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【多选题】
How do enzyme-coupled receptors work?A.
Enzyme-coupled receptors are transmembrane proteins that display their ligand-binding domains on the outer surface of the plasma membrane. The cytoplasmic domain of the receptor either acts as an enzyme itself or forms a complex with another protein that acts as an enzyme.
B.
Enzyme-coupled receptors were discovered through their role in responses to extracellular signal proteins that regulate the growth, proliferation, differentiation, and survival of cells in animal tissues.
C.
Most of these signal proteins function as local mediators and can act at very low concentrations (about 10ˉ 9 to 10ˉ 11 M). Responses to them are typically slow (on the order of hours), and their effects may require many intracellular transduction steps that usually lead to a change in gene expression.
D.
The largest class of enzyme-coupled receptors consists of receptors with a cytoplasmic domain that functions as a tyrosine kinase, which phosphorylates particular tyrosines on specific intracellular signaling proteins. These receptors, called receptor tyrosine kinases (RTKs).
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举一反三
【单选题】What does the author mean by saying 'we are encouraged to hop when we have two perfectly good legs' (Para. 4) ?
A.
We needn't hop because we are healthy.
B.
Only those who have something wrong in their legs can be encouraged to hop.
C.
It's foolish of us to hop when we have two perfectly good legs.
D.
We should use both sides of the brain.