• primarily in proteosomes and lysosomes
• catabolism of cellular protein
• Reasons
• Regulation through turnover
• Quality control: errors, mistranslated, misfolded

Chaperons make an attempt at correction, but lysosome and proteasomes degrade

• Source of amino acids: fuel and anabolic metabolites

Use proteins for fuel and anabolic reactions

• Can also use proteins as anabolic precursors
• Some N-terminal sequences can affect protein longevity
• Serine is long (>20 hours), aspartate very short (< 3 minutes)
• PEST sequence has short halflife

Proline (P), Glutamate (E), Serine (S), Threonine (T) May be sites for phosphorylation that allows its recognition by ubiquitination machinery

• Make sure we degrade the right protein at the right time STEPS
• Ubiquitination
• Transamination