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