Amino Acid Synthesis Part II Flashcards
Pyrimidines vs purines
5’ and 3’ end of DNA/RNA
Pyrimidines: Cytosine, thymine and uracil
One carbon-nitrogen ring
Purines: Adenine and guanine
Two carbon-nitrogen rings
5’ end off PO4
3’ end off ribose 3’ carbon
Purine and pyrimidine synthesis overview
- de novo and salvage synthesis pathways
- purines are built upon ribose phosphate
- pyrimidines are formed from synthesis of orotate attached to ribose phosphate
- reserves of nucleotides kept low, only synthesized when needed
5-phosphoribosyl 1-pyrphosphate importance
Synthesized from ribose-5-phosphate
this molecule is what the built up nitrogenous base is then attached to
Used in all nucleotide synthesis pathways
Amino acid precursors for different nucleotide synthesis pathways
AA act as sources of nitrogen groups for nitrogenous bases
De novo pathway for purines: glutamine, glycine and aspartate
De novo pathway for pyrimidines: aspartate, PRPP and carbamoyl phosphate and glycine (NH2 donation)
De novo synthesis of purines pathway (part I)
1) Ribose-5-phosphate –> 5-phosphoribosyl-1-pyrophosphate (PRPP)
Enzyme: PRPP synthetase
2) PRPP + glutamine –> 5-phosphoribosylamine
Enzyme: glutamine PRPP amidotransferase
3) Addition of glycine giving 3 carbons to form 1st ring of purine
4) Addition of glutamine
5) Addition of aspartate (in higher eukaryotes, a step is skipped using AIR carboxylase)
6) Formation of inosinate (IMP) intermediate
De novo synthesis of guanylate purine (part II)
6a) IMP –> XMP + NADH
Enzyme: IMP dehydrogenase
Cofactors: H2O, NAD+
7a) XMP + Glutamine + ATP –> guanylate (GMP)
Enzyme: XMP-glutamine amidotransferase (ammonia transferase)
Cofactors: glutamine (NH2 donor to carbon 2) and ATP
De novo synthesis of adenylate purine (part II)
6b) IMP + GTP + aspartate –> adenylosuccinate
Enzyme: adenylosuccinate synthetase
7b) adenylosuccinate –> adenylate (AMP) + fumarate
Enzyme: adenylosuccinate lyase
Inhibitors of overall de novo purine synthesis
ADP: inhibits PRPP synthetase
AMP, GMP and IMP: inhibits glutamine PRPP amidotransferase
*Goal is to keep concentrations of AMP/GMP low
Inhibitors of specific de novo purine synthesis
GMP: inhibits IMP dehydrogenase
AMP: inhibits adenylosuccinate synthetase
*Goal is to keep concentrations of AMP/GMP low
De novo synthesis of pyrimidines
1) Aspartate + carbamoyl phosphate (by cytoplasmic carbamoyl phosphate synthetase II)
2) After a few steps Orotate is formed
3) Orotate + PRPP –> Orotidylate
4) Orotidylate –> Uridylate (UMP)
5) UMP + ATP –> UTP
6) UTP + Glutamine + ATP –> CTP
inhibition of de novo pyrimidine synthesis
CTP can inhibit first step of synthesis
Enzyme: aspartate trans carbamoylase
2 pathways for conversion of ribonucleotides to deoxyribonucleotides
H atoms donated from NADPH and added to C2 on ribose
Pathway 1: Reduction of oxidized GSSH –> 2GSH, sulfonation of ribonucleotide reductase
Enzyme: glutaredoxin reductase
Cofactor: H2O for final step
Pathway 2: reduction of FAD –> FADH2, sulfonation of ribonucleotide reductase
Enzyme: Thioredoxin reductase
Cofactor: H2O for final step
Synthesis of dTMP from CDP or UDP
Pathway:
1) CDP/UDP –> dCDP/dUDP
Enzyme: ribonucleotide reductase
2) dCDP/dUDP –> dCTP/dUTP
Enzyme: nucleotide diphosphate kinase
3a) dCTP –> dUTP
Enzyme: deaminase
3b) dUTP –> dUMP
Enzyme: dUTPase
4) dUMP –> dTMP (deoxythymidylate)
Enzyme: thymidylate synthase
*addition of CH3 to carbon 5 of uracil
What provides ring structure in pyrimidines?
Carbamoyl phosphate and aspartate
Then PRPP is added to orotate –> orotidylate
What provides ring structure in purines?
Glutamine aminates PRPP and then later glycine (3 carbons) begins ring formation
Additional glutamine and aspartate add NH3 groups
