Protein degradation

Protein degradation is energy-dependent and occurs under two conditions:

• physiologically:
  • as a means of destroying proteins which have formed abnormally
  • as a means of reducing enzyme concentrations and so altering flux through metabolic pathways
  • within cells which are subject to digestive enzymes
• pathologically e.g. during prolonged starvation

Within cells, protein degradation can occur:

• within lysosomes:
  • particularly in liver
  • contain abundant proteinases
• within reticulocytes: contain cyosolic proteases
• within muscle:
  • peptidases activated by calcium
  • degrade myofibrillar proteins

Amino acids so produced are then available for the normal pathways of amino acid metabolism.

Little is understood about the general control of protein degradation, but the regulation of skeletal muscle is better characterized:

• anabolic hormones such as insulin decrease protein degradation
• glucocorticoids and thyroxine have the reverse effect