Block 2 Lecture 2 -- The Urea Cycle

Question 1

How is uric acid produced?

Answer 1

metabolism of purine nucleic acids

Question 2

How is creatinine produced?

Answer 2

from creatine phosphate

Question 3

From where is PLP derived?

Answer 3

Vitamin B6 (pyridoxine)

Question 4

Vit. B6 name

Answer 4

pyridoxine

Question 5

For what reactions is PLP a cofactor?

Answer 5

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

Question 6

Describe the reactive site of PLP.

Answer 6

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

Question 7

What enzymes are involved in oxidative deamination?

Answer 7

1) Glutamate DH

2) L-AA oxidase

Question 8

What cofactor is involved in Glu DH?

Answer 8

NAD+ (reduced to NADH)

Question 9

What cofactor is involved in L-AA oxidase?

Answer 9

FMN (reduced to FMNH2)

Question 10

Where does the reaction of Glu DH take place?

Answer 10

primarily in mitochondrion of liver for urea cycle

Question 11

Describe Glu DH reaction

Answer 11

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

Question 12

How is Glu DH inhibited?

Answer 12

allosteric inhibition via

1) ATP
2) GTP
3) NADH

Question 13

How is serine converted to pyruvate?

Answer 13

serine dehydratase

Question 14

How is cysteine converted to pyruvate?

Answer 14

cysteine sulfhydrase

Question 15

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

Answer 15

threonine dehydratase

Question 16

What are the types of deamination?

Answer 16

oxidative & non-oxidative

Question 17

Describe the mechanism of a dehydratase/desulfhydrase.

Answer 17

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

Question 18

Describe the mechanism of serine dehydratase.

Answer 18

Ser –> pyruvate + NH4(+)

Question 19

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

Answer 19

Glu, Gln, Ala

Question 20

what is the function of dehydratases/desulfhydrases?

Answer 20

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

Question 21

What are the 2 types of non-oxidative deamination?

Answer 21

1) dehydratase / desulfhydrase

2) hydrolytic deamination (Glutaminase, Asn-ase)

Question 22

Describe the general mechanism of Glutaminase/Asparaginase?

Answer 22

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

Question 23

What is the general function of Gln synthetase?

Answer 23

to detoxify extra-hepatic NH4(+)

Question 24

Describe the reaction of Gln synthetase.

Answer 24

Glu + NH4(+) + ATP –> Gln

Question 25

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

Answer 25

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

Question 26

What is the difference between CPS1 & CPS2?

Answer 26

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

Question 27

What is the 1st enzyme in the urea cycle?

Answer 27

CPS1

Question 28

How is CPS1 activated?

Answer 28

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

Question 29

Describe the reaction of CPS1.

Answer 29

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

Question 30

Where is CPS1 located?

Answer 30

mitochondrion

Question 31

Describe the synthesis of NAG.

Answer 31

Glu + Acetyl-CoA --> NAG

Question 32

Where does the enzymatic activity of Ornithine Transcarbamoylase take place?

Answer 32

mitochondrion

Question 33

Where is ATP used in the urea cycle?

Answer 33

1) CPS1 (2 ATP) | 2) Argininosuccinate synthetase

Question 34

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

Answer 34

hyperammonemia only

Question 35

How does OTCD present?

Answer 35

hyperammonemia + orotate in the blood/urine

Question 36

What is orotate?

Answer 36

An intermediate for CPS2

Question 37

How does ASD present?

Answer 37

hyperammonemiaelevated | citrulline in blood & urine

Question 38

How does ALD present?

Answer 38

hyperammonemiaelevated | argininosuccinate in blood/urine

Question 39

How does Arginase-Deficiency present?

Answer 39

hyperammonemiaelevated | arginine in blood/urine

Question 40

What treatments are used to remove excess ammonia?

Answer 40

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

Question 41

Product & cofactor of benzoate administration?

Answer 41

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

Question 42

Product & cofactor of phenylacetate administration?

Answer 42

``` product = phenylacetylglutamine cofactor = A-CoA, ATP ```

Question 43

Where does L-amino acid oxidase mainly act?

Answer 43

mainly in liver & kidney

Question 44

What cofactor is required to synthesize PLP?

Answer 44

NAD+

Question 45

To what AA is PLP usually bound in an enzyme?

Answer 45

Lys

Question 46

What substances are incorporated in to Carbamoyl-PO4?

Answer 46

NH4 + HCO3 + 2 ATP

Question 47

How does NH4 accumulation become toxic?

Answer 47

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

Question 48

What are the clinical endpoints of ammonia accumulation?

Answer 48

brain damage, coma death

Question 49

How doe kidneys & intestines work together to make Arg?

Answer 49

1) intestines release citrulline | 2) kidney releases Arg

Question 50

What surgical procedure could result in an Arg deficiency?

Answer 50

small intestine resection

Question 51

Describe gluconeogenesis reaction for Ala.

Answer 51

2 Ala --> 1 Glucose + 1 urea

Question 52

How is Ala produced from pyruvate in muscle?

Answer 52

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