Protein Biochemistry 3 Flashcards
When in the disulfide form, cysteine is referred to as _______.
cystine
Why would it be disadvantageous to have free cysteine residues on the outer surface of proteins?
Because they contain oxidizing sulfa portions
The first step in methionine degradation is ______________.
the ATP-fueled charging of methionine by S-adenosylmethionine (SAM) synthase, which adds an adenosyl group to the sulfur atom
S-adenosylmethionine is degraded to S-adenosylhomocysteine by ________________.
methyltransferase
What enzyme makes homocysteine?
Adenosylhomocysteine hydrolase
Methionine is made from _____________.
homocysteine
Methionine synthase requires what co-enzymes?
Folate and cobalamin (hence why having a deficiency in either leads to elevation of homocysteine)
In the generation of methionine, the methyl group passes from ________________.
folate to B12 to homocysteine
Three adverse events result from high levels of homocysteine: ____________________.
increased CVD mortality; impaired wound healing; and increased risk of cervical cancer
B6 (pyridoxine) is needed by what enzyme?
Cystathionine beta-synthase (giving B6 can “force” CBS to generate some cystathionine)
What high-energy group donates methyl groups to norepinephrine (to form epinephrine)?
S-adenosylmethionine
When oxidized, ____________ forms dimers.
glutathione (by the sulfur residue in the middle)
What is the amino acid residue order of glutathione?
Glutamate - cysteine - glycine
Three necessary molecules are produced by tryptophan metabolism: ___________________.
serotonin, melatonin, and niacin
What enzyme converts phenylalanine to tyrosine?
Phenylalanine hydroxylase
When phenylalanine accumulates in the blood of those with PKU, it is converted to ____________, which gets excreted in the urine.
phenylacetate
Homocysteine can proceed to two pathways: ________________.
- 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.
Why do defects in tetrahydrobiopterin or enzymes that reduce it lead to PKU-like syndromes?
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.
True or false: cysteine is an essential amino acid.
False. Methionine is an essential amino acid, but cysteine is synthesized from methionine.
Cystathionine beta-synthase requires homocysteine, B6, and what else to generate cystathionine?
Serine
After SAM is formed, what enzymatic reactions occur to generate homocysteine?
(1) SAM donates a methyl group, leaving SAH.
(2) The enzyme adenosyl-homocysteine hydrolase frees adenosine from homocysteine.
Defects in what enzyme lead to a disorder that looks like Marfan’s?
Cystathionine beta-synthase (the disease is homocystinuria)
Low levels of B12, B6, or folate can make ____________ an essential amino acid.
cysteine (because you’re not able to make it from methionine)
True or false: cysteinuria results from an enzyme deficiency.
False. It results from a defective transporter in the kidneys (that also resorbs ornithine, arginine, and lysine).
What intermediate in the metabolism of methionine has been shown to be useful in epigenetics (among many other things)?
SAM
Methotrexate inhibits ________________.
dihydrofolate reductase
