Enzymes occur in different subcellular locations. Some are specific to a subcellular location. Some are heterogeneously distributed. Is there a reported phenomenon for why they occur in different locations within the cell?
There is a Wikipedia article on the subject of protein targeting. It's a good place to start.
https://en.wikipedia.org/wiki/Protein_targeting
Thanks for your answer Dr. Shapiro. Although, I do have some knowledge about mechanisms of protein targeting, I'm more interested to know about the probable perspectives of why enzymes occur in specific (or multiple) locations rather than how are they targeted.
Hi Abhishek. I don't know if this answers your question, but Intracellular enzymes are not randomly distributed.
Most cell surface receptors have intracellular domains that dictate localized phosphorylation/dephosphorylation enzyme activities that are mostly membrane localized.
Many of these intracellular domains are the same in distinct receptors that serve different functions.
These are on/off switches that depend on occupancy of a given receptor and crosstalk of kinases and phosphorylases.
A particular receptor (oncogene) can overwhelm the intracellular signal mechanism using the same kinases/phosphorylases that are supposed to keep their activity in check.
If these intracellular kinase/phosphorylase processes were organized and highly regulated we would not have cancer metastasis, autoimmune diseases, viral infections and deaths due to cytokine storm.
Proteins are localized to organelles or specific subcellular locations in eukaryotes because that is where they need to be to perform their functions. Hydrolytic enzymes are localized to the lysosomes, where they perform their digestive functions. Enzymes involved in the electron transport system are localized to the mitochondria, where they are involved in energy production. DNA and RNA polymerase are localized to the nucleus because that is where DNA replication and RNA transcription occur. Within the cytoplasm, some proteins are localized to microtubules in order to perform vesicular transport functions. And so on.
Proteins destined for secretion must pass through the ER, Golgi, and secretory vesicles on the journey, so they can be found in multiple locations. Macromolecular assemblies must be built up from individual subunits synthesized elsewhere, so those subunits are found at the site of synthesis and also in the macromolecular assembly. Microtubules assemble and disassemble, so tubulin is found in microtubules, but also in the cytoplasm. These are some examples that can help explain why proteins can be found in multiple locations within the cell.
Many proteins carry signal sequences that bind to specific transport systems for import into organelles. This includes the (H/K)DEL sequence for import into the ER via SRP-receptor, the amphipatic helix for import into the mitochondria via TOM/TIM or the Man-6-P signal for import into the lysosome.
The beta-subunit of Na/K-ATPase directs the enzyme to the basolateral side of the plasma membrane (by interaction with the actin/spectrin network), whilst the beta-subunit of the closely related H/K-ATPase directs to the apical side. Tight junctions prevent proteins from diffusing between these compartments.
Rafts are islands in the plasma membrane that are thicker than the bulk membrane. Proteins that occur in rafts have longer transmembrane helices than those that occur in the bulk membrane. Similarly, the cholesterol concentration increases from ER over Golgi to PM, this leads to an increase in thickness. Again, the length of a protein's transmembrane helix is adapted to its 'preferred' environment.
Insertion of the protein into the membrane occurs such that positive residues preferentially occur on the cytosolic side, this is stabilised by the negative potential of the cytosol compared to the extracellular space.
Mitochondria and plastids have entered the eukaryotic cell as endosymbionts. Thus proteins required for their function are localised inside them, even though most of their proteins are now encoded by nuclear genes.
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