Summary
Highlights
Epinephrine, released during stress, binds to a transmembrane receptor on liver cells, causing a conformational change that activates a G protein. The activated G protein activates adenylyl cyclase, converting ATP to cyclic AMP (a second messenger). Cyclic AMP activates Protein Kinase A, initiating a phosphorylation cascade that ultimately breaks down glycogen into glucose for energy during fight or flight.
After a meal, high blood sugar triggers the pancreas to release insulin. Insulin binds to receptor proteins on target cells, activating them to form a dimer. This activates intracellular proteins, including kinases, leading to various cell responses like lipid metabolism and glycogen synthesis. Critically, it also causes vesicles containing GLUT4 transporter proteins to fuse with the cell membrane, allowing glucose to enter the cell and lower blood sugar.
Ethylene, a gas hormone, plays a role in fruit ripening. In the absence of ethylene, a receptor activates CTR1 kinase, which inhibits the EIN2 protein, preventing ripening. When ethylene binds to the receptor, it inhibits CTR1 kinase, allowing EIN2 to become active. EIN2 then produces a second messenger, leading to a signaling cascade that activates transcription factors in the nucleus, resulting in fruit ripening.
Epidermal Growth Factor (EGF) initiates cell division. When EGF binds to its transmembrane receptor, the receptors dimerize and become active, phosphorylating and activating intracellular proteins. This cascade activates the Ras protein, which then activates a series of kinases (Raf, Mek, Erk). The active Erk protein phosphorylates transcription factors in the nucleus, turning on genes necessary for cell division and growth. Mutations causing Ras to be continually active are linked to uncontrolled cell growth in cancer.
Steroid hormones, like testosterone, are lipid-soluble and can diffuse directly through the cell membrane and nuclear envelope. Their protein receptor is located inside the cell. Once testosterone binds to its receptor, the complex acts as a transcription factor, activating specific gene expression. For example, during puberty, this leads to changes such as voice deepening, increased muscle mass, and hair growth.