Editing Proteolysis (section)

====Rate of intracellular protein degradation====
Different proteins are degraded at different rates. Abnormal proteins are quickly degraded, whereas the rate of degradation of normal proteins may vary widely depending on their functions. Enzymes at important metabolic control points may be degraded much faster than those enzymes whose activity is largely constant under all physiological conditions. One of the most rapidly degraded proteins is [[ornithine decarboxylase]], which has a half-life of 11 minutes. In contrast, other proteins like [[actin]] and [[myosin]] have a half-life of a month or more, while, in essence, [[haemoglobin]] lasts for the entire life-time of an [[erythrocyte]].<ref name="degradation">{{cite book |author=Thomas E Creighton |title=Proteins: Structures and Molecular Properties |edition=2nd |chapter=Chapter 10 - Degradation |pages=[https://archive.org/details/proteinsstructur0000crei/page/463 463–473] |year=1993 |publisher=W H Freeman and Company |isbn=978-0-7167-2317-2 |chapter-url=https://archive.org/details/proteinsstructur0000crei |url=https://archive.org/details/proteinsstructur0000crei/page/463 }}</ref>

The [[N-end rule]] may partially determine the half-life of a protein, and proteins with segments rich in [[proline]], [[glutamic acid]], [[serine]], and [[threonine]] (the so-called [[PEST sequence|PEST protein]]s) have short half-life.<ref>{{cite book |author=Voet & Voet |title=Biochemistry |pages=[https://archive.org/details/biochemistry00voet_0/page/1010 1010–1014] |edition=2nd |year=1995 |publisher=John Wiley & Sons |isbn=978-0-471-58651-7 |url=https://archive.org/details/biochemistry00voet_0/page/1010 }}</ref> Other factors suspected to affect degradation rate include the rate deamination of glutamine and [[asparagine]] and oxidation of [[cystein]], [[histidine]], and methionine, the absence of stabilizing ligands, the presence of attached carbohydrate or phosphate groups, the presence of free α-amino group, the negative charge of protein, and the flexibility and stability of the protein.<ref name="degradation"/> Proteins with larger degrees of [[Intrinsically disordered proteins|intrinsic disorder]] also tend to have short cellular half-life,<ref>{{Cite journal|title = Structural disorder serves as a weak signal for intracellular protein degradation|journal = Proteins|date = 2008-05-01|issn = 1097-0134|pmid = 18004785|pages = 903–909|volume = 71|issue = 2|doi = 10.1002/prot.21773|first1 = P.|last1 = Tompa|first2 = J.|last2 = Prilusky|first3 = I.|last3 = Silman|first4 = J. L.|last4 = Sussman|s2cid = 13942948}}</ref> with disordered segments having been proposed to facilitate efficient initiation of degradation by the [[proteasome]].<ref>{{Cite journal|title = Paradigms of protein degradation by the proteasome|journal = Current Opinion in Structural Biology|date = 2014-02-01|issn = 1879-033X|pmc = 4010099|pmid = 24632559|pages = 156–164|volume = 24|doi = 10.1016/j.sbi.2014.02.002|first1 = Tomonao|last1 = Inobe|first2 = Andreas|last2 = Matouschek}}</ref><ref>{{Cite journal|title = Intrinsically Disordered Segments Affect Protein Half-Life in the Cell and during Evolution|journal = Cell Reports| date=25 September 2014| issn = 2211-1247|pmc = 4358326|pmid = 25220455|pages = 1832–1844|volume = 8|issue = 6| doi = 10.1016/j.celrep.2014.07.055|first1 = Robin|last1 = van der Lee|first2 = Benjamin|last2 = Lang|first3 = Kai|last3 = Kruse|first4 = Jörg|last4 = Gsponer|first5 = Natalia|last5 = Sánchez de Groot|first6 = Martijn A.|last6 = Huynen|first7 = Andreas|last7 = Matouschek|first8 = Monika|last8 = Fuxreiter|first9 = M. Madan|last9 = Babu}}</ref> 
 
The rate of proteolysis may also depend on the physiological state of the organism, such as its hormonal state as well as nutritional status. In time of starvation, the rate of protein degradation increases.