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Summary - Advanced food chemistry
What are the current classes of proteins and what are there properties? Tessa
- Whey proteins: Soluble over pH range, Heat denaturation.
- Soy proteins: insoluble at pH 4-6, Heat denaturation
- Casein: Insuluble at pH 4-6, (random coil)
- Gelatin: Reversilbe with heating (random coil)
- Egg protein: Heat denaturation.
Solubility is an important properties of proteins. Looking at the origin of the animal proteins and their structure, what difference is solubility of fibrillar and globular proteins do you expect?Fibrilar proteins tend to come form muscle tissue. Since muscles need to give structure to the body they would be insoluble under neutral pH (pH of blood). Globular proteins are found for instance in milk or eggs. Since these are aqueous systems, the proteins should have high solublility (under that pH and ionic strength).
How does classification based on classes go?You have 3 different classes based on secondary structure elements.
1. mainly alpha
2. alfa & beta
3. mainly beta (found a lot in lipid membranes)
These classes can be subdivided by there architecture. This is the spatial ordering of sencondary structure.
This can then be subdivided in topology. Somilar folding arrangements.
How does the classification based on super-families go?Different types of proteins are divided based on there sturucture and evolution. For instance goat whey and cow whey.
How does classification based on solubility go?You start with an insoluble product (wheat)
You extract proteins with different solvents and classify them on in winch solvent they solubilize.
How does classification based on insolubility?You start with a soluble product (milk)
You add different slovens and she which with wich solvent the protein becoms insoluble.
How does classification based on structure go?The different structures are:
- globular proteins (e.g. beta-lactoglobulin, soy proteins)
- fibrillar proteins (e.g. collagen, tropomyosine)
- random coil proteins (e.g. caseins)
- other proteins (e.g. gluten)
How doe globular, fibrillar and random coil proteins hide there hydrophobic AA?globular proteins have the hydrophobic AA on the inside of the protein.
fibrillar hides the hydrophobic AA between the proteins. So they hide it between different molecules.
random coil proteins make complexes between different proteins to hide there hydrophobic AA.
2.1 Protein structure
What are the structural properties of ovalbumin?It is one of the only proteins that naturally occurs in the monomeric form.
However this molecule can have different molecular weights, charges and hydrophobisity due to phosporylation and glycolysation.
What are the structural properties of beta-lactoglobulin?In a solution it becomes a dimer. However this can change with temperature, ionic strength, protein concentration and pH.
What are the structural properties of glycinin (soy protein)?At normal conditions (room T, pH7, low ionic strength) glycinin is present in the hexameric form (6 monomeric sub units). They are linked together trough a acid and basic polypeptide chain. Liked by a disulfide bridge.
For different conditions you have different forms with different amoutn of monomers.
Consider ovalbumin, beta-lactoglobullin and glycinin. What would be the difference between these proteins, as studied by SDS-PAGE, or by size exclusion chromaography. First draw schematically the SDS-Page gel for the proteins and then draw the SEC chromatograms that you would expect. Then, discuss how the resulst compare to each other.In the SDS page, the sample preparation and conditions result in dissociation of the proteins into their constituent polypetide chains. For ovalbumin (Ova). one band is observed (Mw-42 kDA), for beta-lactoglublin (BLG) one band is observed (Mw-18KDa), and for glycin tow bands are observed (acidic and basic polypeptide chains with Mw-40 and 20 kDa).
In the SEC, the samples are tipically not treated to induce dissociation. Therefore, the proteins will be present in their associated form. In other words, the quaternary structure is still intact. For ovalbumin this doesn't change anything. There is still one peak with a Mw-42kDa. The BLG still shows one peak, but the apparent Mw-36 KDa since the protein is present as a dimer under 'normal'conditions. In intact (non-dissociated) glycinin, the protein occurs as a hexameer, heaming 6 times the sub-unit that contains one acid and one basic sub-unit. This means that the protein elutes as a molecule with a Mw - 6*(40+20)=360 kDa.
2.2 Sources of proteins and protein isolation
What things can have an effect on the qulaity and content of isolated proteins?Content:
Lipids and sacharides
Lipids, sacharides and phenolic compounds.
Ash only has a small effect
What can be the effect of ash is you take a bigger batch of your isolate?When you have a bigger batch there is more salt in the ash and this can have an effect.
What is the first step in isolation?Removal of other compound groups.
How should you isolate proteins that are in the dry state and how in the dissolved state?
- Dry state: separation should be done by semi-dry separation techniques (wind sifting, or shear separation)
- Wet state: aqueous extraction.
How can you seperate proteins form a sample?
- Solubility in different conditions (precipitation)
- Difference in size (Filtration ,Size-exclusion chromatography)
- Charge (ion-exchange chromatography)
What is the general isolation scheme?See picture.
Why do you make a mass balance?To check if noting is formed or to check if nothing is lost.
for this you need to write down total mass, total volume, dilutions, ect.
What does the yield tell you about the success rate of the isolation?Yield tells something about how much you isolated but you can not extract all the proteins. Due to different pI, matrix entrapment enz.
So it not always is a bad process but you do need to consider it.
How can you extract different groups of proteins form a sample for instance weat?
(second level of protein isolation)The meal is washed with water. Now you get the albumins out
The residue is washed with NaCl 5%. Now you get the globulins out.
The residue is washed with 70% etanol. Now you get the prolamins out.
The thins that are left are the glutelens.
All these things are not one protein but a group of proteins.
How do you prepare a potato for analyzing the protein content?
- juice the potato
- Add disulfite to prevent reactions with phenolic compounds
- dyalise sample to get organic acids out
- adjust the pH
How does the ion exchange chromatography go for potato?This chromatogram has negatively loaded beats.
You go above the pI of the patato proteins to give them a positive charge so they will stick on the column.
To get the porteins off sodium chloride was added and you detect 3 peaks.
The firs one seems to be patatin.
How does size exclusion chromatography go for potato?This chromatography has beats with pors. The small molecules will go into the pores and the big ones will not. Big ones come out first
It the selected peak one type of protein?The SDS-page indicated yes but with iso-electric focusing it was found that you get different types of the same molecule. So resonant structures.
This can be seen because you have bands at different pH and therfore molecules with a different pI.
The resoncance can be due to glycosilation, different carbohydrat chains or phosporilation. POST TRANSLATIONAL modifications
Why do you also have enzyme in a isolation and what are the most important ones?All enzymes are proteins so if you start isolating proteins it is logical you also separate some of them.
- proteases and polypenoloxidases (PPO)
What can be the effect of the presence of enzyme activity (PPO/protease)?
- Discoleration (reaction with polyphenols)
- Change in taste / smell (rancidity of lipids)
- Change in digestibility (due to change in strucutre caused by enzyme)
What can be the effect of the reaction of a phenolic compound with a protein and how can you prevent it?off-color (enzymatic browning), loss of solubility
PPO the enzyme causing this reaction can be inhibited by adding sulfide in the extraction process.
You can also work at low temperatures or low pH since the activity of PPO will go down.
What can be the effect of the presence of non-protein compounds in your isolate?
- Complexation with proteins
- Modification of proteins (lactosylation)
- Measuring yield (non-protein nitrogen).
What happens in a pH parcipitation experiment if you have a protein that is changed by PPO?You have smaller peptieds now that all have there own pI. So you can not parcipitate them anymore.
So if you do sds page you will find nothing.
What happens to solublity when a protein is changed by PPO?It goed down at low pH
What are the high molecular weigh and low molecular weigh anti nutritional components?see picture
What is protease inhibitors?These are proteins that prevent protease from doing its job.
The concenquences are:
- direct: inactivation of endogenous proteases (hindered / decreased digestion)
- indirect: additional production of endogenous (internal) proteases -> loss of endogenous protein
How can you inactivate protein inhibitors and what is the problem?You can inactivate them by heating but this will lower the quality of you protein isolate.
- Need to inhibit protease inhibitors (problems heat inactivation)
- In industry autoclaving (100- 120 °C up to 1 hour)
- Presence of inhibitors --> no hydrolysis by Trypsin/ trypsin-like proteases
What can lectin do?Interact with the epithelial cells, resulting in loss of nutrient uptake.
Why is phytic acid a anti-niturtional factor?It binds calcium, some enzymes that are used for digestion need calcium to work.
How much protease inhibitors are present in plant protein sources?See picture
Latest added flashcards
- The highe solubility of a native protein is aa result of the fact that the pH is then far away from the from the pI. If we hydrolyse, there is not 1 pI anymore. By consequence at any pH there are some peptides that are close to their pI.
- There is exposure of hydrophobic groups upon hydrolysis. This may induce aggregation between peptides, which also decrease the solubility.
For the solubility, the typical argument is that the solubility is lowest around the pI (net-zero charge) and then increases with the total charge of the protein as the pH is further away from the pI. However, as discussed around figure 2.16 the whey protiens (includin geta-lactuglobulin) are charaterised by a high solubility, even at pH values close to the iso-electric point. Therfore, both proteins will have a 'minimal' solubility at the pI. However, it is close to 0 for beta-casein, wile fore beta-lactuglobulin still most of the proteins will be soluble, even at the pI.
pre-crystallisation stage: IV, V, VI, You lose the IV when you heat it up to 32 degrees. Primary nucleation
Solidification stage: V crystals are formed. Secondary nucleation
Why: phospholipids can
- cause oil discoloration (decompose at high T)
- serve as a precursor of off-flavours
- cause problems in other refining processes (emulsifiers)
They are relatively saturated, composition depends on species.
- Lard: swine fat
- (Beef) tallow: more saturated than lard.
- Horse fat: more unsaturated than other animal fats (liquid at 20 degrees)