NUCLEIC ACID METABOLISM A. A. OSUNTOKI, Ph.D.

NUCLEIC ACIDS Polynucleotides i.e. polymers of nucleotides Two types Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA)

NUCLEOTIDES Participate in nearly all biochemical processes. Activated precursors of DNA and RNA. Some derivatives are activated intermediates in biosyntheses. ATP, an adenine nucleotide is the universal currency of energy in biological systems. Adenine nucleotides are components of three major coenzymes; NAD +, FAD and CoA. Involved in metabolic regulation e.g. Cyclic AMP is a mediator of hormonal activity.

Nucleotides contd The nitrogenous base is a purine or pyrimidine derivative. The two major purines are adenine and guanine. The three major pyrimidines are cytosine, uracil and thymine. A nucleoside consists of a base linked to a sugar. A nucleotide consists of a nitrogenous base, a sugar and one or more phosphate groups. A nucleotide is a phosphorylated nucleoside.

PURINE RING

Adenine

Guanine

2 The purine ring is synthesized from amino acids and tetrahydrofolate derivatives and CO 2 N1 is from aspartate N3 and N 9 are from glutamine N7,C4 and C5 are from glycine C2 is from N 10 formyl THF C8 is from N 5,N 10 -methylene THF C6 is from CO 2

PYRIMIDINE RING

The pyrimidine ring is synthesized from carbamoyl phosphate and aspartate. C2, N3 are from carbamoyl phosphate The rest of the ring is from aspartate

NUCLEOTIDE BIOSYNTHESIS Two major pathways De novo and Salvage The deoxyribonucleotides are synthesized by the reduction of ribonucleotides. Deoxythymidylate is formed from the methylation of deoxyuridylate.

PURINE BIOSYNTHESIS The initially synthesized purine derivative is inosine monophosphate (IMP) IMP is the nucleotide of the base hypoxanthine AMP and GMP are synthesized using IMP as intermediate Purines are synthesised as ribonucleotides rather than free bases

Hypoxanthine

IMP

PURINE SALVAGE Most cells have an active turnover of many of their nucleic acids resulting in the release of pure purines These purines are converted to corresponding nucleotides by salvage pathways Salvage pathways are diverse In mammals purines are salvaged mainly by two different enzymes

Adenine phosphoribosyl transferase (APRT) mediates AMP formation through the transfer of adenine to PRPP with the release of PPi Adenine + PRPP ↔ AMP + PPi

Hypoxanthine-guanine phosphorybosyl transferase (HGPRT) catalyses the IMP and GMP formation Hypoxanthine + PRPP ↔ IMP +PPi Guanine + PRPP↔GMP +PPi HGPRT deficiency results in Lesch –Nyhan Syndrome

PYRIMIDINE SYNTHESIS The intermediate pyrimidine is orotic acid In contrast to purine nucleotides, the pyrimidine ring is formed first and coupled to ribose-5-phosphate

PYRIMIDINE SYNTHESIS

PURINE CATABOLISM The end product of purine catabolism in humans is uric acid. Other mammals have the enzyme urate oxidase and excrete the more soluble allantoin as the end product. Humans do not have this enzyme so urate is the end product for. Uric acid is formed primarily in the liver and excreted by the kidney into the urine.

PURINE DEGRADATION

NUCLEOTIDE METABOLISM AND DISEASES The importance of nucleotide biosynthesis is buttressed by the fact that clinical diseases/ syndromes are associated with both biosynthetic pathways and with catabolism. Examples of such conditions are Gout Lesch-Nyhan Syndrome Orotic aciduria (which is associated with defects in pyrimidine nucleotide biosynthesis) Immunodeficiency diseases

Defects in adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) are associated with distint immunodeficiency diseases. A deficiency in ADA is associated with a severe combined immunodeficiency involving both T- cell and B-cell mediated functions. PNP is associated with an immunodeficiency involving T-cells functions.

APPLICATION IN THERAPY Nucleotide synthesis is required for DNA replication and RNA synthesis in dividing cells Drugs that block de novo pathways of nucleotide synthesis are used as antitumour and antiviral agents

DEOXYRIBONUCLEIC ACID (DNA) Is the store of genetic information in all life forms except some viruses where RNA is the genetic material. Double stranded helix. The two strands are anti-parallel. The structure is stabilized by hydrogen bonds between base pairs (AT, GC). The stacking of the bases also help in stabilizing the helix. The double stranded chain structure ensures information retrieval in case of damage to a strand.

BASE PAIRING IN DNA

DNA STRUCTURE

RIBONUCLEIC ACID (RNA) A close chemical relative of DNA. The major difference is the absence of oxygen at the 2 I position of the sugar-phosphate backbone of DNA which is present in RNA. RNA is single stranded while DNA is double stranded. Present in several different forms in living cells

GENE EXPRESSION A gene is said to be expressed when the gene product is being produced. The flow of genetic information is depicted below: DNA RNA PROTEIN i ii iii

REPLICATION Replication is semi conservative. Starts at a unique origin and proceeds in opposite directions. Both strands serve as templates. DNA synthesis is primed by RNA. One strand is synthesized discontinuously. DNA polymerase takes instruction from the template. DNA polymerase corrects mistakes during replication.

TRANSCRIPTION Only one strand of DNA is transcribed in a particular region of the genome. Direction of synthesis is like that of DNA. Transcription is initiated at promoter sites on the DNA template. RNA polymerase takes instructions from a DNA template. Many RNA molecules undergo post-transcriptional modifications.

TRANSLATION The conversion of the information in a gene into a protein molecule. The process of reading the genetic code. Takes place in the ribosome. mRNA is the template. The tRNAs carry amino acids in the activated form to the ribosome for incorporation into a nascent chain. AUG is the start codon while UAA, UAG and UGA are stop codons.