Which nitrogenous bases are purines Why
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The Biosynthesis of Purines
- Tab. 1
Purines and pyrimidines are nitrogenous bases which, together with sugar phosphates (ribose or deoxyribose), are building blocks of the nucleic acids DNA and RNA. Adenine (6-aminopurine) and guanine (2-amino-6-hydroxypurine) are made up of purine skeletons, while thymine, cytosine and uracil contain pyrimidine skeletons. Some alkaloids are also purine derivatives, e.g. caffeine, theophylline or theobromine.
In addition to their function as information carriers for the transmission of genetic material and the translation of DNA into proteins, nucleotides also play an important role in the organism as storage for metabolic energy (ATP, GTP) or as signaling molecules, e.g. in the signaling cascades of G proteins (GTP / GDP binding) or as a Second MessengercAMP. In the form of NAD, FAD or coenzyme A, nucleotides are involved as coenzymes in numerous enzymatic reactions.
- Tab. 2
- Occurrence of purines and pyrimidines
|base||Purine / pyrimidine||Nucleoside (base and sugar)||Nucleotide (base, sugar and phosphate)|
|Adenine (A)||Purine||Adenosine||Adenosine monophosphate (AMP)|
|Thymine (T)||Pyrimidine||Thymidine||Thymidine monophosphate (TMP)|
|Guanine (G)||Purine||Guanosine||Guanosine monophosphate (GMP)|
|Cytosine (C)||Pyrimidine||Cytidine||Cytidine monophosphate (CMP)|
|Uracil (U) *||Pyrimidine||Uridine||Uridine monophosphate (UMP)|
The purines will de novo synthesized as ribonucleotides. The first step in the synthesis is the formation of phosphoribosyl-1-pyrophosphate (PRPP) from ribulose-5-phosphate. The ten steps of purine synthesis and the genes involved are in the animated overview for the intestinal bacterium Escherichia coli shown. The genes are more or less distributed over the entire circular genome of the bacterium. On the following pages, the respective enzymes and the reactions they catalyze up to reaction stage 10 (purH) are presented.
The purines are broken down into uric acid (urate). In humans, this is the end point of the degrading purine metabolism; uric acid is excreted through the kidneys. Other living beings are able to break down uric acid into urea and even down to the ammonium (or). Disturbances of the purine metabolism and the associated accumulation of urate are the cause of diseases, e.g. gout or the formation of urinary stones (urate crystals).
Enzymatic breakdown of uric acid
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