Nitrogen Sources and Amino Acid Metabolism Flashcards
describe how protein is digested and absorbed
digested by proteinases in the stomach into peptides, then in the small intestine into amino acids by peptidases.
absorbed by brush border
how many essential amino acids are there? why are they?
9
Phe, Met, Trp, Lys, Thr, His, Leu, Ile, Val
Met, Lys, Trp are usually rate limiting
do you get more free AAs from protein turnover or diet?
protein turnover
what are the two pathways to generate AAs from protein?
ubiquitin-proteasome and lysosomal
describe the ubiquitin-proteasome pathway
covalent attachement of ubiquitin to lysine resides on protein targeted for destruction. protesome complex recognizes ubiquitin an uses ATP to degrade the protein
describe the lysosomal pathway
degrades bulk proteins/organelles
cytosolic material gets sequestered into membrane bound compartments called autophagosomes. these fuse with lysosomes where proteases and lipases destroy them
how is the lysosomal pathway regulated?
during starvation, mTOR is turned off, signaling autophagy.
after a meal, insulin inhibits autophagy by activating mTOR via Akt
how is protein synthesis regulated?
3 mechanisms:
- starvation- phosphorylation and inhibition of eIF2
- insulin causes phosphorylation of 4E-BP1 through mTOR, and that releases the active form of eIF4
- mTOR also increases phosphorylation of eIF4B, which is a helicase
aminotransferases
removes an amino group from an amino acid, forming a keto acid, which is usually a gluconeogenic precursor. the amino group is added to a-ketoglutarate to form glutamate
reversible- controlled by substrate concentration
requires pyridoxal phosphate (vitamin b6)
aminotransferases convert alanine, aspartic acid, and glutamate into what?
pyruvate, OAA, and a-KG
glutamate dehydrogenase
oxidative deanimation- converts glutamate to a-ketoglutarate and ammonia
w/ high glutamate (after a meal)- generate aKG
w/ low glutamate- generates glutamate
forming aKG generates NADH from NAD
forming glutamate generates NADP from NADPH
how is most ammonia removed from the body?
converted to urea
alkaptonuria
deficiency in homogentisate oxidase, an enzyme the degrades tyrosine.
urine contains homogentisate, but also accumulates in joints leading to severe arthritis
linked to a single recessive gene
maple syryp urine disease
deficiency in branched chain a-keto acid dehydrogenase (follows immediately after aminotransferase in leucine, isoleucine, and valine degradation).
causes neurological defects d/t a build up in branched chain a-keto acids.
also causes urine to smell like maple syrup
homosytinuria
deficiency in cystathionine b-synthase which is important in methionine metabolism.
produces osteoporosis, mental retardation, and increased risk of heart disease
