Protein Biochemistry II

Question 1

Where does the urea cycle occur?

Answer 1

Mitochondria and cytosol via the antiporter ORNT1

Kickstart with ornithine in the mitochondria.

Ornithine is transported in while citrulline is transported out

Question 2

Overall Urea Cycle Rxn

Answer 2

3 ATP + HCO3 + NH4(N) + Aspartate –> 2ADP + 2Pi + PPi + fumarate + urea

Question 3

1st ATP in urea does what?

Answer 3

Activates soluble form of CO2 (carbamoyl phosphate)

Question 4

What is the control point for protein catabolism?

Answer 4

1. Carbamoyl phosphate synthetase I
2. Uses 2 ATPs to add one phosphate and ammonia
3. 10 mutations found on this enzyme lead to early onset of associated |urea cycle disorder”

Question 5

How is carbamoyl phosphate synthetase I regulated?

Answer 5

1. Activator - N-acetylglutamate.This is activated itself by arginine.
2. Two ATP binding sites

Question 6

Transport of ammonia

Answer 6

Performed by glutamine (glutamate can hold 2 nitrogens = glutamine)

Goes from peripheral tissues to:

1. Kidneys - removed in the form of urine ammonia
2. Liver - removed in the form of urea (through blood to kidney)

Muscle: Ala produced for transport to liver

Question 7

Glutamate dehydrogenase

Answer 7

1. Allosterically regulated (ATP & GTP inhibit and ADP & GDP activate) and serves as another control point for catabolism.
2. Genetic mutation in ATP/GTP binding site results in hyperinsulinism-hyperammonemia syndrome (elevated levels of ammonia in blood = gain of fxn mutation)

Question 8

Cross-talk in urea cycle

Answer 8

NO is a neurotransmitter derived from Arginine via NO synthase

NO is a dilator; activator of guanyl cyclase

Arginine –(NO synthase)–> Citrulline + NO

Question 9

Arginine & urea cycle

Answer 9

Arginine –> Creatine phosphate and Ornithine via amidinotransferase, methyltransferase, kinase

Question 10

Glucogenic vs. Ketogenic

Answer 10

Glucogenic - derive carbon source; pyruvate or Krebs cycle intermediate
Ketogenic - derive acetyl-CoA; no NET production of glucose

Question 11

Glucogenic breakdowns

Answer 11

Asparagine –> aspartate via asparginase

Note: this is an anti-cancer drug because it deprives cells of Asn

Question 12

MSUD

Answer 12

Defect in breakdown of branched chain AA
(branched chain alpha keto acid dehydrogenase complex is defective)

Leu - Ketogenic
Ile - Both
Val - Glucogenic

Leads to high concentrations of keto-acids in urine

Question 13

Thyroid chemistry

Answer 13

T4 made from tyrosine; is a prohormone and is converted to T3
Both are transported via TBG in the blood

Made by the suicide of thyroglobulin (Tg) –>Thyrodine peroxidase iodinates Tyr –> proteolyzed and carried through TBG

TSH: stimulates I- uptake; stimulates release of T4, T3
Thyroid peroxidase: Oxidizes I- to I2
Tg: Contains Tyr reisdues iodinated to form T4, T3
TBG: Transports T4, T3

Question 14

Porphyrias

Answer 14

Defects in heme synthesis

Question 15

Porphyrin synthesis

Answer 15

• Heme is a porphyrin produced in liver
• porphyrins are cyclic molecules that bind Fe2+
• porphyrias are inherited defects in heme synthesis

6 enzymes involved in heme production

** Iron binding to porphyrin ring is catalyzed by ferrochelatase to make heme.

Question 16

Regulation of heme synthesis

Answer 16

1. Initial reactions ALAS1, ALAS2 are inhibited by end-product, and ALA dehydratase is Zn dependent
2. Lead posioning –> anemia; can replace Zn in ALA dehydratase and inactivate enzyme, also can replace FE2+ in ferrochelatase

Question 17

Where does heme synthesis occur?

Answer 17

Some in mitochondria, some in cytosol

Question 18

Heme degradation

Answer 18

• degraded to bilirubin
• high bilirubin leads to jaundice
• bilirubin is also an anti-oxidant

Heme–>biliverdin–>bilirubin (more soluble)

Question 19

Glucoronic acid at birth

Answer 19

Not made when you are born so you need to have more soluble forms of biliverdin and bilirubin

If not very soluble, attach it to glucoronic acid to make it more soluble and get it out of the body

Question 20

Bilirubin glucuronyl-transferase

Answer 20

Low in infants, especially premature

- fluorescent light converts bilirubin to more polar products and allows removal similarto conjugated bilirubin