Why does heat cause the egg yolk to coagulate?

Answer 1

Protein molecules are made up of many strands. When an egg is raw, these protein strands are bound to each other in discrete little bundles. Because these bundles are separate from each other they allow light to pass between them, which is why egg whites are transparent when raw.

When heat is applied to these proteins, they being to unwind and become unstuck from one another. As they do so, the bonds that previously stuck them to one another began to waver around freely, and as the now-loosened protein strands run into each other they again stick to each other, but in this case they form not a tight little bundle but a kind of a web similar to a spider web. This web is dense enough to prevent light from passing through, which is why cooked eggs whites are white in color

Answer 2

Temperature breaks down the bonds of egg yolk and it coagulates.