Protein Biochemistry II

Question 1

State the four steps of the urea cycle and the enzymes involved

Answer 1

Ornithine -> Citrulline (Carbamoyl phosphate synthetase I)
Citrulline + Aspartate -> Argininosuccinate (Argininosuccinate sythase)
Argininosuccinate -> Arginine (Argininosuccinate lyase)
Arginine -> Ornithine + Urea (Arginase)

Question 2

Where is Carbamoyl phosphate synthetase I located, what reaction does it drive, and what activates it?

Answer 2

Located in the mitochondria.
Reaction: bicarbonate + ammonia + 2 ATP -> Carbamoyl phosphate.
This is the first step in the Urea cycle
Activator: N-acetylglutamate is allosteric activator

Question 3

How is N-acetylglutamate created and what activates this process?

Answer 3

Acetyl-CoA + glutamate -> N-acetylglutamate

N-acetylglutamate synthase drives this reaction and is activated by Arginine.

Question 4

What is the difference between glutamate and glutamine and what enzyme converts them?

Answer 4

Glutamate has a single amine group attached, glutamine has two amino groups attached. Glutamine synthase is present in most tissues and attaches the second amino group to glutamate, forming glutamine, which then transports that amino group to the liver where it may enter the Urea cycle.

Question 5

What is the role of Glu dehydrogenase?

Answer 5

Glu dehydrogenase is a key regulator of protein metabolism by directing the removal or incorporation of ammonia into amino acids. Removal of ammonia from Glutamate produces a-ketoglutarate and Urea and occurs in the liver and kidneys. Addition of ammonia to a-ketoglutarate produces glutamate in the muscles and peripheral tissues. It is anabolic in the muscles and peripheral tissues and catabolic in the liver and kidneys.

Question 6

How does the export of ammonia differ in muscle from other peripheral tissues?

Answer 6

Most peripheral tissues export ammonia by creating glutamine with glutamine synthase. Muscle exports ammonia by conversion of pyruvate into alanine via ALT (hence ALT’s presence in skeletal muscle). Alanine then moves to the liver where it is reconverted to pyruvate by ALT and a-ketoglutarate. a-ketoglutarate is converted to Glutamate and pyruvate may then enter gluconeogenesis.

Question 7

What is the reaction that ALT drives?

Answer 7

Pyruvate + Glutamate -> Alanine + a-ketoglutarate.
This reaction occurs in the forward direction in muscle, producing alanine for export, and in the reverse direction in the liver, producing pyruvate for gluconeogenesis.

Question 8

What is the course of nitrogen from free ammonia in the muscle to free ammonia in the liver?

Answer 8

Ammonia + a-kg -> Glutamate (Glu dehydrogenase muscle)
Glu + Pyruvate -> a-kg + Alanine (ALT muscle)
Alanine travels from muscle to liver
Alanine + a-kg -> Pyruvate + Glu (ALT liver)
Glu -> a-kg + Ammonia (Glu dehydrogenase liver)

Question 9

What can arginine be processed into?

Answer 9

NO + citrulline
Via NO synthase

Ornithine
Via Arginase

Creatine Phosphate
Via several enzymes

Question 10

What do ketogenic amino acids do and what AA are ketogenic?

Answer 10

Ketogenic amino acids result in no net production of glucose. Lysine and Leucine are ketogenic and their breakdown produces only acetyl-CoA

Question 11

What do glucogenic amino acids do and what AA are glucogenic?

Answer 11

Glucogenic amino acids produce pyruvate or krebs cycle intermediates. Oxaloacetate in the Kreb cycle comes from Aspartate transamination

Question 12

What is the cause of maple syrup urine disease?

Answer 12

A deficiency in the branched chain a-ketoacid dehydrogenase complex results in excess sweet smelling a-keto acids in the urine

Question 13

What are the three branched chain amino acids and what is the process of their breakdown?

Answer 13

Leucine, isoleucine, and Valine
First reaction: deamination by BRANCHED CHAIN AMINOTRANSFERASE produces a-keto acids.
Second reaction: decarboxylation by BRANCHED CHAIN A-KETO ACID DEHYDROGENASE COMPLEX to Acetyl-Coa and Succinyl-CoA

Question 14

What are the three enzymes that serve as control points for protein metabolism?

Answer 14

1 - Transaminases (cytosolic and mitochondrial)
2 - Carbamoyl Phosphate Synthetase 1 (mitochondrial)
3 - Glu dehydrogenase (Mitochondria)

Question 15

How are transaminases (i.e. ALT, AST) regulated?

Answer 15

By the relative concentration of their substrates and products. These may direct reaction in the forward or reverse directions.

Question 16

How is Carbamoyl Phosphate Synthetase 1 regulated?

Answer 16

By N-acetylglutamate which is produced by the Arginine catalyzed reaction:
Acetyl-CoA + glutamate -> N-acetylglutamate.

Question 17

How is the directionality of Glu dehydrogenase regulated?

Answer 17

By the relative concentrations of Glu, a-ketoglutarate, and NH3. Its overall reaction is:
a-kg + NH3 -> Glu

Additionally, it is allosterically inhibited by ATP and GTP
and allosterically activated by ADP and GDP. Thus it is more active in a low energy state and less active in a high energy state.

Question 18

What four proteins are involved in thyroxin production?

Answer 18

Thyroid stimulating hormone
Stimulates I- uptake and T3,T4 release

Thyroid peroxidase
Oxidizes I- to I2

Thyroglobulin
Contains Tyr residues iodinated to T4,T3

Thyroxin binding globulin
Transports T4,T3

Question 19

What is the structure and subunits of the heme ring?

Answer 19

The Heme ring, also called the porphyrin ring (hence “porphyrias” are diseases of heme groups), is comprised of four linked pyrole rings with their Nitrogen atoms facing inwards to bind Fe. Bilirubin is the intermediate linear form between the pyrole units an the final heme ring. Porphyrins bind Fe2+

Question 20

What are the four key steps in heme creation?

Answer 20

Gly + Succinyl Co-A -> ALA (ALA Synthase)
2 ALA -> Porphobilinogen (ALA dehydratase)
Porphobilinogen ->->->-> Protoporphyrin IV (4 enzymes)
Protoporphyrin IX -> Heme (Ferrochelatase)

Question 21

How does lead interfere with heme creation?

Answer 21

Lead inhibits two enzymes in heme production:
d-aminolevulinate dehydratase
Causes build up of ALA
Ferrochelatase
Causes buildup of Protoporphyrin IX

Question 22

What is the process of heme breakdown?

Answer 22

Heme -> biliverdin (green) -> bilirubin (red-orange) -> bilirubin diglucuronide -> urobilinogen -> stercobilin (brown)

Question 23

What are the locations of the main steps in heme breakdown?

Answer 23

Albumin carries bilirubin in the blood.
Bilirubin is conjugated with glucuronic acid in the liver -> bilirubin diglucuronide (or conjugated bilirubin)
Bilirubin diglucuronide is oxidized to stercobilin in the intestines.