Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Individual proteins in the nucleus and cytosol, as well as in the endoplasmic reticulum (ER) and mitochondria, are degraded at widely differing rates that vary from minutes for some regulatory enzymes to days or weeks for proteins such as actin and myosin in skeletal muscle or months for hemoglobin in the red cell.
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