Classification This family includes 3 subfamilies: 1- Non-Condensed (single) heterocycles. 2- condensed heterocycles. 3- Alkaloid with antibiotic activity. “ Best known representitive are nucleoside antibiotics”
They differ from normal purine and pyrimidine nucleosides in three possible ways: a) Their structure contain normal base and abnormal saccharide b) Contain abnormal base linked to normal saccharide. C) abnormal base linked to abnormal saccharides.
Cordycepin Ribose is substituted by abnormal sugar 3-deoxyribose and join to purines.
5-azacytidine It is an example of antibiotic with changed base. (i.e: 5-azacytidine base + normal ribose). It is isolated as secondary metabolite from Streptomyces species.
Plicacetin It is an example of nucleoside antibiotics with abnormal saccharide and abnormal base. Since nucleoside antibiotics are structural analogues of physiologically important purine and pyrimidine nucleosides, they can interfere with reactions in which physiological nucleosides are involved or formed.
Four basic types of actions may be distingushed Inhibition of de novo synthesis of purines and pyrimidines. Inhibition of interconversion of nucleotides. Inhibition of utilization of nucleotides. Incorporation of nucleoside antibiotics into DNA and RNA
Formycin A It is a nucleoside analouge of adenosine
Mode of action of formycin A It inhibits purines, DNA and protein biosynthesis. Formycin-5´- triphosphate incorporated into DNA and codes similar to ATP. It serves as substrate for many enzymes which work on ATP.