Renal Biochemistry: Urea Cycle

Question 1

With respect to humans, NH3 is

Answer 1

TOXIC

Question 2

Ammonia is converted to what in order to reduce the toxicity

Answer 2

Urea

Question 3

What is the majority of urea comprised of

Answer 3

Nitrogen

Question 4

In acid-base chemistry, Ammonia is considered a

Answer 4

Weak-Base

NH3 + H2O <=> NH4+ + OH-

Question 5

Whats the difference between NH3 and NH4+ when it comes to permeability of a membrane.

Answer 5

NH3 can move through a membrane with ease b/c its uncharged.

NH4+ cannot pass through membrane b/c it charged.

Question 6

What is the charge, acid-base activity, and the amount of nitrogens carried by urea

Answer 6

=> 0 charge
=> No acid-base activity
=> 2 nitrogens

Question 7

Protein turnover is the same as saying

Answer 7

Protein degradation

Question 8

Why is the urea cycle so important to human life.

Answer 8

Humans cannot store nitrogens. When protein turnover (degredation) occurs, there is an excess of amino acids. These amino acids carry nitrogens (amino group). A series of enzymatic rxns can remove these nitrogens from amino acids and the urea cycle can then package these nitrogens for excretion out of body.

Question 9

Nitrogen in the body exists primarily as

Answer 9

NH3

Typically NH4+ b/c of pka~ 9.3 and physiological pH=7.4

Question 10

In which organ does the urea cycle take place in

Answer 10

Liver

Question 11

What are the two important rxns that cause ammonia to be RELEASED in mammals

Answer 11

(TRICK THINK OF RXN CASCADE)
Glutamine (1)=> Glutamate (2)=> a-Ketoglutarate

(1) Glutaminase
(2) Glutamate dehydrogenase

*Glutamine dehydrogenase is a reversible reaction

Question 12

What are the three important rxns that cause ammonia to be FIXED in mammals

Answer 12

a-Ketoglutarate (1)=> Glutamate (2)=> Glutamine

(1) Glutamate Dehydrogenase
(2) Glutaminase

HCO3- + NH4+ +2 ATP (3) => Carbamoyl - P + 2 ADP + 2Pi

(3) Carbamoyl Phosphate Synthetase -1 (CPS1)
* DON’T GET BOGGED DOWN IN DETAILS, JUST KNOW NITROGEN IS AFFIXED TO CARBON DIOXIDE BY CPS1

Question 13

Glutamine is remarkable because

Answer 13

It’s a nitrogen donor (typically 2 in rxn cascade)

Question 14

NH3 in the kidney is useful b/c

Answer 14

It can help with excretion of H+ in urine

Question 15

Why can’t NH3 be excreted directly.

Answer 15

NH3 draws water of body. Cause dehydration

Question 16

Gluconeogenisis

Answer 16

Formation of new glucose “de novo”.

Requires a carbon skeleton.

Question 17

What is a good carbon skeleton and how is it derived.

Answer 17

a-Ketoacid.
During a transamination reaction, amine group from an amino acid is released and the formation on a carbon skeleton occurs. Subsequently, that amine will the go and create a new amino acid.

Ex) Amino acid release NH3 and becomes a-ketoacid

NH3 gets affixed to a-ketoglutarate and forms glutamate

Question 18

When excess nitrogens are found in the muscle/tissue, what can happen to these nitrogens

Answer 18

Amino groups from muscle & other tissues are transported out as glutamine

Glutamate (1)=> Glutamine (fixes 1 nitrogen)

(1) Glutamine synthase

Question 19

When does gluconeogenisis activity go up

Answer 19

When in a fasting state

Question 20

If gluconeogenisis goes up, so should

Answer 20

Urea cycle

Question 21

Under metabolic acidosis, the kidney can

Answer 21

Activate glutaminase enzyme to excrete NH4+

Question 22

How many nitrogens does glutamine carry?

Answer 22

Two nitrogens

Question 23

Where does a the urea cycle reactions take place in the cell

Answer 23

1) Mitochondrial Matrix (minority)

2) Cytosol (majority)

Question 24

What is the first step of the urea cycle

Answer 24

CPS1

Question 25

What is the rate limiting/regulating step of the urea cycle

Answer 25

CPS1

Question 26

What is the first source of nitrogen in the urea cycle

Answer 26

CPS1

Question 27

What is the second source of nitrogen in the urea cycle

Answer 27

Aspartate

Question 28

What ensures the spinning of the urea cycle

Answer 28

ATP hydrolysis

Question 29

Last step of the urea cycle

Answer 29

Formation of urea by arginase

Question 30

What is the role of fumarate in the urea cycle.

Answer 30

Fumarate => Malate => OAA => Aspartate

Aspartate can then can go feed back nitrogen into the urea cycle

*OAA => Aspartate is a transamination rxn

Question 31

In the urea cycle, going from arginosuccinate => arginine causes the release of

Answer 31

Fumarate

Question 32

What are the three feed forward mechanism of the urea cycle

Answer 32

1) Allosteric regulation by N-Acetylglutamate
2) Presence of NH4+
3) Metabollic reasosn

Question 33

Explain the role of N-Acetylglutamate in the urea cycle.

Answer 33

The activity of CPS1 is regulated by N-Acetylglutamate

N-Acetylglutamate is a cofactor that binds to the allosteric sites of CPS1

High presence of Glutamate, Aspartate, and acetyl-coA drive N-Acetylglutamate production.

Once bound, N-Acetylglutamate helps drive the enzymatic activity of CPS1

Question 34

Explain the role of NH4+ in the urea cycle.

Answer 34

High concentration of NH4+ drives the urea cycle forward.

Question 35

Explain the role of metabolic processes in the urea cycle.

Answer 35

Fasting => Gluconeogensis => requires a-keto acid skeleton => excretion of nitrogens from amino acids is required => drives urea cycle

High protein diet (Excess of nitrogens)

Increased gene expression of urea cycle enzymes can drive the urea cycle forward.

Question 36

Why is NH3 so toxic to humans

Answer 36

High levels of NH3 can do the following:

Conversion of a-Ketoglutarate to glutamate can mess up TCA cycle. Causes less ATP to be produced in body. Hurts brain.

Glutamate is converted to glutamine
can cause a reduction in other neurotransmitter circulation. Primarily GABA and Glutamate