The wheat protein activated by overmixing tempura batter is gluten.
Tempura batter usually combines wheat flour with cold water and often egg. Mixing hydrates flour proteins, and continued agitation encourages gluten development. In ordinary bread dough that elasticity is useful, but tempura aims for a thin, fragile coating rather than chewiness.
For that reason, cooks mix tempura batter briefly with chopsticks and deliberately leave small lumps. Cold water and last-minute preparation also slow gluten formation. When the batter is overworked, the fried coating can become heavier, softer, and more dough-like instead of light and crisp.
Gluten is a protein network formed mainly from wheat proteins, especially glutenin and gliadin, when flour is hydrated and worked. It is not the same as collagen, the structural protein in animal connective tissue; casein is the main protein in milk, while pectin is a plant cell-wall carbohydrate. Low-gluten flour can further help produce a delicate tempura crust.