How do endothermic processes affect cooking an egg?
The initial denaturaion of the proteins in the egg is the endothermic step. Once that is accomplished, the protein strands congeal together. See Below
The proteins present in eggs are sort of like sweaters (the kind you wear). They are folded up into stable 3D structures, and are generally made of one long polypeptide. This polypeptide has a bunch of structural characteristics like alpha helices and beta sheets, as well as a hydrophobic core that helps to maintain the structure of the intact protein
The cooking of an egg is generally considered a physical change (irreversible), since no chemical bonds are broken in the process (there may be S-S bonds that are broken, but these are sidechain oxydation processes...none of the amino acids are cut).
Using the analogy of the sweater, when the egg gets cooked, first all of the sweaters get pulled apart into one long piece of yarn (assume your sweater is made of a long piece of yarn). THis process is ENDOTHERMIC.
Once all of the yarn is pulled apart and in long ribbons (billions of them), they get hopelessly tangled in a huge congealed mass. This is your cooked egg (and why it is generally not reversible).
The unwinding of the protein (denaturation) is the endothermic step - there is probably an exothermic process when all of the strands find stability in the big mess of unwound strands, but the overall process is endothermic.