1. As activated precursors of DNA & RNA.

2. As activated intermediates in biosynthetic pathways such as UDP-glucose, CDP-diacylglycerides, CDP-choline, S-adenosylmethionine.

3. ATP is the universal currency of energy.

4. GTP is used in protein synthesis

5. Adenine nucleotides are components of coenzymes viz., NAD(P)+, FAD & FMN, CoA

6. Nucleotides are also metabolic regulators e.g., cAMP, cGMP, ATP, G proteins (GTP), ATP/AMP ratio (energy charge).

7. Act as allosteric effectors in nucleic acid biosynthesis.

8. Act as Ribozymes - catalytic RNAs

Functions of Nucleotides

Structures of Nucleotide

Bases

Nucleotide concentrations in cells

Bacteria:mM Human:mM Bacteria:mM Human:mM

dNMP, total

43rNMP, total

270

dATP 0.18 0.013 ATP 3.0 2.8

dGTP 0.12 0.005 GTP 0.92 0.48

dCTP 0.07 0.022 CTP 0.52 0.21

dTTP 0.08 0.023 UTP 0.89 0.48

Three Sources of Nucleotides

1. De novo synthesis. 2. Salvage pathways by major recycling 3. Extracellular diet

John Buchanan at the University of Pennsylvania & MIT and Robert Greenberg at the University of Michigan fed wild pigeons (they excrete waste nitrogen almost entirely as uric acid) with radio labeled nutrients and identified the source of nitrogen and carbon atoms in purine nucleotides.

HC=O

HC=O

PRPP is also used in • pyrimidine biosynthesis.• synthesis of histidine and tryptophan.• NAD+ synthesis.• purine salvage pathway.

Its activity depends onthe state of aggregation of the enzyme.Pyrophosphate and 2,3-bisphosphoglycerate are activators.End products ADP, GDP give mixed inhibition.

5’-phosphoribosyl-1-pyrophosphate (PRPP)

Ribose phosphate pyrophosphokinase, PRPP

synthase

ATP is required for GTP synthesis.

GTP is required for ATP synthesis.

Ribose phosphate pyrophosphokinase is inhibited by ADP & GDP, IMP, AMP, GMP.

Amidophosphoribosyl transferase is inhibited by ATP,ADP, AMP at one site and GTP, GDP, GMP at another site. It is also allosterically stimulated by PRPP.

Genes for the 12 enzymes involved in purine biosynthesis in E. coli are Physically unlinked (not in an operon).

However, they are transcriptionally coregulated by a single repressor.

ATP and GTP may act as corepressors for reactions that precede the IMP branch point.

The purine biosynthetic pathway is conserved in E. coli, yeast, pigeon and human

One enzyme, ribonucleotide reductase,reduces all four ribonucleotides to their deoxyribo derivitives.

A free radical mechanism is involved in the ribonucleotide reductase reaction.

There are three classes of ribonucleotide reductase enzymes in nature:

Class I: tyrosine radical, uses NDP.

Class II: adenosylcobalamin. uses NTPs (cyanobacteria, some bacteria, Euglena).

Class III: SAM and Fe-S to generate radical, uses NTPs. (anaerobes and facultative anaerobes).

R1 SUBUNIT–Specificity Site

–Hexamerization site

–Activity Site

–Five redox-active –SH groups

R2 SUBUNIT

–Tyr 122 radical

–Binuclear Fe(III) complex

CATALYTIC ACTIVITY ALSO VARIES WITH STATE OF OLIGOMERIZATION:

WHEN ATP, dATP, dGTP, dTTP BIND TO SPECIFICITY SITE OF R1 (CATALYTICALLY INACTIVE MONOMER) CATALYTICALLY ACTIVE (R1)2 IS FORMED.

WHEN dATP OR ATP BIND TO ACTIVITY SITE OF DIMERS TETRAMER FORMATION OCCURS

(R1)4a (ACTIVE STATE) == (R1)4b (INACTIVE)

WHEN ATP BINDS TO HEXAMERIZATION SITE, CATALYTICALLY ACTIVE HEXAMERS (R1)6

Scheme 1. Regulation of dNTP synthesis. (1) and (2) represents dCTP deaminase and dCMP deaminase,

respectively. (3) is the nucleoside diphosphate kinase. (4) is the well known thyA encoded -or newly discovered

thyX encoded, FAD dependent- thymidylate synthase. The dotted lines shows the regulation, negative or

positive, of the involved enzymes by nucleotides.

Nucleotides as drugs

1. Anticancer: Methotrexate – a folic acid analog inhibits formation of deoxythymidylate.

2. Antiviral: Azidothymine, AZT, an inhibitor of reverse transcriptase is used to treat AIDS

Arvind Kumar

Assistant professor department

of zoology S. U. College, Hilsa