When older proteins are broken down in the body, they must be replaced. This concept is called protein turnover, and different types of proteins have very different turnover rates. Protein synthesis occurs during the process of translation on ribosomes. Protein breakdown occurs generally in two cellular locations:
- Lysosomal proteases digest endocytosed proteins
- Cytoplasmic complexes, called proteasomes, digest older or abnormal proteins that have been tagged with ubiquitin for destruction.
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A balance between protein synthesis and protein degradation is required for good health and normal protein metabolism. More synthesis than breakdown indicates an anabolic state that builds lean tissues, more breakdown than synthesis indicates a catabolic state that burns lean tissues. According to D.S. Dunlop, protein turnover occurs in brain cells the same as any other eukaryotic cells, but that "knowledge of those aspects of control and regulation specific or peculiar to brain is an essential element for understanding brain function."
Protein turnover is believed to decrease with age in all senescent organisms including humans. This results in an increase in the amount of damaged protein within the body.
Some bodybuilding supplements claim to reduce the protein breakdown by reducing or blocking the number of catabolic hormones within the body. This is believed to increase anabolism. However, if protein breakdown falls too low then the body would not be able to remove muscle cells that have been damaged during workouts which would in turn prevent the growth of new muscle cells.
- Toyama, B. H.; Hetzer, M. W. (2013). "Protein homeostasis: Live long, won't prosper". Nature Reviews Molecular Cell Biology. 14 (1): 55–61. doi:10.1038/nrm3496. PMC 3570024. PMID 23258296.
- Eden, E; Geva-Zatorsky, N; Issaeva, I; Cohen, A; Dekel, E; Danon, T; Cohen, L; Mayo, A; Alon, U (2011). "Proteome half-life dynamics in living human cells". Science. 331 (6018): 764–8. doi:10.1126/science.1199784. PMID 21233346.
- Dunlop, D. S. (1983). Protein Turnover in Brain Synthesis and Degradation. Handbook of Neurochemistry. pp. 25–63. doi:10.1007/978-1-4899-4555-6_2. ISBN 978-1-4899-4557-0.
- Aerobic Exercise Training Increases Skeletal Muscle Protein Turnover in Healthy Adults at Rest by Pikosky et. al
- Level of dietary protein impacts whole body protein turnover in trained males at rest by Gaine, et. al
- Dietary protein intake impacts human skeletal muscle protein fractional synthetic rates after endurance exercise by Bolster et. al