Rational design of therapeutic proteins replaces random mutagenesis with structure-guided edits at named positions. Three things get engineered: the target binding interface, the physicochemical properties that decide whether the molecule can be manufactured, and the in vivo behavior that decides dosing and safety. Change one and you move the others. What rational design of therapeutic proteins actually changes A therapeutic protein has to clear three separate bars before it becomes a product. It has to hit the right target, hard enough and for long enough. It has to survive expression, purification, formulation, storage and shipping. And it has to stay in circulation long enough to work, reach the tissue that…