7C-Purine/Pyrimidine Synthesis

Question 1

What are the 4 uses of nucleotides?

Answer 1

• Nucleotides are the activated precursors of DNA and RNA
• Nucleotides form structural moieties for coenzymes such as NADH FAD and coenzyme A
• Critical elements in energy metabolism Nucleotide derivatives such as the nucleoside triphosphates play central roles in metabolism in which capacity they serve as sources of chemical energy such as ATP and GTP
• Nucleotides also act as second messengers in intracellular signaling such as in cAMP and cGMP

Question 2

How is PRPP made?

Answer 2

PRPP is formed from adenosine triphosphate (ATP) and ribose5-phosphate. The enzyme used is PRPP synthetase.

Question 3

What is the function of PRPP?

Answer 3

• PRPP is the activated substrate in the synthesis of purine and pyrimidine synthesis.
• Purines are built on a ribose base and PRPP is the activated source of ribose PRPP is used to react with glutamine to form 5-phosphoribosyl 1-amine this is the first committed step in making purines
• PRPP synthase is a primary site of regulation of synthesis

Question 4

What is the mechanism of PRPP?

Answer 4

PRPP transfers its ribose 5-phosphate to orotate to produce orotidine 5-phosphate
PRPP is also an activator of carbamoyl phosphate synthetase (CPS-II) which synthesizes the regulated step of pyrimidine synthesis

Question 5

Where are purines and pyrimidines synthesized?

Answer 5

The liver

Question 6

What does PRPP react with to form in the 1st step of purine synthesis?

Answer 6

PRPP reacts with glutamine to form 5-phosphoribosyl 1-amine. (reaction is catalyzed by glutamine phosphoribosyl amidotransferase- this is a highly regulated enzyme)

Question 7

What is added to the molecule in the 2nd step of purine synthesis?

Answer 7

glycine molecule is added to the growing precursor. Glycine provides carbons 4 and 5 and nitrogen 7 of the purine ring. The step required ATP

Question 8

What 5 things are then added to the growing purine during its synthesis to form IMP?

Answer 8

Then, C8 is donated by by n10-formyl FH4.
N3 is donated by glutamine.
C6 is donated by CO2.
N1 is donated by aspartate.
Finally, C2 is donated by formyl tetrahydrofolate

Question 9

What is IMP?

Answer 9

This results in the formation of Inosine monophosphate (IMP), which contains the base hypoxanthine
IMP the first purine nucleotide requires 6 ATP to be synthesized
IMP severs as a branch point from which both adenine and guanine nucleotides can be produced

Question 10

How does IMP form AMP?

Answer 10

In the formation of AMP, IMP and asparate combine to form adenylosuccinate. The reaction catalyzed by the enzyme adenylosuccinate synthetase. This reaction requires energy in the form of GTP. Then fumarate is relased by the enzyme of adenylosuccinase to form AMP.

Question 11

How does IMP form GMP?

Answer 11

The hypoxanthine base is oxidized by IMP dehydrogenase to produce xanithe and xanthine monophosphate (XMP). Glutamine then donates its amide nitrogen to XMP to form GMP catalyzed by GMP synthetase. Second reaction requires an ATP.

Question 12

What are the 4 sites of purine synthesis regulation?

Answer 12

PRPP synthetase
Glutamine phosphoribosyl amidotranserase
adenylosuccinate synthetase
IMP dehydrogenase

Question 13

What is PRPP synthase inhibited by?

Answer 13

PRPP synthase inhibited by GDP and ADP both have separate allosteric binding site and can both bind simultaneous causing synergistic inhibition

Question 14

What inhibits phosphoribosyl aminotransferase?

Answer 14

synthesis of 5-phosophorribosyl 1-amine by glutamine phosphoribosyl amidotransferase. Enzyme strongly inhibited by GMP and AMP the end products of purine biosynthesis

Question 15

What inhibits the enzyme adenylosuccinate synthetase?

Answer 15

AMP

Question 16

What is IMP dehydrogenase inhibited by?

Answer 16

GMP

Question 17

What are the 2 precursors to pyrimidine sythesis?

Answer 17

CO2 and glutamine

Question 18

What is the first step of pyrimidine synthesis,starting from glutamine?

Answer 18

glutamine combines with bicarbonate and ATP to form carbamoyl phosphate- reaction is catalyzed by carbamoyl phosphate synthase II. (this is the regulated step of the pathway)

Question 19

What happens to the carbamoyl phosphate in the 2nd step of pyrimidine synthesis?

Answer 19

the entire aspartate molecule adds to the carbamoyl phosphate via aspartate transcarbamoylase. This goes to close the ring and is oxidized to orotic acid.

Question 20

How is UMP formed from orotic acid?

Answer 20

Orotate phosphoribosyl transferase catalyzes the transfer of ribose 5-phosphate from PRPP to orotate producing orotidine 5-phosphate. Orotdine is decarboxylated by ortidine 5-P decarboxylase and goes to form UMP.

Question 21

How is UTP and CTP formed from UMP?

Answer 21

UMP is phosphorylated to UTP, which obtains an amino group from glutamine to form cytidine triphosphate (CTP). UTP and CTP are used for RNA synthesis.

Question 22

How is pyrimidine synthesis regulated?

Answer 22

Regulation: regulated step is with the enzyme carbamoyl phosphate synthetase II. This is activated by PRPP and inhibited by UTP. SO- as pyrimidines decrease in concentration (UTP levels decrease) CPS II is activated and pyrimidines are made. As cells approach S phase, they become more sensitive to regulation via PRPP activation and less sensitive to UDP inhibition.
This enzyme is also regulated by phosphorylation at allosteric binding sites. When phosphorylated by MAP Kinase , the enzyme is active, when the enzyme has been phosphorylated again by cAMP dependent protein kinase the enzyme is inhibited

Question 23

What would be the Sx of nucleotide phosphorylase deficiency?

Answer 23

It leads to an immune disorder in which T-cell immunity is compromised.
B-cells are a little compromised or they can be fine.

Symptoms: children have recurrent infections, and more than ½ have neurological complications. Sx appear between 6mo-4y/o

Question 24

What is Lesch-Nyhan syndrome? Sx?

Answer 24

Deficiency in HGPRT
In this condition, purine bases cannot be salvaged. Instead they are degraded and form excessive amounts of uric acid.

Symptoms: mental retardation, self-mutilation (including chewing off your fingers).

Question 25

What is editary orotic aciduira? Tx?

Answer 25

Orotate phosphoribosyl transferase and orotidine 5’-phosphate decarboxylase defeciencies
Orotic acid is excreted in the urine because the enzymes cannot convert it to uridine monophosphate.
Therefore pyrimidine cannot be synthesized and normal growth cannot occur.
Treatment- oral uridine. Uridine is converted back to UMP, which bypasses the metabolic block and provides the body with a scource of pyrimidines, as both CTP and dTMP can be produced from UMP.

Question 26

What occurs in the Ornithine transcarbamoylase deficiency?

Answer 26

This is a urea cycle disorder where excess carbamoyl phosphate from the mitochrondria leaks into the cytoplasm
The elevated levels of cytoplasmic carbamoyl phosphate lead to the pyrimidine production.
Orotic aciduria results.

Question 27

What is the cause and Tx of gout?

Answer 27

It is from the excess of uric acid from either overproduction of purine nucleotides or because of a decreased ability to excrete uric acid.
Normally, as cells die, their purine nucleotides are degraded to hypoxanthine and xanthine uric acid by xanthine oxidase.
Treatment- Allopurinol. This binds and inhibits xanthine oxidase by acting as a suicide inhibitor. It reduces the level of uric acid in the tissues and blood.

Question 28

What is the cause, Sx, and Tx of adenosine deaminase deficiency?

Answer 28

Condition is Severe Combined Immunodeficiency Diseased (SCID).
Both T-cells and B-cells are deficieny, leaving the individual without a functional immune system.
Symptoms: children who have SCID don’t have a thymus, death results from kids not being in a sterible environment.
Treatment: administration of polyethylene glycol-modified adenosine deaminase.