Editing Cytosol (section)

===Protein complexes===
Proteins can associate to form [[protein complex]]es, these often contain a set of proteins with similar functions, such as enzymes that carry out several steps in the same metabolic pathway.<ref>{{cite journal |author=Srere PA |title=Complexes of sequential metabolic enzymes |journal=[[Annu. Rev. Biochem.]] |volume=56 |pages=89–124 |year=1987 |pmid=2441660 |doi=10.1146/annurev.bi.56.070187.000513}}</ref> This organization can allow [[substrate channeling]], which is when the product of one enzyme is passed directly to the next enzyme in a pathway without being released into solution.<ref>{{cite journal |author=Perham RN |title=Swinging arms and swinging domains in multifunctional enzymes: catalytic machines for multistep reactions |journal=[[Annu. Rev. Biochem.]] |volume=69 |pages=961–1004 |year=2000 |pmid=10966480 |doi=10.1146/annurev.biochem.69.1.961}}</ref> Channeling can make a pathway more rapid and efficient than it would be if the enzymes were randomly distributed in the cytosol, and can also prevent the release of unstable reaction intermediates.<ref>{{cite journal |vauthors=Huang X, Holden HM, Raushel FM |s2cid=16722363 |title=Channeling of substrates and intermediates in enzyme-catalyzed reactions |journal=[[Annu. Rev. Biochem.]] |volume=70 |pages=149–80 |year=2001 |pmid=11395405 |doi=10.1146/annurev.biochem.70.1.149}}</ref> Although a wide variety of metabolic pathways involve enzymes that are tightly bound to each other, others may involve more loosely associated complexes that are very difficult to study outside the cell.<ref>{{cite journal |vauthors=Mowbray J, Moses V |title=The tentative identification in Escherichia coli of a multienzyme complex with glycolytic activity |journal=Eur. J. Biochem. |volume=66 |issue=1 |pages=25–36 |date=June 1976 |pmid=133800 |doi=10.1111/j.1432-1033.1976.tb10421.x}}</ref><ref>{{cite journal |vauthors=Srivastava DK, Bernhard SA |title=Metabolite transfer via enzyme-enzyme complexes |journal=Science |volume=234 |issue=4780 |pages=1081–6 |date=November 1986 |pmid=3775377 |doi=10.1126/science.3775377|bibcode=1986Sci...234.1081S }}</ref> Consequently, the importance of these complexes for metabolism in general remains unclear.

[[Image:Carboxysome.png|thumb|right|400px|[[Carboxysome]]s are protein-enclosed [[bacterial microcompartment]]s within the cytosol. On the left is an [[electron microscope]] image of carboxysomes, and on the right a model of their structure.]]