Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 432	Biochemistry	Spring 2002
Lecture Notes:: 11 February	© R. Paselk 2002
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Nucleotide Metabolism 1

Nucleoside = nitrogenous base + ribose

Nucleotide = nitrogenous base + ribose + phosphate

We have already seen how important the nucleotides etc. are to life as the monomer residues in nucleic acids (DNA & RNA), energy transfer cofactors, components of essential cofactors (e.g. NADH, CoA), and secondary messengers (e.g. cAMP) etc.

Today we want to look at their anabolism and catabolism.

Recall the basic structures:

Purine are synthesized via IMP:

Pyrimidines are synthesized via UMP:

the other nucleotides are then obtained by modification of IMP and UMP.

The two initial nucleotides are biosynthesized by two quite differnt strategies:

• Purine begin with ribose-5-P and build the nitrogenous ring stepwise onto the sugar. The first step in this pathway is thus the synthesis of an activated form of ribose, PRPP (phosphoribosylpyrophosphate) using ATP Æ AMP. The diagram below shows the origins of the various atoms in the nitrogenous base:

• Pyrimidine synthesis, in contrast, begins with the formation of the ring vias the condensation of aspartate and carbamyl-phosphate, then reduced to give orotate (shown below), followed by the addition of R-5-P (again from PRPP).

The strategies also differ for subsequent modification. Thus for the purines the monophosphte is modified and then phosphorylated:

while for the pyrimidines the monophophate is first phosphorylated, and then modified:

dTMP is derived from dUMP which in turn is modified from dUTP.

Purine Biosynthesis

Purines are synthesized in a multi-step process outlined in the handout in your Chem 431 packet and in Figure 22-1, p 696 of your text.

• IMP is then modified to give AMP by exchanging a carbonyl group for an amine (note that the base is reduced, but aspartate is oxidized) in a two step process catalyzed by adenylosuccinate synthetase and adenylosuccinate lyase:

• or IMP is modified to GMP by oxidation and amination in a two step process catalyzed by IMP Dehydrogenase and GMP Sythetase:

Regulation of Purine Biosynthesis

Purine biosynthesis is highly regulated by both feedback inhibition and feed-forward activation. In mammals regulation is via PRPP and the mononucleotides, as seen below [redrawn from Devlin (1992) Textbook of Biochemistry with Clinical Correlations, Wiley-Liss, p 541]:

The first commited step, PRPP amidotransferase is synergistically inhibited by IMP & GMP binding to one allosteric site, and AMP binding to another. The two enzymes at the IMP branchpoint (adenylosuccinate synthetase and IMP dehydrogenase) are also regulated allosterically. Both enzyme have about the same K_M value for IMP. A second level of control is accomplished by the specific use of energy sources: as seen in the reactions above, ATP is required to synthesize GMP from XMP, while GTP is required to synthesize AMP.

The regulation of purine biosynthesis in E. coli is the classic example of feedback regulation in metabolism. Though similar to the mamalian case, all of the guanine and adenine nucleotides are involved in regulation. A second level of control is again accomplished by the specific use of energy sources: ATP is required to synthesize GMP from XMP, while GTP is required to synthesize AMP. Again, a careful balancing of nucleotide synthesis should result from this system, as diagrammed below:

As noted in your text, this bacterial system also uses the NDP's to regulate the biosynthesis of PRPP from R-5-P by feedback inhibition.

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Pathway Diagrams

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Last modified 11 February 2002