Foodchem Lecture 11

Folding occurs due to non helical regions (random or amorphous) being present and the propensity to reduce the molecular free energy state

Beta sheet regions can form

Random coil regions

• They will prefer to coalesce together (usually internally) to reduce free energy
• They will tend to associate more strongly with each other if they are exposed to a polar environment

• Can form electrostatic bonds
• Can be solvated

Typically, a disulfide linkage (S-S) between two cysteines or phosphodiesters may occur

Its elongation

It then forms alpha helix, beta sheet, and random coil portions to create its overall secondary structure

Protein then folds into a specific tertiary structure depending on its environment and types of bonding available to take a shape which produces the lowest free energy form

The resolution of all the internal and external forces at work

When they are soluble

Two or more tertiary globular proteins associating as a group to form dimers, tetramers, or octamers

Amino acids linked by peptide bonds

• Alpha helix - hydrogen bonding
• Beta pleated sheet - parallel and antiparallel
• Triple suprahelix of collagen

Folding of the alpha helix - electrostatic, hydrogen, hydrophobic, van der waala, and s-s bonds

Unit associations using all bonding forms other than covalent

When proteins are made up of only amino acids

• Their solubility characteristics
• Developed for biological systems, where proteins were sequentially extracted using a series of solutions which would preferentially extract varios protein components

• Simple proteins soluble in neutral distilled water
• Examples: Egg albumin, and whey
• Do not need salt, acid or base to solubilize

• Simple proteins that are soluble in neutral, dilute salt solutions, but not distilled water
• Examples: Beta lactoglobulin in milk, myosin and actin from meat

• Simple proteins that are soluble in dilute acidic or basic solutions, but not neutral pH solutions
• Example: glutenin from wheat

• Simple proteins that are soluble in 50-90% ethaol and insoluble in water
• Example: Gliadin from wheat
• Generally hydrophobic proteins

• Simple proteins that are insoluble in water and neutral salts
• Resistant to enzymatic hydrolysis
• Generally structural proteins
• Examples: Keratin and collagen

• Simple proteins that are basic proteins
• Contain large amounts of lysine and arginine
• Soluble in water and precipitated by ammonia

Albumins and globulins

A large and diverse group of conjugated proteins which contain additional chemical moieties other than just amino acids

The prosthetic group and such groups often drastically modify the physical and chemical properties of a protein

Classified on the basis of the type of prosthetic groups they contain

• Complex proteins that are complexed with lipids
• Generally triglycerides and phospholipids
• Abundant in nature and have good emulsifying properties
• Examples: Egg yolk, and some milk proteins

• Proteins complexed with carbohydrate
• Can range from simple sugars to heterosaccharides to polysaccharides
• The carbohydrate is generally attached to the OH group of serine or threonine by an O-glycosidic bond or the amide group of asparagine, by an N-glycosidic linkage
• Examples: Ovomucoid in eggs and soybean glycoproteins

• Complex proteins which are complexed with a metal ion which is generally loosely chelated
• Examples: Fe++ in myoglobin and hemoglobin where Fe++ is chelated by a porphyrin ring structure
• Numerous enzymes have metal ions as prosthetic groups

• Complex proteins that are conjugated to inorganic phosphates
• Examples: Casein, the enzyme rennin
• Phosphate is generally linked to the proteins by the OH group of serine and threonine

Its sensitivity to its chemical environment

By causing localized environmental changes

• Proteins are much more sensitive than lipids or carbohydrates
• Can undergo rather drastic changes in structure which can lead to denaturation under relatively mild conditions

• In many ways, both good or bad depending on one's viewpoint and objectives
• Examples:
• Unfolding
• Reduction in solubility
• Decrease in enzymatic activity
• Viscosity increase
• No protein crystallization possible

• Tertiary structure unfolds, secondary structure unravels
• Leaves the peptide bonds more accessible to proteolytic enzymes

• Hydrophobic domains become exposed
• Exposed hydrophobic domains bind together
• They aggregate together
• Precipitate out of solution

Gelation may occur

Kinetic energy in the form of heat

• Changes in pH
• Changes in ionic strength (salts)
• Chemical agents - ethanol, acetone or urea
• Surface forces
• (Combinations of the above)

A change in structure or conformation of a protein which occurs without breaking peptide bonds

• A natural conformation which is required for it to function
• Example: Enzyme
• It is a degree of order assigned as 1.0 at 25°C and its completely denatured state a degree of order of 0.0

Some of the relatively weak bonds, like hydrogen bonds, are disrupted by the increased kinetic energy of the molecule

The distribution shifts toward the denatured form, with a general shift toward disorder

Much of the protein will be denatured and will be in a disordered state

One can consider the globular protein to unravel

• The partially denatured protein may still be able to revert back to the native structure
• This phenomena often occurs in enzymes, which can lose activity and then regain it

• Amino acid makeup
• Chain length
• Complexity of structure
• Prosthetic groups
• Environmental conditions

It does not denature readily even when boiled

Temperature

• Water binding or water holding capacity
• Emulsification properties

• Most proteins tend to be stable over a relatively narrow pH range
• Shift in pH affects the overall charge of the molecule, which affects the electrostatic bond contributions to the tertiary structure

• Aspartic acid
• Glutamic acid
• Lysine

Isoelectric point

• Compounds which can effectively compete for hydrogen bonds can readily disrupt protein structure
• Example: Urea can hydrogen bond competitively with the peptide linkage induced hydrogen bonds and disrupt both tertiary and secondary structure hydrogen bonds

Denaturing proteins for analytical purposes (like electrophoresis) to avoid the shape factor in migration of proteins in an electric field

The structure of proteins, especially around charged groups

By partial dehydration of a protein, competing for the water of hydration, however the degree of structural modification tend to not be as severe

To precipitate enzymes from solution for isolation with minimal denaturation effects

• Detergents have both hydrophilic and hydrophobic groups
• They are able to bridge the hydrophobic and hydrophilic regions of the protein and result in the opening of the internal structure of a protein

Often used as a detergent in electrophoresis, much like urea, but based on a different mechanism

• They convert the solvent environment from hydrophilic to hydrophobic
• This can turn the protein macromolecule inside out
• They can even make enzymes work in the totally opposite ways if the environment is carefully controlled
• Example: can make lipase add FFA to glycerol rather than remove it

• Many proteins have surfactant properties (able to reduce surface tension of water)
• This results in the formation of a foam (egg white)

• Air is trapped within a protein/water matrix (bubble)
• Each bubble has thin membrane (interface) which separates the two

Hydrophilic water and hydrophobic air

• At this interface proteins actively rearrange themselves structurally to reduce their free energy, i.e. hydrophobic groups facing air, and hydrophilic groups facing into the water
• This process can cause denaturation at the interface

The changes can be relatively consistet under consistent conditions

• Boiling/frying an egg - gel
• Egg white/sugar beaten to produce a denatured foam - meringue
• Milk and acid - curd
• Heating collagen - gel
• Gluten/gliadin and water - dough

Freezing and drying

No, because no heat is involved

Causes structural changes

In tissue systems denaturation can be extensive due to secondary effects caused by local changes in ionic strength and pH caused by concentration effects due to the removal of water from the micro environment of the protein molecule

Often freezing rate dependent

Loss of water holding capacity (i.e. the development of freezer drip - often resulting in a loss of desirable texture)

One of the best methods of drying in terms of maintaining functionality, nutritional quality and enzyme activity

• Little, if any
• This is due to the low temperature of sublimation

• Freeze drying allows the best water holding capacity when processed in this manner
• This facilitates rehydration - very expensive

• Used extensively for protein solutions - milk and egg white
• Some denaturation may take place due to higher temperatures and due to shear at the nozzle

Some browning in the case of egg white and milk will take place due to the presence of glucose (egg white) and lactose (milk)

• Fairly harsh treatment
• Causes extensive denaturation
• Causes extensive browning if reducing sugars are present

Proteins are attached by proteolytic enzymes, which are abundant in living tissue and secreted by microorganisms

Proteins are reduced in molecular weight and may lose (or gain) functionality

Formation pf bitter hydrophobic peptides

Papain - tenderization of meat