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PROTEIN DENATURATION

The familiar gelatin dessert actually is a good example of the process of coagulation of

proteins into a 3-dimensional latticework that entraps water molecules to produce a

semisolid gel.

Protein denaturation changes the solubility of individual protein molecules,entrapping

solvent water into a semisolid gel structure.Protein are synthesized by polymerizingamino acids.The polymerization occurs by repeatedly forming peptide bonds that link

individual amino acids together into a chain. There are 3 structural features that influence

the three-dimensional shape of water-soluble protein.

Primary structure is the peptide bond between individual amino acids that creates a

long chain of connected amino acids.These long chains of polymerized amino acids have

hydrophobic(water repelling) and hydrophilic(water attracting) projections that are

oriented perpendicular to the chain.

Secondary structure is the helix that the protein chain curls into,as a result of hydrogenbonds and other weak forces.

Tertiary structures is created when the protein molecules fold back on themselves

outside of the helical segments,to put the hydrophobic portions to the interior and thehydrophilic portions to the exterior.

Several helix regions can exist in different portions of the molecules.When the proteinhas folded and refolded to reach its most stable configurations,it will have mostly

hydrophilic amino acid residues on the exterior,and mostly hydrophobic residues directedinto the interior.

When natural proteins are subjected to physical or chemical treatment,their structure

changes,and they become un-native or un-natural.We call that process denaturation.In this example,heating the proteins in solution imparts energy to the protein

molecules.This added energy is enough to break the relatively weak forces that hold the

protein in its refolded and helical tertiary and secondary configurations.As the process of

denaturation proceeds,the protein molecule unfolds more and more and internallydirected hydrophobic regions now become exposed on the outside of the molecule.

The peptide bonds are largely hydrophilic.Once these segments are set from eachother,they attract water molecules.The recruitment of water molecules entraps the water

molecules in close proximity to the protein strands. The hydrophobic portions of the

molecules are also exposed.This situation is unfavored because the hydrophobic portionsof molecules are not stable in an aqueous environment.Hence,upon unfolding,the

hydrophobic regions on individual protein molecules will associate with hydrophobic

regions on other protein molecules.

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This situation encourages the association of these protein molecules into larger andlarger random 3-dimensional structures.The molecules aggregate into very large,water-

insoluble collections that are quite randomly assembled.

As the proteins denature,lattice-work structure grow amorphously and attract the solvent

water molecules into cell-like structures.

The self associated water molecules,in groups,adhere t the surface of hydrophilic

regions of the protein while hydrophobic regions of the protein dissolve into each other

and provide the energy to retain the structure.

As this process continues irreversibly,all of the protein molecules are recruited to this

large insoluble mass in randomly organized structural framework that contains entrapped

water molecules.

One example of the consequences of unfolding and re-associating protein molecules is

coagulation of egg white.Frying an egg is no more complicated than denaturing the eggwhite protein.

The assembly of irreversibly denatured protein molecules results in formation of solid

gel.The gel entraps water molecules inside the white into a semi-solid structure,whichholds its shape under normal conditions.

Other examples of denatured protein assembling into three-dimensional structuresinclude the baking of yeast-risen bread,coagulation of meat proteins by cooking in such

products as hot dogs,and in the solidification of gelatin cooling of a solution.

Denaturation and coagulation of protein is a complex,irreversible process but the study

of denaturation has allowed to us to better understand the three-dimensional structure of

native proteins.