Foodchem Lecture 11
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Why does folding occur in alpha helices that are part of globular proteins?
Folding occurs due to non helical regions (random or amorphous) being present and the propensity to reduce the molecular free energy state
What can folding of alpha helices result in, if the chains are parallel in globular proteins?
Beta sheet regions can form
What does the presence of imino acids (proline and hydroxy proline) result in for globular proteins?
Random coil regions
What happens to the structure if there is a large number of hydrophobic amino acids in the globular proteins?
- 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
What can happen in globular proteins if there are groups present that have opposite charges (such as COO- and NH3+)?
- Can form electrostatic bonds
- Can be solvated
Can covalent bonds form in globular proteins?
Typically, a disulfide linkage (S-S) between two cysteines or phosphodiesters may occur
What represents the primary structure of a globular protein?
What makes up the secondary structure of a globular protein?
It then forms alpha helix, beta sheet, and random coil portions to create its overall secondary structure
What makes up the tertiary structure of a globular protein?
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
What does the tertiary structure of a globular protein represent?
The resolution of all the internal and external forces at work
When can globular structures further associate as discrete units to form quaternary structures?
When they are soluble
What does the quaternary structure of proteins represent?
Two or more tertiary globular proteins associating as a group to form dimers, tetramers, or octamers
What is primary structure?
Amino acids linked by peptide bonds
What is secondary structure?
- Alpha helix - hydrogen bonding
- Beta pleated sheet - parallel and antiparallel
- Triple suprahelix of collagen
What is tertiary structure?
Folding of the alpha helix - electrostatic, hydrogen, hydrophobic, van der waala, and s-s bonds
What is quaternary structure?
Unit associations using all bonding forms other than covalent
What does the classification "simple protein" mean?
When proteins are made up of only amino acids
What are simple proteins classified based on?
- Their solubility characteristics
- Developed for biological systems, where proteins were sequentially extracted using a series of solutions which would preferentially extract varios protein components
What are albumins?
- Simple proteins soluble in neutral distilled water
- Examples: Egg albumin, and whey
- Do not need salt, acid or base to solubilize
What are globulins?
- Simple proteins that are soluble in neutral, dilute salt solutions, but not distilled water
- Examples: Beta lactoglobulin in milk, myosin and actin from meat
What are glutelins?
- Simple proteins that are soluble in dilute acidic or basic solutions, but not neutral pH solutions
- Example: glutenin from wheat
What are prolamines?
- Simple proteins that are soluble in 50-90% ethaol and insoluble in water
- Example: Gliadin from wheat
- Generally hydrophobic proteins
What are schleroproteins?
- Simple proteins that are insoluble in water and neutral salts
- Resistant to enzymatic hydrolysis
- Generally structural proteins
- Examples: Keratin and collagen
What are histones?
- Simple proteins that are basic proteins
- Contain large amounts of lysine and arginine
- Soluble in water and precipitated by ammonia
What are the simple proteins of most interest to food scientists?
Albumins and globulins
What are complex proteins?
A large and diverse group of conjugated proteins which contain additional chemical moieties other than just amino acids
What is the non-amino acid component of a complex protein termed?
The prosthetic group and such groups often drastically modify the physical and chemical properties of a protein
How are conjugated proteins classified?
Classified on the basis of the type of prosthetic groups they contain
What are lipoproteins?
- 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
What are glycoproteins?
- 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
What are metalloproteins?
- 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
What are phosphoproteins?
- 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
What is associated with the structural complexity of a protein?
Its sensitivity to its chemical environment
How does food processing often stress proteins?
By causing localized environmental changes
Are proteins generally stable?
- 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
How does denaturation affect a protein?
- In many ways, both good or bad depending on one's viewpoint and objectives
- Reduction in solubility
- Decrease in enzymatic activity
- Viscosity increase
- No protein crystallization possible
What is unfolding of proteins?
- Tertiary structure unfolds, secondary structure unravels
- 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
- Surface forces
- (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
- 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
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
- Chain length
- Complexity of structure
- Prosthetic groups
- Environmental conditions
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
- Emulsification properties
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?
- Aspartic acid
- Glutamic acid
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?
Formation pf bitter hydrophobic peptides
When can proteolysis be desirable?
Papain - tenderization of meat
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