Protein-protein Interation

The protein-protein interaction and water protein interaction play a major role in determining gel strength and quality. It is obvious that by changing particular parameters (both intrinsic and extrinsic) the gel properties can be changed). Therefore it could be discussed first how protein interactions originate. A part from peptide bonds linking up aminoacids, only one type of covalent bond may impact the interaction between proteins: these are the disulphide bonds between two residues (SH group). As aaminoacids and their side groups can be charged (due to protonation + charge, due to proton loss: -charge), obviously the carge intermediation are also potentially important; moreover these can be attractive or repulsive.  H-bridge formation between  a H donating (eg. OH group) and a H- acceptor (eg. C=O group) support protein protein interaction too. Temporary dipole interactions between phenyl group from an aromatic amino acid such as tryptophan, tyrosine or phenilaanine are also relevant, but already considerably weaker. So are the lipophilic interactions between the side changes of a-polar amino acids such as leucine, alanine, iso leucine, etc.

Having these interactions in mind it is clear that for instance, by changins the PH of a protein solution, protein-protein interactions may change considerably and thus conduce to gel formation. A typical example can be found in the fresh cheese (cottage cheese) production of yogurt. In fact a combination of other charge interactions and an increased lipophilic interaction between the casein proteins are at the origin of this gelation.

By denaturing proteins, new disulphide bonds can be produced between proteins, ot disulphide bonds I proteins can be broken, thus facilitating the interaction with other proteins (because the protein chain has increased its flexibility, it can move freely in the 3D space). Tus gels can be induces. A typical example is boiled egg, resulting from gelation of ovoalbumin.

As the concentration of ions (salts) impacts protein protein interactions (the higher the salt  concentration the higher the protein-protein interaction) the concentration of salt can have an impact on gel formation.

Upon protein degradation (during cheese fermentation) in which part of the proteins ae hydrolised, shorter peptide chains are produced, thus lowering protein protein interactions. These results in gel losing its strength: the interior of the cheese becomes liquid.

Another example of Gel is gelatin: it is a special case because it is produced by parcial hydrolisis of collagen, increasing its solubility in water and therefore decreasing protein-protein interaction in binding tissue.

Protein-protein interactions can also be influenced by adding crosslinking agents, thus influencing gel formatin potential. 

Protein-Water Interaction

Solubility of proteins depends on the Ph. Caseins precipitates at ph of 4, caseins then are unsoluble. Why? Because At 4.5 the caseins reach the isoelectric point (ph at which the proteins have a neutral charge zero). The results is that phosphate groups are neutralized, and calcium bridges fall out of  each other, and the caseins  micelle fall out of each other, and the prteins will start to act differently until they precipitate.

It is possible to enhance the protein solubility by decreasing the ph. Why? Because we give them strong charge. When we protonate, by giving acid, we get positive charge. For casein it is not possible because when the micelles are disintegrated, then it is irreversible.

Water binding capacity: amount of water that can be bound in an amount of protein. Very important for meat products (weight is money). If you cook ham (pink or purple in color), during boiling we lose water binding capacity of the product because of protein denaturation. The apolar group is exposed so the water goes away. If I boil a pig leg the water will be evaporated, so we need to reduce this water loses. For this we need to increase the water binding capacities. It depends on the ph, at high ph we will have more water binding capacity (more water, but less quality)

Protein Denaturation and Consequence

DENATURATION - Loss of native structure

Protein denaturation denotes the loss of the native protein structure due to a change of the physiological conditions to other conditions. Protein structure can be seen on four different levels: primary (amino acid sequence), secondary (presence of alfa helices or beta sheets substructures), tertiary (three dimensionally folded protein) and quaternary structure (protein clusters, dimers etc). Apart from the primary structure these structures originate from the following interactions between amino acids: SS bounds, electrostatic interactions, H-bounding, hydrophobic interactions and Vanderwaals forces (figures could be added to explain this better). If these interactions change due to a change in the environment of the protein (e.g. pH change will induce charge change in protein and thus may influence electrostatic interactions), this may have an effect on these structural elements. As long as the primary structure of the protein does not change (= changes on amino acid level), these changes are considered as protein denaturation. Denaturation can be induced by heat, Insostatic pressure (physically disrupting the structure), pH change, addition of salts (playing with salt concentration), removal of water (e.g. sublimation during freezing), physical shear, etc. (proteins have a isoelectric character, neutral charge). The protein structure is affected by the hydrogen bonds, the more water, the morw hydrogen bonds.

 Heat denaturation though is considered to occur mostly in the food industry. Each protein is characterised by a particular denaturation time and temperature, but this may be affected by the water activity of your product. Protein could also undergo denaturation during cooled storage due to a change in hydrophobic interactions at low temperatures. Since all the above mentioned interactions are not broken at once, denaturation is a step wise process and as such can be reversible or unreversible (figures could be added explaining this).

Denaturation is important because

-           It changes drastically the functionality of proteins (good or bad).

-          Solubility may be affected (coagulation of egg white) (other examples could be given).

-          It may also affect the biological activity of proteins, such as enzymes.

-          It may have a positive impact on the digestibility of proteins, since they become more accessible for digestive proteases.

-          As no changes on amino acids are involved, no negative impact on nutritional value is expected, on the contrary bio-availability increases.

Summary of consequences: 1. Inactivate antimicrobial factors; 2. Digestibility of proteins is higher (proteases can reach the protein in a better way when it is denatured); 3. Nutritional value increases; 4. Changes on functionality which could be good or bad (activation of enzymes, gelatinization, change of physical characteristics - cooked egg). Denaturation is important because it changes drastically the functionality of proteins (good or bad). Solubitlity e.g. may be affected (coagulation of egg white) (other examples could be given). It may also affect the biological activity of proteins, such as enzymes. It may have a positive impact on the digestibility of proteins, sine they become more accessible for digestive proteases. As no changes on amino acids are involved, no negative impact on nutritional value is expected, on the contrary even since bio-availability increases. 

Protein Interaction

Di sulphide bond: Covalent side chain cross link. Interaction between 2 cysteine recidues. They create a lot of rigidity. When we have 2 cysteines we create a disulphide bridge. The more cysteine the more disulphite bonds, the more bridges and the more rigid. This interactions are very strong. 

Ionic interactions: Lisine + aspartic acid (positive and negative attract each other). Attraction between positively and negatively charged. It is the second strongest.

Hydrogen bonds: responsible for peptide bond. It has a strong interaction. Interaction between hidrogent that is covalently bond to an electro-negative atom.

Hydrophobic interaction: not very strong, but exist. Fat lovers

Van der walse: at atomic level, they are weak but they can be at high numbers. They are the weakest.