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Proteins Amino Acids (APK) Acid Amino Image courtesy of Biotech (biotech.chem.indiana.edu/pages/ protein_intro.html) Peptides (APK) O OH O NH 2 H 3C NH Peptide bond CH 3 Proteins (polypeptides) Segment of a protein H N H Peptide bonds H O O H O O H H N H H H H N H H H H O H 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 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 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) 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-www.mit. edu:8001/esgbio/7001main.html Quaternary structure Active Inactive Quaternary structure (hemoglobin) Four subunits Heme group Image courtesy of MIT Biology Hypertextbook (esg-www.mit. edu:8001/esgbio/7001main.html Stabilizing forces in protein structure • • • • • Hydrogen bond Dipole interaction Hydrophobic interaction Disulfide linkage Ionic interaction Hydrogen bond C O H N Dipole interaction OH HCH HCH OH Hydrophobic interaction Two interacting aromatic phenyl (benzene) rings Disulfide linkage RSH HSR Disulfide linkage RS SR Ionic interaction H NH H OOC 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 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 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) 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 Heat Gel structure Association and formation of junction zones trapped water trapped water trapped water trapped water trapped water trapped water Junction zones This is a gel! Denaturation and gelation Protein Denaturation and Gel Formation Go to Slide Show mode and click to begin 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. Proteins as structure forming agents in foods • Casein - in cheese making • Egg proteins - thickening agents, sauces, custards • Grains - gluten formation