8 – Composition and Metabolism of Nucleic Acids – Biochemistry for Nurses

8

Composition and Metabolism of Nucleic Acids

CHAPTER OBJECTIVES
8.1 INTRODUCTION OF NUCLEIC ACIDS

A nucleic acid is a macromolecule composed of chains of monomeric nucleotides. In our body these molecules carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are universal in living things, as they are found in all cells and virus.

Nucleic acids are usually either single-stranded or double-stranded, though structures with three or more strands can form. A double-stranded nucleic acid consists of two single-stranded nucleic acids held together by hydrogen bonds, such as in the DNA double helix. In contrast, RNA is usually single-stranded, but any given strand may fold back upon itself to form secondary structure as in tRNA and rRNA. Within cells, DNA is usually double-stranded, though some viruses have single-stranded DNA as their genome. Retroviruses have single-stranded RNA as their genome.

8.2 THE STRUCTURE OF NUCLEIC ACIDS

Nucleic acids are of two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Their basic structures consist of chains of alternating phosphoric acid and sugar residues. In RNA, the sugar is D-ribofuranose; in DNA, as its name implies, the sugar is 2-deoxy-D-ribofuranose. In deoxyribonucleic acid, de means without oxygen at carbon no.2 position in the D-ribofuranose sugar, whereas RNA has oxygen at carbon no. 2 position. Figure 8.1 illustrates the structure of nucleic acids into (a) phosphate, (b) structure of D-Deoxyribose and D-ribose sugar, and (c) structure of bases.

 

 

Figure 8.1 Structure of Components of Nucleotides in DNA and RNA

8.2.1 Purines and Pyrimidines

Both RNA and DNA contain the purine bases which are adenine and guanine. Figure 8.2 illustrates the structure nucleolides into (a) Pyrimidine ring, a six-membered ring with two nitrogen atoms and three double bonds. (b) Purine ring, which contains a six-membered pyrimidine ring fused to a five-membered imidazole ring. It contains four nitrogen atoms, two in the pyrimidine ring and two nitrogen atoms in the imidazole ring. (c) General structure showing the numbering for the pentose ring. This is a ribonucleotide. (d) Deoxyribonucleotides. The general structure of a purine and the specific structure of adenine and guanine are given here, as is the numbering of the atoms in the purine. Several unusual bases have been found in the transfer RNA's. These include hypoxanthine, 1-methyl hypoxanthine, N2-dimethyl guanine, 1-methyl guanine; N6-(∆2-isopentenyl) adenine and threonylcarbamoyl adenine. Both RNA and DNA also contain the pyrimidine, cytosine, but the two kinds of nucleic acids differ in the fourth nitrogenous base: RNA contains uracil, whereas DNA contains thymine.

 

 

Figure 8.2 Structure of Nucleotides

8.2.2 Bases, Nucleosides and Nucleotides

Bases

Bases present in DNA are adenine, guanine, thymine and cytosine. Bases present in RNA are adenine, guanine, cytosine and uracil. In RNA, thymine is replaced by uracil.

Nucleosides

The combination of a base and sugar is called a nucleoside. The nucleosides are compounds in which purines and pyrimidines are linked to D-ribofuranose or 2-deoxy-D-ribofuranose in an N-β-glycosidic bond, which is the configuration in the polymeric nucleic acids. The point of attachment of the base to the sugar is the hemiacetal hydroxyl on the C-1′ carbon atom of the sugar. In the purines, it is the N-9 nitrogen atom which participates in the N-glycosyl bond. In the pyrimidines, the N-1 nitrogen atom is the point of attachment.

Nucleotides

They are the building blocks of all nucleic acids. Nucleotides have a distinctive structure composed of three components covalently bound together:

  • A nitrogen-containing ‘base’—either a pyrimidine (one ring) or purine (two rings)
  • A 5-carbon sugar—ribose or deoxyribose
  • A phosphate group

Nucleotides also exist in activated forms containing two or three phosphates, called nucleotide diphosphates or triphosphates. If the sugar in a nucleotide is deoxyribose, the nucleotide is called a deoxynucleotide (Figure 8.3), if the sugar is ribose, the term ‘ribonucleotide’ is used (Figure 8.4).

There are five common bases, and four are generally represented in either DNA or RNA. Those bases and their corresponding nucleosides are described in Table 8.1

8.3 STRUCTURE OF DNA

In most living organisms (except for viruses), genetic information is stored in the molecule deoxyribonucleic acid or DNA. DNA resides in the nucleus of living cells. DNA gets its name from the sugar molecule contained in its backbone (deoxyribose); however, it gets its significance from its unique structure. Four different nucleotide bases occur in DNA: adenine (A), cytosine (C), guanine (G) and thymine (T).

The DNA molecule is actually double-stranded. The nucleotides hydrogen is bonded to another nucleotide base in a strand of DNA opposite to the original. This bonding is specific, and adenine always bonds to thymine (and vice versa) and guanine always bonds to cytosine (and vice versa). This bonding occurs across the molecule, leading to a double-stranded system as in Figure 8.5. Though only four different nucleotide bases can occur in a nucleic acid, each nucleic acid contains millions of bases bonded to it. The order in which these nucleotide bases appear in the nucleic acid is the coding for the information carried in the molecule. In other words, the nucleotide bases serve as a sort of genetic alphabet on which the structure of each protein is determined.

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers of nucleotides linked in a chain between the hydroxyl at 3' carbon of one nucleotide and the phosphate at 5' carbon of another nucleotide through a phosphodiester bonds. This leads to formation of the so-called ‘sugar-phosphate backbone’, from which the bases project.

 

 

Figure 8.3 The Structure of a Nucleotides. The Structure of Deoxyguanosine Depicts the Base, Sugar and Phosphate Moieties. The 5' Carbon has an Attached Phosphate Group, While the 3' Carbon has a Hydroxyl Group

 

Successive nucleotide of both DNA and RNA are covalently linked through phosphate group bridges, in which 5'-phosphate group of one nucleotide unit is joint to 3'-hydroxyl of the next nucleotide, creating phosphodiester linkage.

A key feature of all nucleic acids is that they have two distinctive ends, the 5' (5-prime) and 3' (3-prime) ends. This terminology refers to the 5' and 3' carbons on the sugar. For both DNA (Figure 8.6) and RNA, the 5' end bears a phosphate, and the 3' end a hydroxyl group.

In nucleic acid structures, DNA and RNA polymerases add nucleotides to the 3' end of the previously incorporated base. Nucleic acids are synthesized in a 5' to 3' direction. DNA and RNA are synthesized in cells by DNA polymerases and RNA polymerases.

The covalent backbone of DNA and RNA is subject to slow, non-enzymatic hydrolysis of the phosphodiester bonds. In vitro RNA is hydrolyzed rapidly under alkaline condition to 2', 3'-cyclicmonophosphate derivative, then further hydrolyzed to yield a mixture of 2' and 3'-nucleoside monophosphate. DNA is not subjected to hydrolysis under alkaline conditions because the 2'-hydroxyl group is absent in DNA, but in RNA 2'-hydroxyl group is present which is directly involved in the process.

 

 

Figure 8.4 The Structure of Ribonucleotides (in RNA) has an Extra Hydroxyl Group on the 2' Carbon of Ribose. The 5' Carbon has an Attached Phosphate Group, While the 3' Carbon has a Hydroxyl Group

 

Table 8.1 Nomenclature of Nucleotide and Nucleic Acid

 

 

Figure 8.5 The Nucleotide Bases of the DNA Molecule Forms Complementary Pairs

 

Most DNA exists in the famous form of a double helix, in which two linear strands of DNA are wound around one another. The major force promoting formation of this helix is complementary base pairing: A's form hydrogen bonds with T's (or U's in RNA), and G's form hydrogen bonds with C's. If we mix two ATGC's together, the double-stranded DNA is formed as in Figure 8.7. This is always the case for duplex nucleic acids G-C base pairs have three hydrogen bonds, whereas A-T base pairs have two hydrogen bonds. One consequence of this disparity is that it takes more energy (e.g. a higher temperature) to disrupt GC-rich DNA than AT-rich DNA.

In 1953, Watson and Crick discovered the double-stranded structure of DNA. Watson, Crick, Wilkins and Franklin shown that not only is the DNA molecule double-stranded, but the two strands wrap around each other forming a coil, or helix. The true structure of the DNA molecule is a double helix. The double-stranded DNA molecule has the unique ability that it can make exact copies of itself, or self-replicate. When more DNA is required by an organism (such as during reproduction or cell growth) the hydrogen bonds between the nucleotide bases break and the two single strands of DNA separate. New complementary bases are brought in by the cell and paired up with each of the two separate strands, thus forming two new, identical, double-stranded DNA molecules.

The original model proposed by Watson and Crick (Figure 8.8) had 10 base pairs, or 34 Å per turn of the helix. The vertically stacked bases inside the double helix would be 3.4 Å apart; the secondary repeat distance of about 34 Å was accounted for by the presence of 10 base pairs in each complete turn of the double helix. In aqueous solution the structure differs slightly from that in fibres, having 10.5 base instead of 10 base pairs per helical turn, the secondary repeat distance of about 36 Å was accounted for by the presence of 10.5 base pairs in each complete turn of the double helix.

8.4 STRUCTURE OF RNA

RNA is a single-stranded molecule. RNA is the main genetic material used in the organisms called viruses, and RNA is also important in the production of proteins in other living organisms. RNA can move around the cells of living organisms and thus serves as a sort of genetic messenger, relaying the information stored in the cell's DNA out from the nucleus to other parts of the cell where it is used to help make proteins.

RNAs are usually single stranded (Figure 8.9), but many RNA molecules have secondary structure in which intramolecular loops are formed by complementary base pairing. A simple example of this is shown in Figure 8.10, and much more extensive and complex examples are known. Base pairing in RNA follows exactly the same principles as with DNA: the two regions involved in duplex formation are antiparallel to one another and the base pairs that form are A-U and G-C.

 

 

Figure 8.6 DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are Polymers of Nucleotides Linked in a Chain Between the Hydroxyl at 3' Carbon of One Nucleotide and the Phosphate at 5' Carbon of Another Nucleotide, Through a Phosphodiester Bond

 

 

Figure 8.7 The Two Strands of DNA are Arranged Anti-parallel to One Another, Viewed from Left to Right, the ‘Top’ Strand is Aligned 5' To 3', While the ‘Bottom’ Strand is Aligned 3' to 5'

8.5 TYPES OF RNA

All three types of RNA are involved in protein synthesis. RNA has a greater variety of possible structures and chemical properties than DNA due to the diversity of roles it performs in the cell. The following three principal types of RNAs are involved in protein synthesis:

 

 

Figure 8.8 Watson and Crick Model of Double-stranded Structure of DNA

  • Messenger RNA (mRNA) serves as the template for the synthesis of a protein. It carries information from DNA to the ribosome, a specialized structure where the message is then translated into a protein.
  • Transfer RNA (tRNA) is a small chain of about 70–90 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of synthesis. It pairs the amino acid to the appropriate codon on the mRNA molecule.
  • Ribosomal RNA (rRNA) molecules are extremely abundant and make up at least 80 per cent of the RNA molecules found in a typical eukaryotic cell. In the cytoplasm, rRNA molecules combine with proteins to perform a structural role, as components of the ribosome.

 

 

Figure 8.9 The Nucleotide Bases of the RNA Molecule

 

 

Figure 8.10 Secondary Structure of RNA Molecules in Which Intramolecular Loops are Formed by Complementary Base Pairing

8.6 NUCLEIC ACID METABOLISM

Introduction to Nucleic Acid Metabolism

The nucleic acid metabolism is concerned with the pathway by which nucleic acids and their components are anabolized (formed) and catabolized (broken down). The chemistry of nucleic acids and their components has been described in the first phase of this chapter. This phase of the chapter deals in part with the fundamental mechanism for controlling macromolecular synthesis and activity in cells.

8.7 BIOSYNTHESIS OF PURINE NUCLEOTIDEs

In the biosynthesis of purine, a nitrogenous heterocyclic base, for example, adenine, guanine, hypoxanthine and xanthine are involved. De novo synthesis involves a complex, energy-expensive pathway that yields inosine 5'-monophosphate (IMP), a purine ribonucleotide. AMP and GMP are then formed from IMP by separate pathways. Adenine and guanine are found in both DNA and RNA. Hypoxanthine and xanthine are important intermediates in the synthesis and degradation of the purine nucleotides. Two types of pathways lead to the biosynthesis of purine nucleotide: de novo pathway and the salvage pathway. De novo synthesis of nucleotide begins with their metabolic precursors; amino acids, ribose 5-phosphate, CO2, and NH3. Salvage pathway recycles the free bases and nucleosides released from nucleic acid break down.

The mechanism purine biosynthesis has been reviewed by John Buchanan. By studying purine synthesis in cell-free systems of pigeon liver, it has been established that purine biosynthesis is effective de novo from several small molecules. The various components which contribute the synthesis of purine ring are derived from formate, CO2, glutamine, aspartic acid and glycine as shown in Figure 8.11.

Purine biosynthesis by de novo pathway: Nucleotide bases are assembled from simpler compounds. De novo pathways, the bases of the nucleotides are made from scratch by using simpler starting materials (including amino acids), requires ATP hydrolysis.

 

 

Figure 8.11 Sources of the Purine Ring. The Sources of Carbon and Nitrogen are Different and are Shown in Different Shades

 

Two main bases of purine nucleotide are adenine and guanine. De novo synthesis of purine nucleotide is completed into many steps. De novo pathway of purine biosynthesis leads to the complete synthesis of purine ring. Purine is a heterocyclic aromatic organic compound, consisting of a six-membered pyrimidine ring fused to five-membered imidazole ring. Steps of purine biosynthesis of de novo pathway are as follows (Figures 8.12a and 8.12b):

Step 1: The synthesis of inosinic acid begins with D-ribose-5-phosphate, which is formed in the pentose cycle. D-Ribose-5-P is converted to 5-phosphoribosyl-1-pyrophosphate by the action of ATP. This reaction is catalyzed by the kinase enzyme.

Step 2: 5-phosphoribosyl-1-pyrophosphate reacts with glutamine to form 5-phosphoribosyl-1-amine. Glutamine donates amino group which is attached at C-1 of PRPP, with the formation of 5-phosphoribosyl-1-amine. The enzyme catalyzed this reaction is glutamine-PRPP amidotransferase.

Step 3: In this step, 5-phosphoribosylamine reacts with glycine to form Glycinamide ribonucleotide. The enzyme catalyzed this reaction is GAR synthetase and ATP is consumed to activate the glycine carboxyl group.

Step 4: Glycinamide ribonucleotide in converted to Formylglycinamide ribonucleotide (FGAR) by N10-anhydroformyltetrahydrofolic acid (N10-anhydroformyl-FH4) and the enzyme catalyzed this reaction is glycinamide ribotide transformylase.

Step 5: Formylglycinamide ribonucleotide then reacts with glutamine an ATP to form Formylglycinamidine ribonucleotide (FGAM). The enzyme catalyzed this reaction is FGAR amidotransferase.

Step 6: Formylglycinamidine ribonucleotide (FGAM) reacts with ATP undergo dehydration leads to ring closure yield the five-membered imidazole ring of the purine nucleus, as 5-Aminoimidazole ribonucleotide (AIR) The enzyme catalyzed this reaction is FGAM cyclase.

 

 

Figure 8.12a De Novo Synthesis of Purine Nucleotide

 

 

Figure 8.12b De Novo Synthesis of Purine Nucleotide. The Biosynthesis of Inosinic Acid Begins with D-Ribose-5-Phosphate Leads to the Construction of Purine Ring of Inosinate (IMP) Through Many Steps. The Sources of Carbon and Nitrogen of Purine Ring of Inosinate are Shown with the Same Shades as Shown in Figure 8.11

 

Step 7: 5-Aminoimidazole ribonucleotide (AIR) is then carboxylated to Carboxyaminoimidazole rinucleotide by the enzyme aminoimidazole ribonucleotide carboxylase.

Step 8: The compound Carboxyaminoimidazole ribonucleotide reacts with aspartic acid and ATP to form N-Succinyl-5-aminoimidazol-4-carboxamide ribonucleotide. The enzyme catalyzed this reaction is SAICAR synthetase.

Step 9: N-Succinyl-5-aminoimidazol-4-carboxamide ribonucleotide is then cleaved into fumaric acid by the enzyme SAICAR lyase to form 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR).

Step 10: 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) converted to N-Formylaminoimidazole-4-carboxamide ribonucleotide (FAICAR) by the enzyme AICAR transformylase

Step 11: Inosinic acid is finally formed by action of the enzyme IMP synthase upon N-Formylaminoimidazole-4-carboxamide ribonucleotide (FAICAR). The ring closure takes place to yield the second fused ring of purine nucleus.

Biosynthesis of AMP and GMP from IMP

The first intermediate of complete purine ring is inosinate (IMP). Conversion of inosinate to adenylate requires the insertion of amino group derived from aspartate. Formation of adenylate from inosinate is a two step reaction: in the first step, inosinate reacts with aspartate and GTP is the source of high energy phosphate in synthesizing adenylosuccinate. This reaction is very similar to steps 8 and 9 of purine biosynthesis in which aspartate is used to introduce N-1 of the purine ring. The only difference is that, in this reaction in spite of ATP, GTP is used as a source of energy. This reaction is catalyzed by the enzyme adenylosuccinate synthetase. In the second step, adenylosuccinate is converted to adenylate (AMP) with the release of fumarate, catalyzed by the enzyme adenylosuccinate lyase (Figure 8.13).

 

 

Figure 8.13 Biosynthesis of AMP and GMP from IMP

 

Guanylate (GMP) is formed by the NAD+ requiring oxidation of inosinate (IMP) at C-2, followed by addition of an amino group derived from glutamine. This reaction is catalyzed by the enzyme amidotransferase and ATP is cleaved to AMP and PPi (Figure 8.13).

8.8 BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES

The biosynthesis of pyrimidine nucleotides was reviewed by the scientist Reichard. The two major bases of pyrimidine are cytosine and thymine in DNA and cytosine uracil in RNA. The pyrimidine, like the purines, are synthesized as the nucleotides. The discovery that in particular aided in the study of pyrimidine synthesis was that orotic acid (6-carboxyuracil) could satisfy the pyrimidine of several bacteria. The biosynthesis of pyrimidine nucleotide is completed in many steps. The steps are as follows (Figure 8.14):

Step 1: The first reaction in the synthesis of pyrimidine nucleotide is the reaction of carbamoyl phosphate with aspartic acid to form N-carbamoylaspartate. This reaction is catalyzed by the enzyme aspartate transcarbamoylase (ATCase).

Step 2: The enzyme dihydroorotase act on N-carbamoylaspartate to give ring closure with the formation of dihydroorotate.

Step 3: Dihydroorotate is then oxidized to orotate by the enzyme dihydroorotate dehydrogenase which requires NAD+.

Step 4: Orotate is converted into orotidylate by reacting with 5-phosphoribosyl 1-pyrophosphate. This reaction is catalyzed by the enzyme orotate phosphoribosyl-transferase.

Step 5: Orotidylate is decarboxylated to uridylate (UMP) by the enzyme orotidylate decarboxylase.

Step 6: Uridylate (UMP) is converted into uridine 5'-triphosphate (UTP) through phosphorylation by ATP and kinases.

Synthesis of Cytidine 5'-triphosphate (CTP)

The formation of cytidine 5'-triphosphate is through amination of UTP with NH3 to form CTP. Amino group is derived from glutamine. This reaction is catalyzed by the enzyme cytidylate synthetase.

Synthesis of TTP thymidine 5'-triphosphate

The synthesis of thymine nucleotide involves both conversion of ribonucleotide to deoxyribonucleotide and also methylation of pyrimidine ring and enzyme from E. coli converted deoxyuridylic acid to thymidylic acid in the presence of serine, tetrahydrofolic acid, ATP and Mg++. It was found that N10–hydroxymethyltetrahydrofolic acid could replace the serine and tetrahydrofolic acid requirement (Figure 8.15).

 

 

Figure 8.14 The Biosynthesis of Pyrimidine Nucleotides. The Biosynthesis of Pyrimidine Nucleotide Starts With the Reaction of Carbamoyl Phosphate With Aspartic Acid to form N-carbamoylaspartate. Carbamoyl Phosphate, Shown in Dark Shade and Aspartic Acid Shown in Light Shade. The N-carbamoylaspartate is Shown in Two Shades Indicating that the Dark Part is of Carbamoyl Phosphate and the Light Part is of Aspartic Acid

 

 

Figure 8.15 Synthesis of Thymidine Nucleotide

8.9 CATABOLISM OF PURINE NUCLEOTIDEs

Adenylate is degraded to uric acid through various steps. Adenylate yields adenosine, which is deaminated to inosine by the enzyme adenosine deaminase.

Inosine is hydrolyzed to hypoxanthine, hypoxanthine is oxidized to xanthine by the enzyme xanthine oxidase, and then xanthine to uric acid. Uric acid is the excreted end product of purine catabolism in primate birds, and some other animals. In most mammals and vertebrates, uric acid is further degraded to allantoin by the action of enzyme urate oxidase (Figure 8.16).

Catabolism of GMP

The end product of GMP is also uric acid. GMP is first hydrolyzed to guanosine, which is then cleaved to form guanine by the action of the enzyme nucleosidase. Guanine undergoes hydrolytic removal of its amino group to yield xanthine which is converted to uric acid by the action of enzyme xanthine oxidase.

8.10 CATABOLISM OF PYRIMIDINE NUCLEOTIDEs

Degradation of pyrimidine nucleotide lead to NH4+ production and ultimately leads to urea synthesis. Thymine nucleotide is first degraded to dihydrothymine by the enzyme dihydrouracil dehydrogenase, then to β-ureidoisobutyrate by the enzyme dihydropyrimidanase, then β-ureidoisobutyrate is degraded to β-aminoisobutyrate by the enzyme β-ureidopropionase, then β-aminoisobutyrate is further degraded to methylmalonyl-semialdehyde by the enzyme aminotransferase. The end product of thymine catabolism is methylmalonyl-semialdehyde. Methylmalonyl-semialdehyde is an intermediate product of valine catabolism. It is further degraded to propionyl-CoA and methylmalonyl-CoA to succinyl-CoA (Figure 8.17).

 

 

Figure 8.16 Catabolism Of Purine Nucleotide: Note that Primates Excrete Much More Nitrogen as Urea Via the Urea Cycle than as Uric Acid from Purine Degradation. Similarly, Fish Excrete Much More Nitrogen as NH4 than as Urea Produced by the Pathway Shown Here

8.11 SALVAGE PATHWAY

Salvage pathway recycle already used bases by reattaching them to a ribose. Purine and pyrimidine bases are recycled by salvage pathways. Free bases of both purine and pyrimidine nucleotide are released in cells during the degradation of nucleotide. Free purines, in large part, are salvaged. Salvage pathway for adenine is very simple as compared to de novo pathway of purine biosynthesis. Free adenine reacts with PRPP to form AMP and pyrophosphate by the enzyme adenosine phosphoribosyltransferase. Free guanine and hypoxanthine, which is the deamination product of adenine are salvaged in the same way as that of the adenine by the enzyme hypoxanthine-guanine phosphoribosyltransferase.

The salvage pathway for pyrimidine bases is similar to that of the purine bases.

 

 

Figure 8.17 Degradation of Pyrimidine Nucleotide Leads to NH4+ Production and Ultimately Leads to Urea Synthesis. Thymine Nucleotide is First Degraded to Dihydrothymine and the end Product of Thymine Catabolism is Methylmalonyl-semialdehyde

8.12 METABOLIC DISORDER OF NUCLEIC ACIDs

High uric acid overproduction results from the inability to recycle either hypoxanthine or guanine, which interrupts the inosinate cycle producing a HPRT deficiency, lack of feedback control of synthesis results in rapid catabolism of hypoxanthine or guanine bases to uric acid.

Lesch–Nyhan Syndrome

Purines can be synthesized in 2 ways:

  • De novo—‘from scratch’
  • Salvage pathway—by recycling the nucleotides.

In Lesch–Nyhan syndrome patients, HPRT (hypoxanthine-guanine phosphoribosyltransferase) is absent which is the key enzyme in salvaging and recycling old purines from DNA and RNA degradation. Nucleotides made by the salvage pathway provide feedback inhibition to the de novo pathway to conserve energy. Without HPRT, the salvage pathway is impaired and de novo synthesis goes into over production due to lack of negative inhibition. Excess amounts of purines are broken down into uric acid, by means of the enzyme xanthine oxidase, leading to hyperuricemia. In the Lesch–Nyhan syndrome, children with this genetic disorder are sometimes poorly coordinated and mentally retarded.

Symptoms of Hyperuricemia

  • Formation of urate crystals
  • Formation of kidney stones

Impaired kidney function leads to renal failure and gout-like arthritis.

Xanthinuria and Xanthine lithiasis

Increased excretion of hypoxanthine and xanthine are associated with xanthine oxidase deficiency leading to genetic defect and severe liver damage. Patients with a severe enzyme deficiency may exhibit xanthinuria and xanthine lithiasis.

Gout

Various genetic defects in PRPP synthetase present clinically as gout. It results in overproduction and overexcretion of purine catabolites. When serum urate levels exceed the gout solubility limit, sodium urate crystallizes in soft tissues and joints and causes an inflammatory reaction, gouty arthritis. However, most cases of gout reflect abnormalities in renal handling of uric acid.

Characteristics of Gout

A painful form of arthritis in which usually the big toe is painful and swollen. This disease is also known as the ‘disease of kings’ as it is caused by rich foods and alcohol. In this disease, the body does not process uric acid properly so, blood has high level of uric acid. Uric acid precipitates out in the joints, causing swelling. Certain foods are high in purines, which increase uric acid production.

Treatment of Gout

Gout is effectively treated by not eating purine rich food and alcohol. Drug treatment is also given, like drug allopurine is given which is a xanthine oxidase inhibitor, which is a structural analogue of hypoxanthine and xanthine and has a higher affinity for xanthine oxidase. Allopurinol inhibits xanthine oxidase, the enzyme that catalyzes the conversion of purines to uric acid. Allopurinol is a substrate of xanthine oxidase, which converts allopurinol to oxypurinol. When xanthine oxidase is inhibited, the excreted products of purine metabolism are xanthine and hypoxanthine, both are more water soluble than uric acid and less likely to form crystalline deposits.

EXERCISES
  1. Give a brief account of nucleotides.
  2. Differentiate between nucleosides and nucleotides.
  3. Draw the structure of purine and pyrimidine ring along with the structure of their bases.
  4. What is the difference between DNA and RNA? Explain with the help of a diagram.
  5. Write the biosynthesis of purine nucleotide.
  6. Write the break down of purine nucleotide.
  7. Write in detail the degradation of pyrimidine nucleotide.
  8. What is the difference between de novo pathway and salvage pathway of purine nucleotide?
Multiple Choice Questions
  1. The best role of purine and pyrimidine nucleotides is to serve as the monomeric precursors of
    1. RNA
    2. DNA
    3. Both of the above
    4. None of the above

     

    Ans. c

  2. The purine nucleotides act as the components of
    1. FAD+
    2. NAD+
    3. NADP+
    4. All of the above

     

    Ans. d

  3. The chemical name of thymine
    1. 2 – oxy – 4 – aminopyrimidine
    2. 2, 4 – dioxy – 5 – methylpyrimidine
    3. 2, 4 – dioxypyrimidine
    4. None of the above

     

    Ans. b

  4. The chemical name 2-amino-6-oxypurine is said to be
    1. Adenine
    2. Xanthine
    3. Gaunine
    4. Hypoxanthine

     

    Ans. c

  5. The most abundant intracellular-free nucleotide
    1. ATP
    2. FAD+
    3. NAD+
    4. NADP+

     

    Ans. a

  6. The epimerization of galactose to glucose and vice versa takes place by
    1. UTP
    2. CTP
    3. GTP
    4. ATP

     

    Ans. a

  7. The biosynthesis of phosphoglycerides in animal tissue requires
    1. ATP
    2. CTP
    3. GTP
    4. TPP

     

    Ans. b

  8. The chemical name 4-hydroxypyrazole pyrimidine is used for
    1. Thioguanine
    2. Mercaptopurine
    3. Azathioprine
    4. Allopurinol

     

    Ans. d

  9. Guanosine nucleotide is held by the cytosine nucleotide by the number of hydrogen bonds
    1. 1
    2. 2
    3. 3
    4. 4

     

    Ans. c

  10. DNA is denatured by
    1. Heat
    2. Acid
    3. Alkali
    4. All of the above

     

    Ans. d

  11. Each transfer RNA molecule contains the number of nucleotides
    1. 70
    2. 75
    3. 80
    4. 85

     

    Ans. b

  12. The carbon atoms at positions 4 and 5 and the N atom at position 7 of purine base are supplied from
    1. Valine
    2. Alanine
    3. Glycine
    4. Serine

     

    Ans. c

  13. 5- phosphoribosylamine reacts with glycine to produce glycinamide ribosylphosphate by glycinamide kinosynthetase in presence of
    1. ATP
    2. GTP
    3. UTP
    4. CTP

     

    Ans. a