lecture 1 part 2

Where does amino acid nitrogen enter metabolism from?

Via glutamate and glutamine, through specific aminotransferases, and via the urea cycle in the liver

What metabolic pathways provide the carbon backbones for amino acids?

Glycolysis, the pentose phosphate pathway, and the citric acid cycle

How many of the 20 common amino acids have a non-ionizable R group?

13 of 20 do not ionize (only 7 have an ionizable R group)

5 common reversible post-translational modifications

Phosphorylation, O-GlcNAc glycosylation, methylation, acetylation, and farnesylation

Which residues are typically phosphorylated, and by what?

Serine, threonine, or tyrosine residues, by specific protein kinases

Methylarginine and methyllysine — where are these modifications especially relevant?

Modifications found on histone proteins, important in regulating gene expression

Ornithine's full role in the urea cycle

Enters the mitochondria, condenses with carbamoyl phosphate to form citrulline → citrulline exits and condenses with aspartate to form argininosuccinate → cleaved to form arginine (which is then cleaved into urea + ornithine again)

Why is arginine considered non-essential?

Because it's an intermediate of the urea cycle, the body can synthesize it

Selenocysteine's biosynthesis pathway (in brief)

Serine is first charged onto a specific tRNA (forming sec-tRNA) by seryl ligase, then enzymatically converted to selenocysteine using selenium from H2Se, before being incorporated into selenoproteins during translation

SAM/AdoMet's other pathway roles besides methylation

The AdoMet pathway also produces cysteine (via the transsulfuration pathway) and polyamines (via a separate branch)

Glycine titration curve key points (0.1M glycine + 0.1M NaOH)

pK1 (carboxyl group) corresponds to pH 2.34; pK2 (amine group) corresponds to pH 9.6; the isoelectric point (pI) of 5.97 marks the region of greatest buffering power