Purines and pyrimidines

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Chapter 12 Purines and pyrimidines

Chapter contents

Nucleosides and nucleotides 89
Metabolism of purines and pyrimidines 91
DNA and RNA 92

Purines and pyrimidines are known as nitrogenous bases. The two groups have different features (Figure 12.1):

• Purines: a six-membered and a five-membered nitrogen-containing ring fused together.
• Pyrimidines: a six-membered nitrogen-containing ring.
image

Figure 12.1 The structure of purines and pyrimidines.

These nitrogenous bases are crucial not only to the storage and transmission of genetic information, but also as a means of storing mobile energy. They can be synthesized in the body from basic components. The degraded bases can be salvaged and reused (see Chapter 37 ‘Metabolic disorders’, p. 295).

Nucleosides and Nucleotides

When a sugar is added to a nitrogenous base, a nucleoside is formed (Figure 12.2).

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Figure 12.2 The basic structure of nucleosides and their inclusion in the nucleotide structures of RNA and DNA.

When phosphate groups attach to the nucleoside, a nucleotide is formed (Figure 12.2) with one phosphate (monophosphate), two phosphate (diphosphate) or three phosphate (triphosphate) groups attached.

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Figure 12.3 The structure of the coenzyme nicotinamide adenine dinucleotide phosphate (NADP+), the secondary messenger cyclic adenosine monophosphate (cAMP) and the energy carrier adenosine triphosphate (ATP), all of which are derived from purines and pyrimidines.

Nucleotides

Nucleotides are very important chemicals. they:

• Store genetic information in the nucleus in the form of DNA.
• Transmit genetic information in the form of messenger RNA.
• Are responsible for the storage of energy in chemicals adenosine triphosphate (ATP) and guanosine triphosphate (GTP).
• Are involved in signal transduction, which is the movement of signals from outside the cell to its inside (see Figure 19.5, p. 141).
• Act as secondary messengers in cellular communication in the form of cyclic adenosine monophosphate (cAMP).
• Are required for coenzyme function (coenzymes enable enzymes to work properly), e.g. nicotinamide adenine dinucleotide phosphate (NADP+).

The Importance of Hydrogen Bonds in DNA (Figure 12.4)

The two strands of the DNA structure are held together by hydrogen bonds between the nitrogenous bases (see Chapter 3 ‘Bonds found in biological chemistry’, p. 18) (Figure 12.4). This allows the structure to be pulled apart and put back together in protein synthesis (see Figure 11.6, p. 85).

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Figure 12.4 Hydrogen bonding between the nitrogenous bases in DNA. This is where alkylating agents used for chemotherapy work.

Metabolism of Purines and Pyrimidines

Every body organ has a different capacity to synthesize purines and pyrimidines. The liver is responsible for synthesizing the majority of purines from their basic components (de novo synthesis).

Those purines and pyrimidines that are not resynthesized are removed from the body and their breakdown results in the production of uric acid, primarily by the liver, which is excreted by the kidneys. A defect in this process will result in a clinical condition, as in the case of gout (see Chapter 37 ‘Metabolic disorders’, p. 295).

DNA and RNA

It is important to note that although there is a turnover of RNA within a cell, this does not occur with DNA. Instead, enzymes excise the damaged parts during a process of repair. If this process goes wrong then cell metabolism malfunctions. This is considered when treating viral infections and using chemotherapy to damage the viral or aberrant cell DNA (see Chapter 39 ‘Chemotherapy’, p. 309).

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