Proteins - Purdue University · Alpha helix • Examples – Myosin - a muscle protein –...
Transcript of Proteins - Purdue University · Alpha helix • Examples – Myosin - a muscle protein –...
Proteins
Image courtesy of Biotech (biotech.chem.indiana.edu/pages/protein_intro.html)
Amino Acids (APK)
Amino
Acid
Peptides (APK)
O
NHCH3
OHO
NH2
CH3Peptide bond
Proteins (polypeptides)
O
NH
H
HH
O
NHH
HHH
O
N
O
O
H
HH
H
H
HSegmentof 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
Levels of protein structure (APK)• Primary
• Secondary
• Tertiary
• Quaternary
Primary (APK)
Aa1-aa2-aa3-aa4-aa5-aa6-aa7-….aanA list of the amino acids from one end of theprotein 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 andextensible 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
Inactive
Active
Quaternary structure (hemoglobin)
Foursubunits
Image courtesy of MIT Biology Hypertextbook (esg-www.mit.edu:8001/esgbio/7001main.html
Hemegroup
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
HNHH
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)
Fibroushigher viscosity
Globularlower viscosity
Denaturation
Denaturing agent
The protein depicted here is crambin, a plant seed protein.Native state Denatured
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 structureDenaturation
H eat
Gel structureAssociation and formation of junction zones
This is a gel!
trapped water
trapped water
trapped water
trapped water
trapped water
trapped water
Junction zones
Denaturation and gelationProtein Denaturation and Gel
Formation
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Isoelectric point
protein protein protein
+ +
+
++
+
+
+
+ +
+ +
-
-
-
-
- -
- --
- -
-
Isoelectricpoint (zero net charge)
Proteins dispersions are least stable (most likely toform 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