Sulfite oxidase is an enzyme that is essential for normal neurological development in children. This video shows the active site of the enzyme and its molybdenum cofactor visible as a faint ball-and-stick representation buried within the protein. The positively charged channel (blue) at the active site contains a chloride ion (green) and three water molecules (red). As the protein oscillates, one can see directly down the positively charged channel. At the bottom is the molybdenum atom of the active site (light blue) and its oxo group (red) that is transferred to sulfite to form sulfate in the catalytic reaction.