Nucleic Acids in Chemistry and Biology

or ammonia. The leaving group can be inorganic phosphate, pyrophosphate or AMP, while the displacing
nucleophile can be ammonia, the side chain of glutamine or the -amino group of aspartate (Figure 3.74).
Steps in the biosynthesis of purine nucleotides furnish good examples of a standard control mechanism
in metabolic pathways. This is feedback inhibition, where an enzyme catalysing an early step in the pathway

118 Chapter 3

NH 2

Ribose-5P

Ribose-5P

HN

CO

H 2 C

NH 3

OHCHN

H 2 N N

N

O

Ribose-5P

Ribose-5P

HN

CO

H 2 C

NHCHO

N

N

Ribose-5P

H 2 N

O

H 2 N

Ribose-5P

HN

CNH

H 2 C

NHCHO

N

N

Ribose-5P

H 2 N

CO 2

N

N

Ribose-5P

H 2 N

O

CH HN

H 2 C

COO

OOC

N

HN N

N

O

Ribose-5P

N

N

Ribose-5P

H 2 N

O

O

12 3 4

phosphoribosylamine

glycinamide

ribonucleotide

formylglycinamide

ribonucleotide

formylglycinamidine

ribonucleotide

5-aminoimidazole

ribonucleotide

5

8 7 6

9

inosine 5'-monophosphate (IMP)

5-aminoimidazole-

4-carboxylateribonucleotide

5-aminoimidazole-

4-N-succinocarboxamide

ribonucleotide

5-aminoimidazole

-4-carboxamide

ribonucleotide

5-formamidinoimidazole

-4-carboxamide

ribonucleotide

N^5 , N^10 -methylene

tetrahydrofolate

glutamine

N^10 - formyl

tetrahydrofolate

aspartate

glycine

Figure 3.72 Formation of the purine ring; biosynthesis of IMP

N

N N

N

NH

Ribose-5P

CH

CH 2 COO

OOC

HN

N

H

N

N

O

Ribose-5P

O

HN

N N

N

O

Ribose-5P

N

N N

N

NH 2

Ribose-5P

HN

N N

N

O

Ribose-5P

H 2 N

adenylosuccinate

xanthosine 5'-monophosphate

(XMP)

GMP

GTP

aspartate

oxidation

"NH 3 "

fumarate

IMP

AMP

ATP

Figure 3.73 Formation of AMP and GMP from IMP

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