01:694:301 Lecture 23: Chapter 23 - Protein Turnover and Amino Acid Catabolism (Revised)

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Chapter 23‘Protein Turnover and Amino Acid Catabolism
Digestion of dietary protein in intestine and degradation of proteins ! supply aa to cell
o Cellular proteins are constantly being degraded/resynthesized in response to metabolic
demand; others are damaged or unfolded and need to be degraded
Primary use of aa are as building blocks for protein synthesis and nitrogenous cmpds (i.e.
o Surplus aa used as metabolic fuel; α-amino acids are removed leaving carbon skeleton
" These amino groups converted to urea by Urea Cycle
Amino carbon skeletons transformed to acetyl CoA, acetoacetyl CoA, PYR, or
intermediates used CAC
" Other carbon skeletons ! glucose, glycogen, fats
Dietary protein is vital source of amino acids; especially those containing ‘essential aa’
" Phe Val Trp Met Arg Thr His Ile Leu Lys
Dietary protein is hydrolyzed by pepsin in the stomach, and then in the small intestine.
o Pepsin is active and functions in acidic env. (pH 2) of stomach which denatures protein
o Variety of proteoyltic enzymes further digest proteins
Essentially all protein consumed orally is broken down to amino acids, which is why money
spent on most "enzyme pills" (like Superoxide Dismutase) is wasted.
Ubiquitin is a small protein (76 aa) used as a
for cellular protein turnover.
o –COOH terminal gly residue of Ub covalently attaches to ε-amino group of lys
residue forming
isopeptide bond
and thereby marking protein
o Marked proteins are unfolded and broken down by the proteasome, a "garbage
can" shaped structure which is related to Gro-EL, a folding chaperone (the
opposite job!).
" ATP driven multisubunit
digest ubiquitinated proteins
but spares ubiquitin
(which gets recycled)
Subunits are arranged in 4 rings of 7 subunits stacked ! ‘barrel’
Proteolytic active sites found interior of barrel;
o Both ubiquitin pathway an proteasome appear in eukaryotes; archaeal proteasomes differ
(all subunits are identical)
o Ubiquitin not found in prokaryotes, but there exist molecular ancestor’s to Ub
Protein degradation can be used to regulate biological function (i.e. circadian rhythms (TIM
PER) !
eless and per
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