HETEROCYCLIC COMPOUNDS. NUCLEINIC ACIDS

1. KARAGANDA STATE MEDICAL UNIVERSITY HETEROCYCLIC COMPOUNDS. NUCLEINIC ACIDS Karaganda 2011 y.

2. Brief contents • Functions of nucleic acids. • Tautomeric forms of purines and pyrimidines. • Nomenclature of nucleotides. • Purine, pyrimidine and nucleotide analogs.

3. There are two types of nucleic acids, namely deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Primarily, nucleic acids serve as repositories and transmitters of genetic information. Brief history. DNA was discovered in 1869 by Johann Friedrich Miescher, a Swiss researcher. The demonstration that DNA contained genetic information was first made in 1944, by Avery, Macleod and MacCary. Functions of nucleic acids. DNA is the chemical basis of heredity and may be regarded as the reserve bank of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. Further, every aspect of cellular function is under the control of DNA. The DNA is organized into genes, the fundamental units of genetic information. The genes control the protein synthesis through the mediation of RNA, as shown below DNA RNA Protein

4. The interrelationship of these three classes of biomolecules (DNA, RNA and proteins) constitutes the central dogma of molecular biology or more commonly the central dogma of life. Components of nucleic acids. Nucleic acids are the polymers of nucleotides (polynucleotides) held by 3' and 5' phosphate bridges. In other words, nucleic acids are built up by the monomeric units -nucleotides (It may be recalled that protein is a polymer of amino acids). Nucleotides. Nucleotides are composed of a nitrogenous base, a pentose sugar and a phosphate. Nucleotides perform a wide variety of functions in the living cells, besides being the building blocks or monomeric units in the nucleic acid (DNA and RNA) structure. These include their role as structural components of some coenzymes of B-complex vitamins (e.g. FAD, NAD+), in the energy reactions of cells (ATP is the energy currency), and in the control of metabolic reactions.

5. STRUCTURE OF NUCLEOTIDES As already stated, the nucleotide essentially consists of base, sugar and phosphate. The term nucleoside refers to base + sugar. Thus, nucleotide is nucleoside + phosphate.

6. Purines and pyrimidines. The nitrogenous bases found in nucleotides (and, therefore, nucleic acids) are aromatic heterocyclic compounds. The bases are of two types purines and pyrimidines. Their general structures are depicted in Fig.1. Purines are numbered in the anticlockwise direction while pyrimidines are numbered in the clockwise direction. And this is an internationally accepted system to represent the structure of bases. Major bases in nucleic acids The structures of major purines and pyrimidines found in nucleic acids are shown in Fig.2. DNA and RNA contain the same purines namely adenine (A) and guanine (G). Further, the pyrimidine cytosine (C) is found in both DNA and RNA. However, the nucleic acids differ with respect to the second pyrimidine base. DNA contains thymine (T) whereas RNA contains uracil (U). As is observed in the Fig.2, thymine and uracil differ in structure by the presence (in T) or absence (in U) of a methyl group.

7. Fig.2: Structures of major purines (A, G) and pyrimidines (C,T,U) found in nucleic acids.

8. Tautomeric forms of purines and pyrimidines The existence of a molecule in a keto (lactam) and enol (lactim) form is known as tautomerism. The heterocyclic rings of purines and pyrimidines with oxo functional groups exhibit tautomerism as simplified below. The purine-guanine and pyrimidines-cytosine, thymine and uracil exhibit tautomerism. The lactam and lactim forms of cytosine are represented in Fig.3. At physiological pH, the lactam (keto) tautomeric forms are predominantly present. Fig. 3: The tautomeric forms of cytosine

9. Minor bases found in nucleic acids: Besides the bases described above, several minor and unusual bases are often found in DNA and RNA. These include 5-methylcytosine, N4-acetyl-cytosine, N6-methyladenine, N6, N6-dimethyl-adenine, pseudouracil etc. It is believed that the unusual bases in nucleic acids will help in the recognition of specific enzymes. Other biologically important bases: The bases such as hypoxanthine, xanthine and uric acid (Fig.4) are present in the free state in the cells. The former two are the intermediates in purine synthesis while uric acid is the end product of purine degradation. Purine bases of plants: Plants contain certain methylated purines which are of pharmacological interest. These include caffeine (of coffee), theophylline (of tea) and theobromine (of cocoa).

10. Fig.4. Structuresoi'somebiologicallyimportantpurines. Sugars of nucleic acids The five carbon monosaccharides (pentoses) are found in the nucleic acid structure. RNA contains D-ribose while DNA contains D-deoxyribose. Ribose and deoxyribose differ in structure at C2. Deoxyribose has one oxygen less at C2 compared to ribose (Fig.5).

11. Fig. 5. Structures of sugars present in nucleic acids (noose is found In RNA and deoxyribose in DNA; Note the structural difference at С2). Nomenclature of nucleotides The addition of a pentose sugar to base produces a nucleoside. If the sugar is ribose, ribonucleosides are formed. Adenosine, guanosine, cytidine and uridine are the ribonucleosides of A, G, С and U respectively. If the sugar is a deoxyribose, deoxyribo-nucleosides are produced. The term mononucleotide is used when a single phosphate moiety is added to a nucleoside. Thus adenosine monophosphate (AMP) contains adenine + ribose + phosphate.

12. The principal bases, their respective nucleosides and nucleotides found in the structure of nucleic acids are given in Table 1. Note that the prefix 'd' is used to indicate if the sugar is deoxyribose (e.g. dAMP). The binding of nucleotide components. The atoms in the purine ring are numbered as 1 to 9 and for pyrimidine as 1 to 6 (See Fig.1). The carbons of sugars are represented with an associated prime (') for differentiation. Thus the pentose carbons are 1' to 5'.

13. Table 1. Principal bases, nucleosides and nucleotides

15. The pentoses are bound to nitrogenous bases by β-N-glycosidic bonds. The N9 of a purine ring binds with C1(1') of a pentose sugar to form a covalent bond in the purine nucleoside. In case of pyrimidine nucleosides, the glycosidic linkage is between N1 of a pyrimidine and C'1 of a pentose. The hydroxyl groups of adenosine are esterified with phosphates to produce 5'- or З'-monophosphates. 5'-Hydroxyl is the most commonly esterified, hence 5' is usually omitted while writing nucleotide names. Thus AMP represents adenosine 5'-monophosphate. However, for adenosine З'-monophosphate, the abbreviation З'-AMP is used. The structures of two selected nucleotides namely AMP and TMP are depicted in Fig.6.

16. Fig. 6: The structures of adenosine 5'-monophosphate (AMP) and thymidine 5'-monophosphate (TMP) [*-Addition of second or third phosphate gives adenosine diphosphate (ADP) and adenosine triphosphate (ATP) respectively).

17. Nucleoside di- and triphosphates Nucleoside monophosphates possess only one phosphate moiety (AMP, TMP). The addition of second or third phosphates to the nucleoside results in nucleoside diphosphate (e.g. ADP) or triphosphate (e.g. ATP), respectively. The anionic properties of nucleotides and nucleic acids are due to the negative charges contributed by phosphate groups.

18. PURINE, PYRIMIDINE AND NUCLEOTIDE ANALOGS It is possible to alter heterocyclic ring or sugar moiety, and produce synthetic analogs of purines, pyrimidines, nucleosides and nucleotides. Some of the synthetic analogs are highly useful in clinical medicine. The structures of selected purine and pyrimidine analogs are given in Fig.7. Fig. 7: Structures of selectedpurine andpyrimidine analogs.

19. The pharmacological applications of certain analogs are listed below Allopurinol is used in the treatment of hyperuricemia and gout (For details, Refer Chapter 17). 5-Fluorouracil, 6-mercaptopurine, 8-aza-guanine, 3-deoxyuridine, 5- or 6-azauridine, 5- or 6-azacytidine and 5-idouracil are employed in the treatment of cancers. These compounds get incorporated into DNA and block cell proliferation. Azathioprine (which gets degraded to 6-mercaptopurine) is used to suppress immunological rejection during transplantation. Arabinosyladenine is used for the treatment of neurological disease, viral encephalitis. Arabinosylcytosine is being used in cancer therapy as it interferes with DNA replication. The drugs employed in the treatment of AIDS namely zidovudine or AZT (3-azido 2',3'-dideoxythymidine) and didanosine (dideoxy-inosine) are sugar modified synthetic nucleotide analogs.