Folic acid (THF) is essential for the synthesis of purine nucleotides (reactions 4 and 10)-Refer to Figure 11.2. Sulphonamides are the structural analogues of para aminobenzoic acid (PABA). These sulpha drugs can be used to inhibit the synthesis of folic acid by micro-organisms. This indirectly reduces the synthesis of purines and therefore the nucleic acids (DNA and RNA). Sulphonamides have no influence on humans since folic acid is not synthesised and is supplied through diet. The structural analogues of folic acid (example: methotrexate) are widely used to control cancer. They inhibit the synthesis of purine nucleotide viz reaction 4 and 10 refer Figure 11.2 and thus inhibits the nucleic acid synthesis. Both these reactions are concerned with the transfer of one carbon moiety (formyl group). These inhibitors also affect the proliferation of normally growing cells. This causes many side effects including anemia, baldness, scaly skin, and so on. Azaserine is an antagonist of glutamine, and this can inhibit the reaction 5. Diaz Norleucine inhibits reaction 2. 6-mercaptopurine inhibits reactions 13 and 14. Mycophenolic acid inhibits reaction 14.