Prosthetic groups

[dawn1070]

What are prosthetic groups and why are they necessary? I understand that they are non polypeptide structures (usually inorganic metals) that are tightly bound and key for the activity of an enzyme or other protein. Essentially they are cofactors. But why are they necessary for the function of an enzyme? I cannot think of an explanation other than they provide a protein with properties that are necessary for its function that it would not otherwise have. (E.g. O2 binding capability of iron in Haem portion of haemoglobin.) Does anyone have a good explanation for this that they could share?

[JackBean]

the are not inorganic ions. Actually in vast majority they are small organic molecules.

The point is that they contain some reactive groups, which participate in the reaction (like NAD+ or FAD in the redox reactions)