When you crack an egg into a pan, you can immediately see three parts. There is the yolk, a thin watery white, and a thick gelled white.
Egg white contains several mucoproteins, where the protein is attached to carbohydrates. In the egg, these serve as nutrients for the growing embryo, and as support and protection.
Over half of the protein in egg white is of one type – ovalbumin. It denatures at 176° Fahrenheit, forming the solid white mass we see at breakfast.
The next most prevalent protein in egg white is ovotransferin (also called conalbumin), which makes up about 12% of the proteins in the white of the egg. It denatures at a lower temperature, about 145°.
The third most prevalent protein is ovomucoid, at 11%. It is found close to the yolk, mixed with other proteins, thickening them.
When you crack an egg into the frying pan, the thin part of the egg white has less ovomucin, and the thick part of the white has 2 to 4 times as much. Ovomucin is the main gelling agent in egg white.
The first protein to denature when heating an egg white is the ovotransferin. As it unfolds it binds not only to other unfolded ovotransferin molecules, but to other proteins that are not yet denatured. The ovomucoid and ovomucin molecules, which do not coagulate with heat by themselves, can thus be incorporated into a strong gel with the ovotransferin and ovalbumin.
The remaining proteins make up less than a quarter of the protein in egg white, but some of them bear mentioning here. Avidin makes up a very small portion of the egg white (less than a tenth of a percent) but it binds very tightly to the essential nutrient biotin, making the biotin unavailable as a food source. This effect is destroyed when the protein is denatured by heat or beating, but can be a problem in a diet that contains a lot of raw egg white.