Vitamin B12 and coenzyme. Molecular model of vitamin B12 in its cofactor (active) form. Cofactors are compounds that are bound to a coenzyme. An enzyme is a protein that catalyses metabolic reactions. Coenzymes need specific molecules to bind to them in order to function. Atoms are represented as spheres and are colour- coded: carbon (blue), hydrogen (gold), cobalt (dark red, centre), nitrogen (dark blue), oxygen (red) and phosphorus (purple). Vitamin B12 is an essential nutrient that humans are unable to produce and so need to obtain from their diet. It is found in meat, fish poultry and dairy products. It is needed for producing red blood cells and for a healthy nervous system. Deficiency can lead to pernicious anaemia, with symptoms including tiredness, dizziness and a reduced sense of taste.