Coenzymes are small organic non-protein molecules that carry chemical groups between enzymes. Coenzymes are sometimes referre to as cosubstrates. These molecules are substrates for enzymes and do not form a permanent part of the enzymes' structures. This distinguishes coenzymes from prosthetic groups, which are non-protein components that are bound tightly to enzymes - such as iron-sulfur centers, flavin or haem groups. Both coenzymes and prosthetic groups are types of the broader group of cofactors, which are any non-protein molecules (usually organic molecules or metal ions) that are required by an enzyme for its activity.