🔹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).
