ROS and Urea Cycle

Question 0

Triplet Oxygen

Answer 0

The oxygen that we breathe. No double bond, one unpaired electron on each oxygen spinning in the same direction. Unreactive. If it gains an electron, it can become a superoxide radical, can gain another to be peroxide. Will become H2O2, will split after it gains an electron to form H2O and OH radical. OH radical can gain an electron and eventually become water.

Question 1

Oxidative Stress

Answer 1

Imbalance between antioxidants/oxygen radicals

Question 2

Singlet Oxygen

Answer 2

O2 with a single bond and unpaired electrons on each oxygen spinning in the opposite direction. Dangerous, can insert across double bonds.

Question 3

Hydrogen Abstraction

Answer 3

Oxygen radical can steal a hydrogen, generating another radical.

OH.+LH –> H2O + L.

Question 4

Disproportionation

Answer 4

Where two radicals come together, but instead of terminating, one is oxidized and one is reduced. O2.+O2. +2H–>O2+H2O2

Question 5

Haber Weiss and Fenton Reactions

Answer 5

Fe-based generation of OH radicals, using H2O2.

Question 6

Why are ROS toxic?

Answer 6

1) They can affect proteins (Oxidize SH, fragment aromatic aa’s).
2) They can affect lipids (destroy cell membranes).
3) They can affect DNA (cleave backbone, excise bases, oxidize bases).

Question 7

Autooxidation of Quinones

Answer 7

If ubi gains electron, it becomes semiquinone radical, which can create O2 radical. If it doesn’t semiquinone can gain another electron and become ubiquinol.

Question 8

Superoxide Dismutase

Answer 8

Enzyme that protects against ROS.

O2.+O2. +2H -> O2 + H2O

SOD 1 uses Copper/Iron, SOD2 uses Manganese.

Question 9

Catalase Reaction

Answer 9

2H2O2 –> 2H2O + O2

Question 10

Peroxidase Reaction

Answer 10

H2O2 + SH2 –> S + 2H2O

Question 11

How are ROS beneficial?

Answer 11

Can mount respiratory burst in macrophages/phagocytes to kill foreign bacteria. Needed for ribonucleotide reductase, also needed for methylmalonyl CoA mutase.

Question 12

Reperfusion Injury

Answer 12

When a tissue is hypoxic, Adenine can be turned into Hypoxanthine which can be turned into Xanthine (or generated from guanine) using Xanthine Dehydrogenase. Xanthine become uric acid via another xanthine dehydrogenase reaction. When oxygen is reintroduced into this tissue, Xanthine reductase can generate ROS.

Question 13

Desferal and Allopurinol

Answer 13

Desferal is an iron chelator that stops the fenton reaction. Allopurinol is a Xanthine oxidase inhibitor.

Question 14

Essential Amino Acids

Answer 14

Phenylalanine
Valine
Tryptophan
Threonine
Isoleucine
Methionine
Histidine
Arginine
Leucine
Lysine

Question 15

When does the Urea Cycle occur?

Answer 15

When there are excess AA’s in the diet. Under starvation conditions and protein breakdown. Occurs in liver and kidney.

Question 16

How does the urea cycle begin?

Answer 16

Most aa’s will react with AKG via transamination to create their alpha ketoacid and Glutamate.
This glutamate will travel to the liver and release the NH3 (regenerating AKG) via Glutamate Dehydrogenase.

Question 17

Glutamate Dehydrogenase

Answer 17

Helps glutamate release NH3 in liver. Glu becomes AKG.

Question 18

How are Ser Thr, Cys, His metabolized?

Answer 18

They create NH3 in muscles. So this NH3 is attached to Glutamate using ATP via Glutamine Synthase. Glutamine then travels to the liver, where glutaminase disassembles it back into glutamate and NH3.

Question 19

Urea Cycle reaction

Answer 19

In Liver Mitochondria:
HCO3 + NH3 –> Carbamoyl Phosphate (using 2 ATP’s via Carbamoyl Phosphate Synthetase AKA CPS1).

Carbamoyl Phosphate + Ornithine —> Citrulline (via ornithine transcarbamoylase).

Citrulline migrates out of mitochondria and + aspartate –> argininosuccinate (using ATP-> AMP via argininosuccinate synthetase).

Argininosuccinate –> Arginine + Fumarate (via argininosuccinate lyase).

Fumarate goes to the TCA cycle.

Arginine + H2O –> Urea + Ornithine (via arginase).

Question 20

How many times does glutamate feed into the urea cycle?

Answer 20

Twice.

1) By donating its NH3, which is used in step one of the urea cycle.
2) By undergoing transamination with OAA to create aspartate, which combines with citrulline to form argininosuccinate.

Question 21

How is NH3 overdose treated?

Answer 21

Eliminate glycine using benzoate or eliminate glutmate using phenylacetate.

Question 22

GOT

Answer 22

Glutamic Oxoacetic Transaminase. Catalyzes Asp + AKG –> OAA + Glu.

Question 23

GPT

Answer 23

Glutamic Pyruvate Transaminase. Catalyzes Alanine + AKG –> Pyruvate + Glutamate