Week 8: Synthesis and Breakdown of Nucleotides

Are dietary nucleotides necessary for survival?

No

Which enzymes are involved in the hydrolysis of ingested nucleic acids (in functional order)?

Endonucleases, phosphodiesterases, nucleoside phosphorylases

What is produced by the action of endonucleases (nucleic acids to ___)?

Oligonucleotides

What is produced by the action of phosphodiesterases (oligonucleotides to ___)?

Free nucleosides

What is produced by the action of nucleoside phosphorylases (free nucleosides to ___)?

Ribose-1-phosphate and free bases

What is the ultimate product of purine degradation?

Uric acid

What is the ultimate product of pyrimidine degradation?

Beta-alanine, beta-aminoisobutyrate, NH3, CO2

What are the three processes of nucleotide biosynthesis?

De novo synthesis, phosphoribosylation of purines, phosphorylation of purine nucleosides

What is the first process in de novo nucleic acid synthesis?

Formation of PRPP

What is PRPP synthesized from?

Ribose-5-phosphate, using ATP

What is the rate-limiting step in the formation of IMP?

PRPP and glutamine form 5-phosphoribosylamine

What is the rate-limiting enzyme in the formation of IMP?

PRPP amidotransferase

What is the parent molecule for AMP and GMP production?

Inosine monophosphate

What controls the direction of the AMP/GMP synthesis pathway?

Amount of each nucleotide

Which enzymes are required for AMP synthesis?

Adenylosuccinate synthetase and lyase

Which enzymes are required for GMP synthesis?

IMP dehydrogenase I, GMP synthetase

What is the general process by which ATP and GTP are formed from AMP and GMP?

Phosphorylation

What is the general mechanism for salvage pathways?

Nucleotides are synthesized from breakdown intermediates

Which enzyme produces AMP via a salvage pathway?

Adenine phosphoribosyltransferase (APRT)

Which enzyme produces IMP and GMP via salvage pathways involving hypoxanthine and guanine?

Hypoxanthine-guanine phosphoribosyl transferase (HGPRT)

Which enzyme is important for purine salvage in rapidly dividing cells?

Adenosine deaminase (ADA)

What does ADA catalyze?

Deamination of adenosine to inosine

What does ADA deficiency result in?

Severe combined immunodeficiency (SCID)

Which rate-limiting enzymes catalyze the first two steps in purine biosynthesis?

PRPP synthetase and PRPP amidotransferase

What inhibits PRPP synthetase?

AMP and GMP

What inhibits PRPP amidotransferase?

ATP, ADP, AMP at one site, GTP, GDP, GMP at another

What stimulates PRPP amidotransferase?

PRPP

List the compounds in the breakdown of GMP to uric acid

GMP, guanosine, guanine, xanthine, uric acid

List the compounds in the breakdown of AMP to uric acid

AMP, IMP, inosine, hypoxanthine, xanthine, uric acid

What characterizes gout?

Hyperuricemia

What are common causes of hyperuricemia?

Increased production, decreased excretion, cancer

Defect in what enzyme is typically associated with hyperuricemia?

PRPP synthetase

What enzyme is deficient in Lesch-Nyhan syndrome?

HGPRT

Deficiency in HGPRT causes what condition?

Lesch-Nyhan syndrome

What symptoms are characteristic of Lesch-Nyhan syndrome?

Gout, behavioral disorders, learning disorders, aggressiveness, hostility, self-harm

Deficiency of glucose-6-phosphatase causes what condition?

Von Gierke’s disease

Von Gierke’s disease is caused by a deficiency in which enzyme?

Glucose-6-phosphatase

Which processes are impaired due to Von Gierke’s disease?

Glycogenolysis and gluconeogenesis

What biochemical findings are associated with Von Gierke’s disease?

Hypoglycemia, hyperuricemia, hyperlipidemia, ketosis, lactic acidosis

Which compound is produced as glucose-6-phosphate accumulates (and through which process), ultimately leading to hyperuricemia?

Ribose-5-phosphate (through PPP)

Which molecules contribute to de novo purine synthesis?

Aspartate, glutamine, glycine, THF, CO2

What does aspartate contribute to a purine?

N1

What does THF contribute to a purine?

C2, C8

What does glutamine contribute to a purine?

N3, N9

What does glycine contribute to a purine?

C4, C5, N7

What does CO2 contribute to a purine?

C6

What is a synthesized pyrimidine base derived from?

Carbamoyl-phosphate and aspartate

What is required to convert UTP to CTP?

Glutamine and ATP

What does carbamoyl-phosphate contribute to a pyrimidine base?

C2, N3

What does aspartate contribute to a pyrimidine base?

C4, C5, C6, N1

What forms the carbamoyl-phosphate needed for pyrimidine biosynthesis?

Glutamine and CO2, using ATP

Is the pyrimidine ring structure built on PRPP?

No, assembled as a free base

When does PRPP enter the pyrimidine biosynthesis pathway?

Added to the first formed base (orotic acid) to form OMP

What is the fate of OMP?

Decarboxylated to UMP

List the sequence of molecules in the initial reactions of pyrimidine synthesis

Carbamoyl-phosphate, carbamoyl aspartate, dihydroorotic acid, orotic acid, orotidine-5-monophosphate, uridine monophosphate

What is UTP a precursor for?

Cytosine triphosphate (CTP)

What is required for the conversion of UTP to CTP? (converted to ___)

Glutamine (converted to glutamate)

What are the activities of the multifunctional eukaryotic enzyme for pyrimidine synthesis?

Carbamoyl-phosphate synthetase, aspartate transcarbamoylase, dihydroorotase

What are the two main control mechanisms for the multifunctional pyrimidine synthesis enzyme?

Synthesis of the enzyme itself and feedback inhibition by pyrimidine nucleotides

What does degradation of cytosine and uracil produce?

Beta-alanine

What does degradation of thymine produce?

Beta-amino isobutyrate

What are additional byproducts of pyrimidine catabolism?

NH3 and CO2

Deficiency of which enzymes causes type 1 orotic aciduria?

Orotate phosphoribosyl transferase and ortidylate decarboxylase

Deficiency of which enzyme causes type 2 orotic aciduria?

Ortidylate decarboxylase

What conditions are associated with type 1 orotic aciduria?

Retarded growth, megaloblastic anemia, macrocytosis

What condition is associated with type 2 orotic aciduria?

Megaloblastic anemia

What is typical treatment for orotic aciduria?

Oral uridine