Urea Cycle

Question 1

What is the important role of alanine in the urea cycle?

Answer 1

• major amino acid substrate for gluconeogenesis during fasting
• major way of removing ammonia

Question 2

What is the important role of glutamine in the urea cycle?

Answer 2

• major way of removing ammonia

- nitrogen carrier

Question 3

What is the important role of glutamate in the urea cycle?

Answer 3

Ammonium is incorporated into glutamate

Question 4

What are the sources of NH4 in the urea cycle?

Answer 4

Brain
Muscle
Serine
Threonine
Histidine
Gut
Asparagine
Glutamine
Glutamate>>a-ketogluterate

Question 5

What are the different fates of ammonia in the cell?

Answer 5

• reductive aminaiton of a-ketogluterate to glutamate

- ATP dependent amidation of carboxyl of glutamate to glutamine

Question 6

What are transamination reactions?

Answer 6

transfer of an amino group from an a-amino acid to an a-keto acid

Question 7

How are amino acids degraded?

Answer 7

• removal of a-amino group

- carbon chains are altered for entry into central pathway of carbon metabolism (TCA cycle)

Question 8

Which amino acids are essential to humans?

Answer 8

Essential:
Alanine
Arginine
Aspartic acid
Cysteine
Glutamic Acid
Glutamine
Glycine
Proline
Serine
Tyrosine
Asparagine

Question 9

Which amino acids are glucogenic?

Answer 9

Glu            Asp
Gln             Ala
Arg             Ser
Pro             Cys
His              Gly
Met
Val

CHAPS GAG MAGV

Question 10

Which amino acids are ketogenic?

Answer 10

Leu

Lys

Question 11

Which amino acids are glucogenic and ketogenic?

Answer 11

Phe
Tyr
Ile
Thr
Trp

PITTT

Question 12

What is unique about degradation of branched chain amino acids?

Answer 12

Beginning Leu/Val/Iso are all degraded using the same three enzymes for the first three steps.

1. Transamination
2. Oxidative decarboxylation
3. dehydrogenation

TODD

Question 13

What are some other fates of amino acids (other than being made into proteins)? How do you make NO?

Answer 13

• precursors for many other structures

- Arginine >Nitric oxide synthase>citrulline + *N=O

Question 14

What is the glucose/alanine cycle?

Answer 14

• exchange of glucose and alanine between muscle and liver

- provides an indirect means for muscle to eliminate nitrogen (and pyruvate)

Question 15

How is the urea cycle regulated?

Answer 15

“Feed forward” - regulated by substrate availability
-high protein diet

NAG allosterically activates CPS1

induction/repression of synthesis of urea cycle enzymes
E-carbamoyl phosphate synthetase I
E- ornithine transcarbamoylase
E- argininosuccinate sythetase
E- argininosuccinate lyase
E- arginase

Question 16

Role and location of each enzyme

Answer 16

1. Carbamoyl phosphate synthetase 1
2. ornithine transcarbamoylase
3. argininosuccinate synthetase
4. argininosuccinate lyase
5. arginase

Question 17

How are benzoic acid and phenylbutyrate used?

Answer 17

treatment for urea cycle defects