nitrogen sources and disposal Flashcards
breakdown of dietary proteins? Where and how?
in stomach, digested in di- and tripeptidase by proteinases
peptidases on the surface of intestinal cells (in the brush boarder) then break these down into amino acids
absorption of amino acids depends on?
depends on active transport by sodium symport systems in plasma membrane of intestinal cells
essential AA are? Where do we get them?
9: phe, met, trp, lys, thr, his, leu, ile, val
protein from different foods are equally effective as a source of energy and nitrogen but nutritional value varies based on AA composition - low content of one or more of the essential AA increases the amount of that protein required to maintain the nitrogen balance
protein turnover pathways between meals?
between meals or in protein-free diet degrade cellular protein
by ubiquitin-proteaosome pathway or lysosome degradation pathway
ubiquitin proteasome pathway
depends on covalent attachment of ubiquitin to lysine residues of protein targeted for degradation
polyubiquinated protein then recognized by proteasome complex and degraded in ATP dependent process
responsible for degradation of targeted proteins
insulin (via IRS1 receptor) inhibits this process
lysosome degradation pathway
responsible for degradation of bulk cellular proteins and organelles
cytosolic material sequestered into membrane bound compartment - autophagy
autophagosomes fuse with lysosomes/vacuoles where proteases and lipases can degrade the contents
high insulin levels after meals inhibit process, starvation stimulates process
regulation of protein synthesis
regulated at ribosomal level
under starvation: initiation factor eIF2 is sequestered and/or phosphorylated to inhibit it
insulin and increased AA levels result in phosphorylation of 4E-BP1 - triggers release of eIF4
amino acid catabolism
Routes?
free AA from AA pools catabolized to lipogenic and gluconeogenic precursors depending on metabolic status
two routes:
aminotransferase reactions
oxidative deaminiation
aminotransferase reactions
reversible
aka transaminases
specific enzyme for each AA and keto-acid pair
most occur in liver, also in muscle during starvation
transamination usually last step in synthesis of AA and first in degradation
require pyridoxal phosphate (PLP; derivative of vitamin B6)
common examples:
alanine aminotransferase:
alanine + alpha-ketoglutamate => pyruvate + glutamate
aspartate aminotransferase:
aspartate + alpha-ketoglutamage => oxaloacetate + glutamate
catabolism can create excess amino groups which can be funneled to glutamate by conversion of alpha-keto glut to glutamate - allows these to be used for metabolism
step in aminotransferase reaction
need pyridoxal phosphate (PLP) at active site
catalytic mechanism transfers the amino group of an amino acid to the coenzyme => pyridoxamine phosphate intermediate
hydrolysis releases an alpha-keto acid
intermediate reacts with another alpha-keto acid to form an amino acid (often glutamate) and regenerates the bound PLP
equilibrium constant for most aminotransferases near one so reaction controlled by substrate concentration
AA that can make alpha-ketoglutarate
arginine, glutamate, glutamine, histidine, proline
AA that can make succinyl coA
isoleucine
methionine
threonine
valine
AA that can make fumarate
aspartate
phenylalanine
tyrosine
AA that can make oxaloacetate
asparagine
aspartate
AA that can make pyruvate
alanine cysteine glycine serine threonine tryptophan
