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Drew 922 > Block 2 Lecture 2 -- The Urea Cycle > Flashcards

Block 2 Lecture 2 -- The Urea Cycle Flashcards

1
Q

How is uric acid produced?

A

metabolism of purine nucleic acids

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2
Q

How is creatinine produced?

A

from creatine phosphate

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3
Q

From where is PLP derived?

A

Vitamin B6 (pyridoxine)

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4
Q

Vit. B6 name

A

pyridoxine

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5
Q

For what reactions is PLP a cofactor?

A

1) TA
2) non-oxidative deamination (Gln-ase, Asn-ase)
3) decarboxylation

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6
Q

Describe the reactive site of PLP.

A

Aldehyde…forms Schiff base when attacked by NH3 (enzymatic Lys or AA)

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7
Q

What enzymes are involved in oxidative deamination?

A

1) Glutamate DH

2) L-AA oxidase

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8
Q

What cofactor is involved in Glu DH?

A

NAD+ (reduced to NADH)

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9
Q

What cofactor is involved in L-AA oxidase?

A

FMN (reduced to FMNH2)

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10
Q

Where does the reaction of Glu DH take place?

A

primarily in mitochondrion of liver for urea cycle

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11
Q

Describe Glu DH reaction

A

Glu + NAD(+) + H2O –> a-KG + H(+) + NH4(+)

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12
Q

How is Glu DH inhibited?

A

allosteric inhibition via

1) ATP
2) GTP
3) NADH

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13
Q

How is serine converted to pyruvate?

A

serine dehydratase

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14
Q

How is cysteine converted to pyruvate?

A

cysteine sulfhydrase

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15
Q

How is threonine converted to alpha-KB + NH4(+)?

A

threonine dehydratase

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16
Q

What are the types of deamination?

A

oxidative & non-oxidative

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17
Q

Describe the mechanism of a dehydratase/desulfhydrase.

A

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

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18
Q

Describe the mechanism of serine dehydratase.

A

Ser –> pyruvate + NH4(+)

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19
Q

How is NH4(+) produced extra-hepatically transferred to the liver?

A

Glu, Gln, Ala

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20
Q

what is the function of dehydratases/desulfhydrases?

A

gluconeogenesis! (generate pyruvate, alpha-KB becomes CoASH)

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21
Q

What are the 2 types of non-oxidative deamination?

A

1) dehydratase / desulfhydrase

2) hydrolytic deamination (Glutaminase, Asn-ase)

22
Q

Describe the general mechanism of Glutaminase/Asparaginase?

A

1) Enzyme generates imine
2) H2O spontaneously attacks imine
3) Glu/Asp generated

23
Q

What is the general function of Gln synthetase?

A

to detoxify extra-hepatic NH4(+)

24
Q

Describe the reaction of Gln synthetase.

A

Glu + NH4(+) + ATP –> Gln

25
Q

How are Gln synthetase / Gln-ase regulated in the liver?

A

They are compartmentalized to control flow of NH3 to end-products

26
Q

What is the difference between CPS1 & CPS2?

A

CPS1: 1st step in urea cycle
CPS2: cytosolic enzyme involved in pyrimidine synthesis

27
Q

What is the 1st enzyme in the urea cycle?

A

CPS1

28
Q

How is CPS1 activated?

A

1) NAG—- NAG synthesis activated by Arg
2) Glu—- involved in NAG synthesis
3) NH4(+)—- reagent in CPS1

29
Q

Describe the reaction of CPS1.

A

NH4(+) + HCO3(-) + 2 ATP —-> CarbP + 2 ADP + 2 Pi

30
Q

Where is CPS1 located?

A

mitochondrion

31
Q

Describe the synthesis of NAG.

A

Glu + Acetyl-CoA –> NAG

32
Q

Where does the enzymatic activity of Ornithine Transcarbamoylase take place?

A

mitochondrion

33
Q

Where is ATP used in the urea cycle?

A

1) CPS1 (2 ATP)

2) Argininosuccinate synthetase

34
Q

Describe the presentation of CPSD (Carbamoyl-P Synthetase)?

A

hyperammonemia only

35
Q

How does OTCD present?

A

hyperammonemia + orotate in the blood/urine

36
Q

What is orotate?

A

An intermediate for CPS2

37
Q

How does ASD present?

A

hyperammonemiaelevated

citrulline in blood & urine

38
Q

How does ALD present?

A

hyperammonemiaelevated

argininosuccinate in blood/urine

39
Q

How does Arginase-Deficiency present?

A

hyperammonemiaelevated

arginine in blood/urine

40
Q

What treatments are used to remove excess ammonia?

A

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

41
Q

Product & cofactor of benzoate administration?

A 
product = hippurate (benzoylglycine)
cofactor = A-CoA, ATP
42
Q

Product & cofactor of phenylacetate administration?

A 
product = phenylacetylglutamine
cofactor = A-CoA, ATP
43
Q

Where does L-amino acid oxidase mainly act?

A

mainly in liver & kidney

44
Q

What cofactor is required to synthesize PLP?

A

NAD+

45
Q

To what AA is PLP usually bound in an enzyme?

A

Lys

46
Q

What substances are incorporated in to Carbamoyl-PO4?

A

NH4 + HCO3 + 2 ATP

47
Q

How does NH4 accumulation become toxic?

A

1) GDH depletes a-KG
2) Gln synthetase depletes Glu
3) Glu depletion depletes GABA
4) TCA stops, ATP decreases

48
Q

What are the clinical endpoints of ammonia accumulation?

A

brain damage, coma death

49
Q

How doe kidneys & intestines work together to make Arg?

A

1) intestines release citrulline

2) kidney releases Arg

50
Q

What surgical procedure could result in an Arg deficiency?

A

small intestine resection

51
Q

Describe gluconeogenesis reaction for Ala.

A

2 Ala –> 1 Glucose + 1 urea

52
Q

How is Ala produced from pyruvate in muscle?

A

1) makes Glu from a-KG via transamination with TA of another protein
2) TA excess pyruvate to Ala

Drew 922 (12 decks)

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