Conversion of IMP to AMP IMP is converted to AMP in two enzymatic steps Step-1: Donation of amino group by aspartate: Amino group of aspartate is enzymatically linked to the IMP (C6 of purine) coupled with GTP hydrolysis to form adenylosuccinate with the help of enzyme- adenylosuccinate synthetase. Step-2: Eliminates fumarate group to form AMP: Adenylosuccinate is enzymatically converted to AMP by the removal of fumarate group with the help of enzyme adenylosuccinate lyase.

Conversion of IMP to AMP and GMP Note: GTP is used for AMP synthesis. IMP is the precursor for both AMP and GMP.

Conversion of IMP to GMP IMP is converted to GMP in two enzymatic steps Step-1:Dehydrogenation of IMP: IMP is enzymatically dehydrogenated to form Xanthosine Monophosphate (XMP) with the enzyme IMP dehydrogenase. The H+ ions released are accepted by NAD+. Step-2: Amidation of XMP: In the second step, XMP is amidated with the amide group from glutamine with the presence of H2O and hydrolysis of ATP yields GMP (Guanosine monophosphate); catalyzed by the enzyme GMP synthetase.

ADP, ATP, GDP and GTP biosynthesis kinase kinase AMP ADP ATP ATP ADP ATP ADP kinase kinase GMP GDP GTP ATP ADP ATP ADP

Synthesis Of Purine Nucleotides By Salvage Pathway Dr. Shumaila Asim Lecture # 2

Salvage Pathways Since De novo synthesis is expensive and A few cells (e.g. RBCs) lack some enzymes involved in de novo synthesis (e.g. PRPP-amido-transferase) Hence cells use salvage pathway (requiring far less energy and enzymes than de novo synthesis)

Salvage Pathway for Purine Nucleotides Two pathways are available a. One step pathway (phospho-ribosylation of purines) (direct phospho-ribosylation of adenine, guanine and hypoxanthine)

Guanine Phosphoribosyl Tranferase

Salvage Pathway for Purine Nucleotides b. A minor pathway is also available (phosphorylation of nucleoside) ATP ADP Nucleoside kinase Purine – Ribose A or G Purine–Ribose–Phosphate AMP or GMP

Synthesis of Deoxyribonucleotides: Deoxyribonucleotides are synthesized from their corresponding ribonucleotides by the reduction of ribose sugar at position C2’. The Enzymes in the formation of deoxyribonucleotides by the reduction of the corresponding ribonucleotides are called ribonucleotide reductases (RNRs). The enzyme is ribonucleotide reductase complex.Active only when cells are actively synthesizing DNA. It requires thioredoxin, thioredoxin reductase and NADPH.

Synthesis of Deoxy-ribonucleotide

Regulation of ribonucleotide reductase

Regulatory Control of Purine Biosynthesis Above the level of IMP production: Independent control Synergistic control Feedforward activation by PRPP Below level of IMP production Reciprocal control Total amounts of purine nucleotides controlled Relative amounts of ATP, GTP controlled

Purine nucleotide biosynthesis is regulated by feedback inhibition

Summary of purine biosynthesis IMP

Catabolism of Purine Nucleotides

Degradation of purines Uric acid is the end product in humans Uric acid = alloxan + urea Excreted mainly through kidneys (600-800 mg/day) Some mammals contain uricase, which converts uric acid to a highly water-soluble product, allantoin 

Assignment first week (Practical Copy) Tabulate the biochemical characteristics of adenosine, guanosine, cytidine and uridine derivatives------5 marks Draw and label the sources of atoms of Purine and Pyrimidine nucleus-----5 marks Flow chart showing conversion of IMP to AMP and GMP with enzymes and co-enzymes-----5 marks A flow sheet describing Purine degradation with enzymes and co-enzymes-----5 marks