Cells constantly create new proteins in order to maintain their normal function, this is called protein turnover.
Why is that? Do the old molecules wear out as time passes, so that they need a replacement?
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$\begingroup$ Clarification: creating new proteins is only part of protein turnover. A more accurate succinct description would be that protein turnover is the replacement of old proteins with new ones. The original, unedited version of the question (which focused specifically on new protein creation) makes more sense than the current version. Turnover is part of the answer to that question. $\endgroup$– John BollingerMay 28, 2022 at 14:27
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2 Answers
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Biology is an intricate orchestration of chemical reactions and their products. Generally, this fete is accomplished by enzymatic facilitation of certain reactions that would otherwise occur too slowly.
However, "unwanted" reactions occur spontaneously all the time, too. One important mechanism for these reactions is the presence of free radicals causing oxidative stress: reactive molecules are present in cells as a consequence of the energy necessary for metabolism, and these can react with proteins and other cellular components and change their structure. Modified proteins may fold incorrectly and lose their function (or gain harmful function). There is very little that can be done to prevent these reactions from occurring except to clean up afterwards.
Half-lives vary by protein, but for most proteins are measured in the scale of hours (Chen et al, 2016) to a couple days (Boisvert et al, 2012). By constantly degrading and replacing proteins, cells ensure that their proteins are functional and fresh. You might consider this as analogous to performing regular maintenance on a machine like an automobile, using replacement parts as the originals become worn.
Both protein degradation (e.g., via the ubiquitin-proteasome system) and synthesis are highly regulated.
Boisvert, F. M., Ahmad, Y., Gierliński, M., Charrière, F., Lamont, D., Scott, M., ... & Lamond, A. I. (2012). A quantitative spatial proteomics analysis of proteome turnover in human cells. Molecular & Cellular Proteomics, 11(3).
Chen, W., Smeekens, J. M., & Wu, R. (2016). Systematic study of the dynamics and half-lives of newly synthesized proteins in human cells. Chemical science, 7(2), 1393-1400.
answered May 25, 2022 at 16:29
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4$\begingroup$ "Half-lives vary by protein, but for most proteins are measured in the scale of hours"Wow that's wild $\endgroup$ May 26, 2022 at 17:57
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$\begingroup$ @AzorAhai-him- Indeed it is, though I want to be careful that I'm not exaggerating things too much. I've added a second reference with a different range of estimates; still hours, but possibly more in the range of tens of hours. Some can be much longer, though; this Biology.SE answer: biology.stackexchange.com/a/67698/27148 refers to a couple of those. $\endgroup$– Bryan Krause ♦May 26, 2022 at 22:52
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2$\begingroup$ Good answer. tl;dr Proteins turn over because they're consumed, modified, or escape homeostasis and require regulation or degradation. Semi-unrelated note, as a tangential rant, how does nobody think to provide keratin/spider silk as an example of a long-lived protein? Keratin (main structural protein composing hair, wool, feathers, nails, horns...) in its native environment can easily and routinely last (tens/hundreds of) thousands of years. If it's a dry place, you'll find it, especially since it's built in bulk. Hooves. Spider silk. Claws. Feathers. Keratin strong. $\endgroup$– S PrMay 27, 2022 at 11:11
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1$\begingroup$ Still, my haven't-taken-bio-since-HS self would have been off by an order of magnitude $\endgroup$ May 27, 2022 at 17:26
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Many aspects of cellular function are related to feedbacks and cycles wherein proteins need to fluctuate in abundance to maintain cellular function.
One example of protein cycling would be cell cycle dynamics. Another would be circadian protein cycling.
answered May 25, 2022 at 15:20