1. Antibiotics as nucleoside analouges

2. Nucleoside antibiotics are structural analouges for physiologically important purine and pyrimidines nucleosides.

3. Inhibition of denovo synthesis of purines

4. Inhibition of interconversion of nucleotides

5. Inhibition of utilization of nucleotides due to incorporation of nucleoside analouges antibiotics into polynucleotide.

6. Exsamples
Formycin A is a nucleoside analouge for adenosine that inhibits:
Purine biosynthesis
DNA and protein synthesis.

7. Formycin- 5- triphosphate which incorporated into DNA and codes similar to ATP.
Both antibiotics serve as substrates for many enzymes which act on adenosine or ATP.

8. Formycin B is a nucleoside analouge for inosine (is not phosphorylated) and inhibit purine phosphrylase and can supress viral growth.

9. Antibiotics and inhibition of DNA synthesis
e.g. Azaserine and diazooxo-L- nor leucine (DON) as an analouge to L. glutamine.
Azaserine
DON

10. e.g. formycin , tubercidin as adenosine analouges.
Incorporation of several nucleoside antibiotics resulting in the formation of ineffective nucleic acids.
e.g. formycin , tubercidin as adenosine analouges.
Tubercidin

11. Cordycepin, after intracellular phosphorylation to triphosphate becomes incorporated at the growing 3-end of RNA chains and as it lacks the free hydroxyl group at the 3 carbon, further elongation of the chain is blocked.

12. Interference with polymerization process:
Some antibiotics are known to bind polymerization enzymes and inhibits their activities.
RNA polymerase
DNA polymerase

13. Some antibiotics cross-linked by covalent bond with DNA.
e.g. Mitomycin

14. Hadacidin is a structural analouge of L-aspartic acid and it inhibits adenyl succinate synthetase enzyme which catalyses the first step in the conversion of IMP ………… AMP

16. Intercalation:
i.e Binding with DNA itself
e.g. Actinomycin D and anthracyclins.
Inhibition of transcription:
Chromomycin and distamycin A inhibits transcription.
Degradation of DNA
e.g. Streptozotocin