Protein Biochemistry 3

Question 1

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

Answer 1

cystine

Question 2

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

Answer 2

Because they contain oxidizing sulfa portions

Question 3

The first step in methionine degradation is ______________.

Answer 3

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

Question 4

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

Answer 4

methyltransferase

Question 5

What enzyme makes homocysteine?

Answer 5

Adenosylhomocysteine hydrolase

Question 6

Methionine is made from _____________.

Answer 6

homocysteine

Question 7

Methionine synthase requires what co-enzymes?

Answer 7

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

Question 8

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

Answer 8

folate to B12 to homocysteine

Question 9

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

Answer 9

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

Question 10

B6 (pyridoxine) is needed by what enzyme?

Answer 10

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

Question 11

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

Answer 11

S-adenosylmethionine

Question 12

When oxidized, ____________ forms dimers.

Answer 12

glutathione (by the sulfur residue in the middle)

Question 13

What is the amino acid residue order of glutathione?

Answer 13

Glutamate - cysteine - glycine

Question 14

Three necessary molecules are produced by tryptophan metabolism: ___________________.

Answer 14

serotonin, melatonin, and niacin

Question 15

What enzyme converts phenylalanine to tyrosine?

Answer 15

Phenylalanine hydroxylase

Question 16

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

Answer 16

phenylacetate

Question 17

Homocysteine can proceed to two pathways: ________________.

Answer 17

• 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.

Question 18

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

Answer 18

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.

Question 19

True or false: cysteine is an essential amino acid.

Answer 19

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

Question 20

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

Answer 20

Serine

Question 21

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

Answer 21

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

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

Question 22

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

Answer 22

Cystathionine beta-synthase (the disease is homocystinuria)

Question 23

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

Answer 23

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

Question 24

True or false: cysteinuria results from an enzyme deficiency.

Answer 24

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

Question 25

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

Answer 25

SAM

Question 26

Methotrexate inhibits ________________.

Answer 26

dihydrofolate reductase