Function[ edit ] Reversible phosphorylation of proteins is an important regulatory mechanism that occurs in both prokaryotic and eukaryotic organisms. Many enzymes and receptors are switched "on" or "off" by phosphorylation and dephosphorylation. Reversible phosphorylation results in a conformational change in the structure in many enzymes and receptorscausing them to become activated or deactivated. Phosphorylation usually occurs on serinethreoninetyrosine and histidine residues in eukaryotic proteins.
To balance this potential differencenegative chloride ions also exit the cell, through selective chloride channels. The loss of sodium and chloride ions compensates for the osmotic effect of the higher concentration of organic molecules inside the cell.
Here, a signal such as a hormone or an action potential opens calcium channels so that calcium floods into the cytosol. These cells were also able to synthesize proteins if given ATP and amino acids, implying that many of the enzymes in cytosol are bound to the cytoskeleton. This crowding effect can produce large changes in both the rates and the position of chemical equilibrium of reactions in the cytosol.
A well-studied example of these are the "calcium sparks" that are produced for a short period in the region around an open calcium channel.
Carboxysomes are protein-enclosed bacterial microcompartments within the cytosol. On the left is an electron microscope image of carboxysomes, and on the right a model of their structure.
Protein compartments[ edit ] Some protein complexes contain a large central cavity that is isolated from the remainder of the cytosol. One example of such an enclosed compartment is the proteasome.
Since these would be damaging if they mixed freely with the remainder of the cytosol, the barrel is capped by a set of regulatory proteins that recognize proteins with a signal directing them for degradation a ubiquitin tag and feed them into the proteolytic cavity. These microdomains could influence the distribution of large structures such as ribosomes and organelles within the cytosol by excluding them from some areas and concentrating them in others.
Examples of these processes include signal transduction from the cell membrane to sites within the cell, such as the cell nucleus or organelles.
This is relatively simple for water-soluble molecules, such as amino acids, which can diffuse rapidly through the cytosol. For instance, in mammals about half of the proteins in the cell are localized to the cytosol. On the direction of pyridine nucleotide oxidation-reduction reactions in gluconeogenesis and lipogenesis.
Control of energy metabolism, edited by B.Play a game of Kahoot! here.
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