Conjugated proteins contain a non-protein prosthetic group.Lipids combine with proteins to form lipoproteins.Carbohydrates combine with proteins to form glycoproteins.Metal ions and molecules derived from vitamins also form prosthetic groups.Haem is the non-protein group attached to each of the alpha and beta subunits of the haemoglobin protein. The enzyme catalase also contains the haem group.Haem contains iron Fe2+.This allows it to bind molecular oxygen.