Biliverdin is a green pigment formed as a byproduct of hemoglobin breakdown. It consists of four linearly connected pyrrhole rings (a tetrapyrrhole). Biliverdin is formed when the heme group in hemoglobin is cleaved at its alpha-methene bridge. The resulting Biliverdin is then reduced to Bilirubin, a yellow pigment, by the enzyme Biliverdin Reductase. The changing color of a bruise from deep purple to yellow over time is a graphical indicator of this reaction.