We report experimental evidence that the pH value of the micro-environment of the protein is the key for the conformational changes of transferrin, rather than the ligated for unligated state of the binding sites. The "open" and "closed" conformation of protein, suggested by X-ray crystallography in static condition, is triggered by the pH value and the effect is independent of the metal ion being transported. We propose a simple description of the structure-function relationship that allows this protein to deliver, in particular, iron or aluminum ions in the biological cycle.