Protein Biochemistry III

Question 1

Which 2 AA contain sulfur?

Answer 1

1. Methionine - Essential

2. Cysteine - Non-essential

Question 2

Describe the relationship between methionine, cysteine, and cystine

Answer 2

• can use methionine to make cysteine

- cystine is the oxidized version of cysteine

Question 3

Disulfide bond importance?

Answer 3

• makes protein more stable
• allows for proteins to fold
• Note different redox environements

Question 4

Redox environments in/out of cells

Answer 4

Outside: Oxidizing
Inside: Reducing

Question 5

Sulfer AA degradation

Answer 5

1. 1st step of Met degradation produces activated sulfur (adoMet) –> SAM
2. SAM is more prevalent than ATP

Can go on and make Cys (Met + Ser = Cys)

Question 6

Steps from met to cys

Answer 6

Met -(SAM synthetase)-> SAM -(MTases)-> SAH-(AdoHcyhydrolase)->homocysteine ->cysteine

Could also go from homocysteine back to Met via THF

Question 7

Regeneration of met

Answer 7

• Requires 2 coenzymes
  1. THF
  2. Vit B12 (methyl group is transferred from THF to B12 to homocysteine)

Question 8

How is PLP used here?

Answer 8

Holds again between:

1. Homocysteine and Cystathione
2. Cystathione and Cysteine

Note: alpha-ketobutyrate can go on to make succinyl CoA

Question 9

Hyperhomocysteinemia and homocystinuria

Answer 9

come from 130 mutations identified in cystathione B-synthase

Can also come from low values of B6 and B12

Hyperhomocysteinemia: Tx is folate, B6 and B12
Homocystinuria: Tx with B6
Cysteinuria: Tx with acetazolamide to make cysteine more soluble

Question 10

Homocysteine is bad

Answer 10

1. Vascular disease (NMDA)
2. Impaired wound healing
3. High correlation to cancer (cervical)

Question 11

MTHFR

Answer 11

If this is broken, can get elevated levels of homocysteine

Question 12

SAM

Answer 12

“energy storage unit” similar to ATP

• used for tx of lots of things because of methylation
• epigenetics; host defense
• cancer (methotrexate)
• depression

Question 13

SAM and methylation

Answer 13

Methylation of NorE->Epi

Methylation of cytosine residues in DNA

Question 14

Modified forms of THF do what?

Answer 14

Transfer carbons in different oxidation states

Question 15

THF

Answer 15

1. Produced from Vit B9 (folic acid) by dihydrofolate reductase (DHFR)
2. Essential for synthesis of AA and nucleic acids

Note: Methotrexate can block this and nail cancer cells.

Question 16

How are redox states controlled?

Answer 16

Glutathione (GSH)

Question 17

GSH synthesis

Answer 17

Glutamate –> gamma-Glu-Cys –> GSH

Can form disulfides through Cys and can control redox chemistry of cell

Question 18

Why is GSH important?

Answer 18

• more soluble compared to Cys
• Thiol acts as redox buffer to maintain reduced forms
• Cofactor for several enzymes
• reduce hydrogen peroxide to water and provides general protection against ROS

Question 19

GSH in hemoglobin

Answer 19

• Keeps iron in frrous state

If Fe2+–>Fe3+, then cannot bind oxygen

Question 20

3 important enzymes related to GSH

Answer 20

1. Glutathione peroxidase: GSH –> GSSG
   - mutations lead to high BC risk
2. Glutathione reductase: reduces GSSG –> GSH
   - mutations are rare but problematic for heme
3. Glutathione S-transferase: conjugation of GSH to agents
   - detoxification, drug resistance, upregulated in tumors

Question 21

3 aromatic residues

Answer 21

1. Trp
2. Phe
3. Tyr

Question 22

Trp Metabolism

Answer 22

• metab to pyruvate or acetyl-CoA
• Trp –> serotonin, melatonin, niacin
• Tetrahydrobiopterin (BH4) cofactor required

Question 23

Phe & Tyr metabolism

Answer 23

1. Phe –> Tyr via phenylalanine hydroxylase
2. Phe, Tyr metabolized to fumerate and acetoaceteate
3. Need BH4 as cofactor
4. Tyr –> catecholamines, melanin

PKU - defect in phenylalanine hydroxylase
Tyrosinemia - defect in one of 3 enzymes:
- fumarylacetoacetate
- Tyr aminotransferase
- hydroxyphenylpyruvate dioxygenase

Question 24

PKU alternative pathway

Answer 24

• use a different transaminase (phenylalanine -> ketoacid; phenylpyruvate and phenylacetate - smelly in urine)
• Tyr becomes essential

Question 25

Parkinson’s

Answer 25

1. Treat with Dopa
2. Treat with MAOs or Mtase inhibitors
   Block deamination to produce a buildup to offset ramifications of parkinson’s