Download Proteins

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