Vitamins as coenzymes

Many enzymes are conjugated proteins, formed by combination of a simple protein, called the apoenzyme (apo means off, separated from), and one or more other molecules or ions, called coenzymes. Some coenzymes are metal ions. An example is the carbonic anhydrase in human erythrocytes. The apoenzyme has molecular mass 28,000 d. It combines with one zinc ion, Zn" " , to form the active enzyme, which catalyzes the decomposition of carbonic acid to water and carbon dioxide. Another example is the amylase in human saliva, which helps digest starch. It has molecular mass 50,000 d, and requires one calcium ion, Ca" " , as coenzyme. Many enzymes have vitamins or derivatives of vitamins as coenzymes. 

Why are B vitamins necessary If we consider the action of B vitamins as coenzymes, the answer is clear. There is no electrophilic functional group in the 20 amino acids of proteins. Coenzymes are therefore required for enzymes to carry out electrophilic and radical catalysis. Thiamin diphosphate is by itself a... 

---