Leaves the peptide bonds more accessible to proteolytic enzymes
During denaturation, how is there reduction in solubility of the protein?
Hydrophobic domains become exposed
Exposed hydrophobic domains bind together
They aggregate together
Precipitate out of solution
What may happen as viscosity increases during denaturation of a protein?
Gelation may occur
What is the most common contributing factor to denaturation?
Kinetic energy in the form of heat
Besides heat, what are some other contributing factors to denaturation of proteins?
Changes in pH
Changes in ionic strength (salts)
Chemical agents - ethanol, acetone or urea
(Combinations of the above)
What is the definition of denaturation?
A change in structure or conformation of a protein which occurs without breaking peptide bonds
What is the "native state" of a globular protein?
A natural conformation which is required for it to function
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
As some thermal energy is put into the system, what happens?
Some of the relatively weak bonds, like hydrogen bonds, are disrupted by the increased kinetic energy of the molecule
As thermal energy is put into the system that brings the temperature close to the denaturation point, what happens?
The distribution shifts toward the denatured form, with a general shift toward disorder
What happens when the temperature of the protein becomes significantly higher than the denaturation temperature, what happens?
Much of the protein will be denatured and will be in a disordered state
What happens if the tertiary structure is involved in the denaturation?
One can consider the globular protein to unravel
What if there is a distribution of forms during denaturation?
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
What does sensitivity of proteins to denaturation depend on?
Amino acid makeup
Complexity of structure
Why is it important that casein has little secondary structure and is covalently bonded by phosphate?
It does not denature readily even when boiled
What is globular egg albumin very sensitive to?
What can meat proteins lose during denaturation?
Water binding or water holding capacity
What are the effects of pH on denaturation?
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
Proteins containing significant quantities of what are especially susceptible to changes in pH?
Many soluble proteins tend to precipitate when they are close to their what?
How can hydrogen bond breakers disrupt protein structure?
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
What is urea commonly used for?
Denaturing proteins for analytical purposes (like electrophoresis) to avoid the shape factor in migration of proteins in an electric field
What is water hydrogen bonding integral to and help stabalize?
The structure of proteins, especially around charged groups
How can alcohol and acetone disrupt H-bonds?
By partial dehydration of a protein, competing for the water of hydration, however the degree of structural modification tend to not be as severe
When are alcohol and acetone commonly as chemical agents in food science?
To precipitate enzymes from solution for isolation with minimal denaturation effects
How can detergents have 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
What is sodium dodecyl sulfate (SDS) often used for?
Often used as a detergent in electrophoresis, much like urea, but based on a different mechanism
How do organic solvents have denaturation effects?
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
How do surface forces have denaturation effects?
Many proteins have surfactant properties (able to reduce surface tension of water)
This results in the formation of a foam (egg white)
What is a foam?
Air is trapped within a protein/water matrix (bubble)
Each bubble has thin membrane (interface) which separates the two
What are the environments for a foam?
Hydrophilic water and hydrophobic air
What occurs at the interface between the 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
Under relatively consistent conditions, is it possible to make complex changes remain cosistent?
The changes can be relatively consistet under consistent conditions
What are some examples of complex protein denaturation changes that we can control through 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
What are two water removal processes that also affect proteins?
Freezing and drying
Does freezing affect the nutritional quality of proteins?
No, because no heat is involved
What is the major change to proteins that freezing causes?
Causes structural changes
How does freezing affect proteins in tissue systems?
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
What does the degree of denaturation of proteins in tissue systems depend on most?
Often freezing rate dependent
In meat and especially fish what are the biggest changes to proteins during freezing?
Loss of water holding capacity (i.e. the development of freezer drip - often resulting in a loss of desirable texture)
What is freeze drying?
One of the best methods of drying in terms of maintaining functionality, nutritional quality and enzyme activity
Is there usually browning during freeze drying?
Little, if any
This is due to the low temperature of sublimation
What is the best way to dry meat?
Freeze drying allows the best water holding capacity when processed in this manner
This facilitates rehydration - very expensive
When/why is spray drying used in food processing?
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
Is there usually browning during spray drying?
Some browning in the case of egg white and milk will take place due to the presence of glucose (egg white) and lactose (milk)
What is drum drying like in terms of protein denaturation?
Fairly harsh treatment
Causes extensive denaturation
Causes extensive browning if reducing sugars are present
What is proteolysis?
Proteins are attached by proteolytic enzymes, which are abundant in living tissue and secreted by microorganisms
How are proteins affected by proteolysis?
Proteins are reduced in molecular weight and may lose (or gain) functionality
What is a negative effect of proteolysis of proteins in cheese?