Protein Biochemistry 3 Flashcards Preview

MS2 - Digestive, Endocrine, and Metabolic Systems > Protein Biochemistry 3 > Flashcards

Flashcards in Protein Biochemistry 3 Deck (26)
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1
Q

When in the disulfide form, cysteine is referred to as _______.

A

cystine

2
Q

Why would it be disadvantageous to have free cysteine residues on the outer surface of proteins?

A

Because they contain oxidizing sulfa portions

3
Q

The first step in methionine degradation is ______________.

A

the ATP-fueled charging of methionine by S-adenosylmethionine (SAM) synthase, which adds an adenosyl group to the sulfur atom

4
Q

S-adenosylmethionine is degraded to S-adenosylhomocysteine by ________________.

A

methyltransferase

5
Q

What enzyme makes homocysteine?

A

Adenosylhomocysteine hydrolase

6
Q

Methionine is made from _____________.

A

homocysteine

7
Q

Methionine synthase requires what co-enzymes?

A

Folate and cobalamin (hence why having a deficiency in either leads to elevation of homocysteine)

8
Q

In the generation of methionine, the methyl group passes from ________________.

A

folate to B12 to homocysteine

9
Q

Three adverse events result from high levels of homocysteine: ____________________.

A

increased CVD mortality; impaired wound healing; and increased risk of cervical cancer

10
Q

B6 (pyridoxine) is needed by what enzyme?

A

Cystathionine beta-synthase (giving B6 can “force” CBS to generate some cystathionine)

11
Q

What high-energy group donates methyl groups to norepinephrine (to form epinephrine)?

A

S-adenosylmethionine

12
Q

When oxidized, ____________ forms dimers.

A

glutathione (by the sulfur residue in the middle)

13
Q

What is the amino acid residue order of glutathione?

A

Glutamate - cysteine - glycine

14
Q

Three necessary molecules are produced by tryptophan metabolism: ___________________.

A

serotonin, melatonin, and niacin

15
Q

What enzyme converts phenylalanine to tyrosine?

A

Phenylalanine hydroxylase

16
Q

When phenylalanine accumulates in the blood of those with PKU, it is converted to ____________, which gets excreted in the urine.

A

phenylacetate

17
Q

Homocysteine can proceed to two pathways: ________________.

A
  • It can react with methionine synthase (and B12 and folate) to make methionine.
  • It can transform to cystathionine (via cystathionine beta-synthase) and then cysteine.
18
Q

Why do defects in tetrahydrobiopterin or enzymes that reduce it lead to PKU-like syndromes?

A

Because BH4 (tetrahydrobiopterin) is a necessary cofactor for phenylalanine hydroxylase. Without it (or without BH4 reductase), then phenylalanine accumulates.

BH4 is also a cofactor in the degradation of tryptophan, so it presents with a worse phenotype than PKU.

19
Q

True or false: cysteine is an essential amino acid.

A

False. Methionine is an essential amino acid, but cysteine is synthesized from methionine.

20
Q

Cystathionine beta-synthase requires homocysteine, B6, and what else to generate cystathionine?

A

Serine

21
Q

After SAM is formed, what enzymatic reactions occur to generate homocysteine?

A

(1) SAM donates a methyl group, leaving SAH.

(2) The enzyme adenosyl-homocysteine hydrolase frees adenosine from homocysteine.

22
Q

Defects in what enzyme lead to a disorder that looks like Marfan’s?

A

Cystathionine beta-synthase (the disease is homocystinuria)

23
Q

Low levels of B12, B6, or folate can make ____________ an essential amino acid.

A

cysteine (because you’re not able to make it from methionine)

24
Q

True or false: cysteinuria results from an enzyme deficiency.

A

False. It results from a defective transporter in the kidneys (that also resorbs ornithine, arginine, and lysine).

25
Q

What intermediate in the metabolism of methionine has been shown to be useful in epigenetics (among many other things)?

A

SAM

26
Q

Methotrexate inhibits ________________.

A

dihydrofolate reductase

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