Block 2 Lecture 2 -- The Urea Cycle Flashcards
How is uric acid produced?
metabolism of purine nucleic acids
How is creatinine produced?
from creatine phosphate
From where is PLP derived?
Vitamin B6 (pyridoxine)
Vit. B6 name
pyridoxine
For what reactions is PLP a cofactor?
1) TA
2) non-oxidative deamination (Gln-ase, Asn-ase)
3) decarboxylation
Describe the reactive site of PLP.
Aldehyde…forms Schiff base when attacked by NH3 (enzymatic Lys or AA)
What enzymes are involved in oxidative deamination?
1) Glutamate DH
2) L-AA oxidase
What cofactor is involved in Glu DH?
NAD+ (reduced to NADH)
What cofactor is involved in L-AA oxidase?
FMN (reduced to FMNH2)
Where does the reaction of Glu DH take place?
primarily in mitochondrion of liver for urea cycle
Describe Glu DH reaction
Glu + NAD(+) + H2O –> a-KG + H(+) + NH4(+)
How is Glu DH inhibited?
allosteric inhibition via
1) ATP
2) GTP
3) NADH
How is serine converted to pyruvate?
serine dehydratase
How is cysteine converted to pyruvate?
cysteine sulfhydrase
How is threonine converted to alpha-KB + NH4(+)?
threonine dehydratase
What are the types of deamination?
oxidative & non-oxidative
Describe the mechanism of a dehydratase/desulfhydrase.
1) enzyme bound as Schiff to PLP
2) NH3 attacks Schiff
3) AA bound to PLP
4) H bonding removes H2O
5) Enzyme (Lys) attacks Schiff, kicks out AA
Describe the mechanism of serine dehydratase.
Ser –> pyruvate + NH4(+)
How is NH4(+) produced extra-hepatically transferred to the liver?
Glu, Gln, Ala
what is the function of dehydratases/desulfhydrases?
gluconeogenesis! (generate pyruvate, alpha-KB becomes CoASH)
What are the 2 types of non-oxidative deamination?
1) dehydratase / desulfhydrase
2) hydrolytic deamination (Glutaminase, Asn-ase)
Describe the general mechanism of Glutaminase/Asparaginase?
1) Enzyme generates imine
2) H2O spontaneously attacks imine
3) Glu/Asp generated
What is the general function of Gln synthetase?
to detoxify extra-hepatic NH4(+)
Describe the reaction of Gln synthetase.
Glu + NH4(+) + ATP –> Gln
How are Gln synthetase / Gln-ase regulated in the liver?
They are compartmentalized to control flow of NH3 to end-products
What is the difference between CPS1 & CPS2?
CPS1: 1st step in urea cycle
CPS2: cytosolic enzyme involved in pyrimidine synthesis
What is the 1st enzyme in the urea cycle?
CPS1
How is CPS1 activated?
1) NAG—- NAG synthesis activated by Arg
2) Glu—- involved in NAG synthesis
3) NH4(+)—- reagent in CPS1
Describe the reaction of CPS1.
NH4(+) + HCO3(-) + 2 ATP —-> CarbP + 2 ADP + 2 Pi
Where is CPS1 located?
mitochondrion
Describe the synthesis of NAG.
Glu + Acetyl-CoA –> NAG
Where does the enzymatic activity of Ornithine Transcarbamoylase take place?
mitochondrion
Where is ATP used in the urea cycle?
1) CPS1 (2 ATP)
2) Argininosuccinate synthetase
Describe the presentation of CPSD (Carbamoyl-P Synthetase)?
hyperammonemia only
How does OTCD present?
hyperammonemia + orotate in the blood/urine
What is orotate?
An intermediate for CPS2
How does ASD present?
hyperammonemiaelevated
citrulline in blood & urine
How does ALD present?
hyperammonemiaelevated
argininosuccinate in blood/urine
How does Arginase-Deficiency present?
hyperammonemiaelevated
arginine in blood/urine
What treatments are used to remove excess ammonia?
1) levulose to acidify colon (NH3 –> NH4)
2) antibiotic enema (less NH3 production)
3) Sodium benzoate or Sodium phenylacetate
4) Replace missing intermediates w/ supplements
5) low-protein diet replaced with a-keto acids
Product & cofactor of benzoate administration?
product = hippurate (benzoylglycine) cofactor = A-CoA, ATP
Product & cofactor of phenylacetate administration?
product = phenylacetylglutamine cofactor = A-CoA, ATP
Where does L-amino acid oxidase mainly act?
mainly in liver & kidney
What cofactor is required to synthesize PLP?
NAD+
To what AA is PLP usually bound in an enzyme?
Lys
What substances are incorporated in to Carbamoyl-PO4?
NH4 + HCO3 + 2 ATP
How does NH4 accumulation become toxic?
1) GDH depletes a-KG
2) Gln synthetase depletes Glu
3) Glu depletion depletes GABA
4) TCA stops, ATP decreases
What are the clinical endpoints of ammonia accumulation?
brain damage, coma death
How doe kidneys & intestines work together to make Arg?
1) intestines release citrulline
2) kidney releases Arg
What surgical procedure could result in an Arg deficiency?
small intestine resection
Describe gluconeogenesis reaction for Ala.
2 Ala –> 1 Glucose + 1 urea
How is Ala produced from pyruvate in muscle?
1) makes Glu from a-KG via transamination with TA of another protein
2) TA excess pyruvate to Ala
