• get rid of surplus: regulate a pathway
• correct genetic mistakes from transcription and translation
• obtain needed amino acids
• signal transduction
• programmed cell death

lysosome

proteosome

time where 1/2 of the protein pool is broken down

long: serve an important purpose ex. hemoglobin

short: need to control levels quickly

7 amino acids long

attacks lysine on target protein: forms isopeptide bond using ATP

same in most life forms

mono: location of protein changes

poly: 4 or more add to itself, linear or branched. signal for degradation of protein

E1: Ub-activating enzyme

E2: Ub-conjugating enzyme (carrier)

E3: Ub-protein ligase (adds to protein)

Uses ATP

1) Attach AMP to C terminus of ubiquitin

release PPi

2) AMP is released and a thioester bond is formed with E1 complex (cystein residue)

Ub is transferred from E1 to E2

still attaches to sulfhydryl group (cysteine)

specific target protein is recognised by F box protein and attaches to E3 with lysine

Ub-E2 complex attaches also

Ubiquitin is transferred to the target protein; attaches Ub to lysine on target

the halflife of a cytoplasmic protein is determined by its amino-terminal residue

some amino acids have high stabilty and low turnover

others have low stability and high turnover

garbage can structure

19S caps on each end

20S catalytic core

uses ATP to unfold protein so it can enter core

each cap has 6 ATPases

does not break every peptide bond; makes fragments

serine protease mechanism

Ubiquitin is removed by isopeptidase to be reused

fragments contain amino acids that can be broken down to make glucose or fatty acids

they can also be used to resynthesize new proteins