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Proteins

Image courtesy of Biotech (biotech.chem.indiana.edu/pages/protein_intro.html)

Amino Acids (APK)

Amino

Acid

Peptides (APK)

Peptide bond

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Proteins (polypeptides)

Segmentof 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

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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 theprotein to the other.

Secondary (APK)

• Alpha helix

• Beta pleated sheet

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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)

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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

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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

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Hydrogen bond

C O H N

Dipole interaction

OH

HCH

HCH

OH

Hydrophobic interaction

Two interacting aromatic phenyl (benzene) rings

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Disulfide linkage

RSH HSR

Disulfide linkage

RS SR

Ionic interaction

HNHH

OOC

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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

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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

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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)

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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

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Gel structureAssociation and formation of junction zones

This is a gel!

Denaturation and gelation

This is available from the web site on the PowerPointanimation page

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.

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Proteins as structure forming agents in foods

• Casein - in cheese making

• Egg proteins - thickening agents, sauces, custards

• Grains - gluten formation