In the catobilsm of purine nucleotides, IMP is further degraded by hydrolysis with nucleotidase to inosine and then phosphorolysis to hypoxanthine. Adenosine does occur but usually arises from S-Adenosylmethionine during the course of transmethylation reactions. Adenosine is deaminated to inosine by an adenosine deaminase. Deficiencies in either adenosine deaminase or in the purine nucleoside phosphorylase lead to two different immunodeficiency diseases by mechanisms that are not clearly understood.
With adenosine deaminase deficiency, both T and B-cell immunity is affected. The phosphorylase deficiency affects the T cells but B cells are normal.
In September, , a 4 year old girl was treated for adenosine deaminase deficiency by genetically engineering her cells to incorporate the gene. The treatment,so far, seems to be successful. Whether or not methylated purines are catabolized depends upon the location of the methyl group.
If the methyl is on an -NH2, it is removed along with the -NH2 and the core is metabolized in the usual fashion. If the methyl is on a ring nitrogen, the compound is excreted unchanged in the urine. Bases to Uric Acid Both adenine and guanine nucleotides converge at the common intermediate xanthine. Hypoxanthine, representing the original adenine, is oxidized to xanthine by the enzyme xanthine oxidase.
Guanine is deaminated, with the amino group released as ammonia, to xanthine. If this process is occurring in tissues other than liver, most of the ammonia will be transported to the liver as glutamine for ultimate excretion as urea.
Xanthine, like hypoxanthine, is oxidized by oxygen and xanthine oxidase with the production of hydrogen peroxide. In man, the urate is excreted and the hydrogen peroxide is degraded by catalase. Xanthine oxidase is present in significant concentration only in liver and intestine.
The pathway to the nucleosides, possibly to the free bases, is present in many tissues. Gouts and Hyperuricemia Both undissociated uric acid and the monosodium salt primary form in blood are only sparingly soluble. Hyperuricemia is not always symptomatic, but, in certain individuals, something triggers the deposition of sodium urate crystals in joints and tissues. In addition to the extreme pain accompanying acute attacks, repeated attacks lead to destruction of tissues and severe arthritic-like malformations.
The term gout should be restricted to hyperuricemia with the presence of these tophaceous deposits. In gouts caused by an overproduction of uric acid, the defects are in the control mechanisms governing the production of - not uric acid itself - but of the nucleotide precursors.
The only major control of urate production that we know so far is the availability of substrates nucleotides, nucleosides or free bases. One approach to the treatment of gout is the drug allopurinol, an isomer of hypoxanthine. Allopurinol is a substrate for xanthine oxidase, but the product binds so tightly that the enzyme is now unable to oxidized its normal substrate.
Uric acid production is diminished and xanthine and hypoxanthine levels in the blood rise. These are more soluble than urate and are less likely to deposit as crystals in the joints. Another approach is to stimulate the secretion of urate in the urine. Summary In summary, all, except ring-methylated, purines are deaminated with the amino group contributing to the general ammonia pool and the rings oxidized to uric acid for excretion. Since the purine ring is excreted intact, no energy benefit accrues to man from these carbons.
Pyrimidine Catabolism In contrast to purines, pyrimidines undergo ring cleavage and the usual end products of catabolism are beta-amino acids plus ammonia and carbon dioxide. Pyrimidines from nucleic acids or the energy pool are acted upon by nucleotidases and pyrimidine nucleoside phosphorylase to yield the free bases.
The 4-amino group of both cytosine and 5-methyl cytosine is released as ammonia. Atoms 2 and 3 of both rings are released as ammonia and carbon dioxide. The rest of the ring is left as a beta-amino acid. Beta-amino isobutyrate from thymine or 5-methyl cytosine is largely excreted. Beta-alanine from cytosine or uracil may either be excreted or incorporated into the brain and muscle dipeptides, carnosine his-beta-ala or anserine methyl his-beta-ala.
General Comments Purine and pyrimidine bases which are not degraded are recycled - i. This recycling, however, is not sufficient to meet total body requirements and so some de novo synthesis is essential. There are definite tissue differences in the ability to carry out de novo synthesis.
De novo synthesis of purines is most active in liver. Non-hepatic tissues generally have limited or even no de novo synthesis. Pyrimidine synthesis occurs in a variety of tissues. For purines, especially, non-hepatic tissues rely heavily on preformed bases - those salvaged from their own intracellular turnover supplemented by bases synthesized in the liver and delivered to tissues via the blood.
The bases generated by turnover in non-hepatic tissues are not readily degraded to uric acid in those tissues and, therefore, are available for salvage. The liver probably does less salvage but is very active in de novo synthesis - not so much for itself but to help supply the peripheral tissues.
De novo synthesis of both purine and pyrimidine nucleotides occurs from readily available components. De Novo Synthesis of Purine Nucleotides We use for purine nucleotides the entire glycine molecule atoms 4, 5,7 , the amino nitrogen of aspartate atom 1 , amide nitrogen of glutamine atoms 3, 9 , components of the folate-one-carbon pool atoms 2, 8 , carbon dioxide, ribose 5-P from glucose and a great deal of energy in the form of ATP.
In de novo synthesis, IMP is the first nucleotide formed. PRPP Since the purines are synthesized as the ribonucleotides, not as the free bases a necessary prerequisite is the synthesis of the activated form of ribose 5-phosphate. The enzyme is heavily controlled by a variety of compounds di- and tri-phosphates, 2,3-DPG , presumably to try to match the synthesis of PRPP to a need for the products in which it ultimately appears.
Commitment Step De novo purine nucleotide synthesis occurs actively in the cytosol of the liver where all of the necessary enzymes are present as a macro-molecular aggregate. The first step is a replacement of the pyrophosphate of PRPP by the amide group of glutamine. The product of this reaction is 5-Phosphoribosylamine.
The amine group that has been placed on carbon 1 of the sugar becomes nitrogen 9 of the ultimate purine ring. This is the commitment and rate-limiting step of the pathway. The enzyme is under tight allosteric control by feedback inhibition. This is a fine control and probably the major factor in minute by minute regulation of the enzyme. Another ATP molecule causes an intermolecular reaction that produces an imidazole ring 5-aminoimidazole ribonucleotide.
The next step of the pathway is adding bicarbonate to make carboxyaminoimidazole ribonucleotide by using ATP it only happens in fungi and bacteria; high eukaryotes simply add CO2 to form the ribonucleotide. As we have just seen, a six-step process links glycine, formate, bicarbonate, glutamine, and aspartate to lead to an intermediate that contains almost all the required atoms to synthesize a purine ring.
This intermediate removes fumarate, and a second formyl group from THF is added. The compound gets cycled and forms inosinate after a sort of intermolecular reactions. Inosinate is the first intermediate in this synthesis pathway to have a whole purine ring. Enzymes taking part in IMP synthesis constitute a multienzyme complex in the cell.
Evidences demonstrate that there are multifunctional enzymes, and some of them catalyze non-sequential steps in the pathway . Laboratory synthesis of purine[ edit ] History[ edit ] The name 'purine' purum uricum was coined by the German chemist Emil Fischer in Eukaryotic organisms contain a multifunctional enzyme with carbamoylphosphate synthetase, aspartate transcarbamoylase, and dihydroorotase activities.
Two mechanisms control this enzyme. First, control at the level of enzyme synthesis exists; the transcription of the gene for the enzyme is reduced if an excess of pyrimidines is present. Secondly, control exists at the level of feedback inhibition by pyrimidine nucleotides.Glutamine phosphoribosyl pyrophosphate amidotransferase is also sensitive to inhibition by the glutamine analog azaserine Figure Figure 1 The second reaction is ring closure to form dihydroorotic acid. Bacteria have but one CPS, and its carbamoyl phosphate product is incorporated into arginine as well as pyrimidines by the enzyme dihydroorotase. In man, the urate is excreted and the hydrogen peroxide is degraded by catalase.
But, of course, the three forms are in equilibrium. The PNP products are merged into xanthine by guanine deaminase and xanthine oxidase, and xanthine is then oxidized to uric acid by this latter enzyme. Glycine contributes C-4, C-5, and N-7 of the purine. This intermediate removes fumarate, and a second formyl group from THF is added. Some iron-containing porphyrins are called hemes.
As one of the older chemotherapy drugs, it has been used for many years.
The amino group is now nitrogen 1 of the final ring. These are transferred from the coenzyme tetrahydrofolate as formyltetrahydrofolate, along with a carbon atom from bicarbonate 1. Azaserine has been employed as an antitumor agent because it causes inactivation of glutamine-dependent enzymes in the purine biosynthetic pathway. Binding of dATP to the overall activity site then shuts the enzyme down. One can legitimately speak of a pool of nucleotides in equilibrium with each other.
Oxidized glutaredoxin is re-reduced by two equivalents of glutathione g-glutamylcysteinylglycine; Figure There are two distinct pathways possible for salvaging the bases.
The name porphyrin comes from a Greek word for purple. Next, an amide bond is formed between the activated carboxyl of glycine and the b-amine.
Pyrrolysine abbreviated as Pyl or O is a naturally occurring amino acid similar to lysine, but with an added pyrroline ring linked to the end of the lysine side chain. Nonribosomal peptide antibiotics for example, actinomycin D , cytostatics, and immunosuppressants are used commercially.
Lesch-Nyhan patients have very high blood uric acid levels because of an essentially uncontrolled de novo synthesis. The NRPS genes for a certain peptide are usually organized in one operon in bacteria and in gene clusters in eukaryotes. Evidences demonstrate that there are multifunctional enzymes, and some of them catalyze non-sequential steps in the pathway .
Figure 1 The second reaction is ring closure to form dihydroorotic acid by the enzyme dihydroorotase.
Control of De Novo Synthesis Control of purine nucleotide synthesis has two phases. After removing excess formamide
In gouts caused by an overproduction of uric acid, the defects are in the control mechanisms governing the production of - not uric acid itself - but of the nucleotide precursors. The structural gene for HGPRT is located on the X chromosome, and the disease is a congenital, recessive, sex-linked trait manifested only in males. Non-standard amino acids that are found in proteins are formed by post-translational modification, which is modification after translation during protein synthesis. The kinetics are sigmoidal. Catabolism of the different purine nucleotides converges in the formation of uric acid. The amino group is provided by aspartate in a mechanism similar to that used in forming nitrogen 1 of the ring.
The result is a maintenance of an appropriate balance of the deoxynucleotides for DNA synthesis. Thus, the fate of IMP is determined by the relative levels of AMP and GMP, so that any deficiency in the amount of either of the principal purine nucleotides is self-correcting. In multifunctional enzymes, such products remain bound and are channeled directly to the next active site, rather than dissociated into the surrounding medium for diffusion to the next enzyme. Lesch-Nyhan patients have very high blood uric acid levels because of an essentially uncontrolled de novo synthesis. Rapidly proliferating cell types such as lymphocytes are particularly susceptible if DNA synthesis is impaired. The eukaryotic version of dihydroorotate dehydrogenase is a protein component of the inner mitochondrial membrane; its immediate e- acceptor is a quinone, and the reducing equivalents drawn from dihydroorotate can be used to drive ATP synthesis via oxidative phosphorylation.