionic strength Presence of detergents The last two are primarily of interest in laboratory investigation of proteins. Possible results of denaturation Decrease in protein solubility Increase in dispersion viscosity Increased reactivity of R groups Loss of enzymatic activity Increased digestibility of proteins Coagulation/gel formation Gel structure Denaturation 12

13 Gel structure Association and formation of junction zones This is a gel! Denaturation and gelation This is available from the web site on the PowerPoint animation page Isoelectric point protein protein protein Isoelectric point (zero net charge) Proteins dispersions are least stable (most likely to form a gel) at their isoelectric point, due to the absence of electrical repulsion. 13

14 Proteins as structure forming agents in foods Casein - in cheese making Egg proteins - thickening agents, sauces, custards Grains - gluten formation 14