Catabolism of DNA by Deoxyribonucleases
The nucleic acids exist mainly in the nucleus as nucleoproteins. These nucleoproteins in the diet are degraded by HCl to nucleic acids and proteins.
Nucleoproteins----------------(HCL)-----------------> Nucleic acids + proteins
Nucleic acids pass as such from the stomach and their catabolism starts in duodenum by several enzymes (nucleases, nucleotidases and nucleosidases) which degrade the nucleic acids to purines, pyrimidines and pentoses. Three different types of enzymes which degrade the nucleic acids are discussed one by one.
The enzymes which degrade the nucleic acids are known as nucleases. Some are specific for RNA and thus known as ribonucleases, and others for DNA and thus known as dexoyribonucleases, while still some others are capable of attacking DNA as well as RNA.
Deoxyribonucleases are further classified in to 2 categories.
1. Exonucleases are the nucleases that attack only the internucleotide bondslocated at the ends of the nucleic acid.
Exonucleases are further classified into 2 groups.
i. Those, which attack the 3' end of single strand DNA called 3' exonuclease
ii. Those, which attack the 5' end of the single strand DNA called 5' exonuclease
These 2 enzymes are non specifically called as phosphodiesterases.
2. Endonucleases are the nucleases that attack only the internucleotide bondslocated throughout the length of the nucleic acid chain (in the middle).
These enzymes hydrolyse the nucleotides to the corresponding nucleosides and inorganic phosphate molecules.
The nucleosides obtained above either absorbed or degraded into bases and sugars by nucleosidases.