Examples of G protein in the following topics:
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- G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein.
- All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
- Once the G-protein binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP.
- One or both of these G-protein fragments may be able to activate other proteins as a result.
- Heterotrimeric G proteins have three subunits: α, β, and γ.
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- When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated.
- G proteins are proteins separate from receptors that are found in the cell membrane.
- When a hormone is not bound to the receptor, the G protein is inactive and is bound to guanosine diphosphate, or GDP.
- After binding, GTP is hydrolyzed by the G protein into GDP and becomes inactive .
- The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase.
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- There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
- G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein.
- The activated G-protein then interacts with either an ion channel or an enzyme in the membrane.
- All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
- In a signaling pathway, second messengers, enzymes, and activated proteins interact with specific proteins, which are in turn activated in a chain reaction that eventually leads to a change in the cell's environment.
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- Activation of RTKs initiates a signaling pathway that includes a G-protein called RAS, which activates the MAP kinase pathway described earlier.
- These pathways are controlled by signaling proteins, which are, in turn, expressed by genes.
- Mutations in these genes can result in malfunctioning signaling proteins.
- The genes that regulate the signaling proteins are one type of oncogene: a gene that has the potential to cause cancer.
- The gene encoding RAS is an oncogene that was originally discovered when mutations in the RAS protein were linked to cancer.
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- Common in cancer cells, silencing genes, which occur through epigenetic mechanisms, include modifications to histone proteins and DNA.
- Mechanisms of epigenetic silencing of tumor suppressor genes and activation of oncogenes include: alteration in CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins.
- Silencing genes through epigenetic mechanisms is very common in cancer cells and include modifications to histone proteins and DNA that are associated with silenced genes.
- In cancer cells, the DNA in the promoter region of silenced genes is methylated on cytosine DNA residues in CpG islands, genomic regions that contain a high frequency of CpG sites, where a cytosine nucleotide occurs next to a guanine nucleotide .
- Mechanisms can include modifications to histone proteins and DNA associated with these silencing genes.
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- Transcription elongation begins with the release of the polymerase σ subunit and terminates via the rho protein or via a stable hairpin.
- Near the end of the gene, the polymerase encounters a run of G nucleotides on the DNA template and it stalls.
- As a result, the rho protein collides with the polymerase.
- As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C–G nucleotides.
- The mRNA folds back on itself, and the complementary C–G nucleotides bind together.
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- Both the packaging of DNA around histone proteins, as well as chemical modifications to the DNA or proteins, can alter gene expression.
- How the histone proteins move is dependent on signals found on both the histone proteins and on the DNA.
- This occurs within very specific regions called CpG islands.
- This modification changes how the DNA interacts with proteins, including the histone proteins that control access to the region.
- Histone proteins and DNA nucleotides can be modified chemically.
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- Proteomics, the analysis of proteins, plays a prominent role in the study and treatment of cancer.
- An individual protein that indicates disease is called a biomarker, whereas a set of proteins with altered expression levels is called a protein signature .
- For a biomarker or protein signature to be useful as a candidate for early screening and detection of a cancer, it must be secreted in body fluids (e.g. sweat, blood, or urine) such that large-scale screenings can be performed in a non-invasive fashion.
- Some examples of protein biomarkers used in cancer detection are CA-125 for ovarian cancer and PSA for prostate cancer.
- Protein signatures may be more reliable than biomarkers to detect cancer cells.
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- Intron sequences in mRNA do not encode functional proteins.
- This is supported by the fact that separate exons often encode separate protein subunits or domains.
- All introns in a pre-mRNA must be completely and precisely removed before protein synthesis.
- The spliceosome cleaves the pre-mRNA's sugar phosphate backbone at the G that starts the intron and then covalently attaches that G to an internal A nucleotide within the intron.
- Initially, the conserved G which starts an intron is cleaved from the 3' end of the exon upstream to it and the G is covalently attached to an internal A within the intron.
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- This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways.
- The glycogen is hydrolyzed into the glucose monomer, glucose-1-phosphate (G-1-P), if blood sugar levels drop.
- Glycogen is broken down into G-1-P and converted into glucose-6-phosphate (G-6-P) in both muscle and liver cells; this product enters the glycolytic pathway.
- Galactose is converted in the liver to G-6-P and can thus enter the glycolytic pathway.
- Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units.