A calcium-binding protein is a protein that binds calcium ions, changing shape to carry out or regulate a cellular function. Examples include calmodulin, which translates calcium signals into enzyme activation, and the calcium-binding proteins in the gut that ferry calcium across intestinal cells during absorption. In dairy science, casein proteins bind calcium phosphate into micelles, keeping milk’s calcium soluble and bioavailable. These proteins are central to muscle contraction, nerve signalling, blood clotting, and bone metabolism. Understanding them explains why calcium’s effects depend not just on intake but on the transport and signalling machinery that handles it.