Protein Biochemistry 2

Question 1

The relative directionality of the reactions catalyzed by AST and ALT is determined by ____________.

Answer 1

the concentrations of the reactants

Question 2

Because ___________ is toxic, glutamine is used to transport it from the tissues to the liver.

Answer 2

ammonia

Question 3

The enzyme _____________ is located in peripheral tissues.

Answer 3

glutamine synthetase

Question 4

After glutamine gets to the liver, what happens?

Answer 4

Glutaminase breaks off one NH3 which goes into the production of urea.

Question 5

Two things happen in the kidneys regarding urea: _____________.

Answer 5

ammonia is removed from glutamine and urea is disposed

Question 6

What enzyme in the urea cycle is an anti-cancer drug?

Answer 6

Asparaginase (which deprives cells of asparagine)

Question 7

What enzyme is defective in someone with maple-syrup urine disease?

Answer 7

Branched-chain alpha-keto-acid dehydrogenase

Question 8

Thyroxin is made from ______________.

Answer 8

tyrosine

Question 9

The active form of thyroxin is cleaved be which enzyme?

Answer 9

Deiodinase (which activates T3 from T4)

Question 10

Trace the urea cycle.

Answer 10

NH4 + HCO3
(Mitochondrial carbamoyl phosphate synthetase) Rate-limiting; uses ATP
Carbamoyl phosphate
(Mitochondrial ornithine transcarbamoylase) Ornithine enters*
Citrulline
(Cytosolic arginosuccinate synthetase) Aspartate enters; requires ATP
Arginosuccinate
(Cytosolic arginosuccinate lyase)
Arginine and fumarate

Fumarate
(Fumarase) 
Malate
(Malate dehydrogenase) 
Oxaloacetate
(AST) 
Aspartate

Arginine
(Arginase)
Urea and Ornithine

Question 11

_____________ is an allosteric activator of carbamoyl phosphate synthetase.

Answer 11

N-acetylglutamate

Question 12

Why do muscles use ALT instead of AST?

Answer 12

Because muscle accumulates pyruvate, which can be shuttled into alanine

Question 13

Describe how ALT and AST “feed” the urea cycle.

Answer 13

ALT produces glutamate, which can release an NH4+ molecule (that enters the urea cycle) by glutamate dehydrogenase. AST can produce aspartate (when the concentration is low) which enters the urea cycle.

Question 14

What protein helps transport urea-cycle reactants to their necessary locations?

Answer 14

ORNT1, which moves citrulline out of the mitochondria and ornithine in.

Question 15

Glutamine transport in the brain is necessary because _______________.

Answer 15

glutamate cannot cross the BBB

Question 16

Glutamate dehydrogenase goes both ways and thus serves as a control point for protein catabolism. What things allosterically stimulate the breakdown of glutamate?

Answer 16

All things that signal low-energy states: ADP, GDP

Question 17

__________ degrades arginine to citrulline and a neurotransmitter.

Answer 17

NO synthase

Question 18

Ornithine and creatine phosphate (a source of energy for muscle) are synthesized from what two reactants?

Answer 18

Arginine and glycine

Question 19

What protein transports T4 and T3 out of cells?

Answer 19

Thyroxin binding globulin