Gprotein-coupled receptors are important molecules involved in signal transduction.
which statement about g protein-coupled receptors is true?
a. they are activated only by steroid hormones.
b. they bind both adenine and guanine nucleotides.
c. they generally contain seven membrane-spanning α helices.
d. they bind only guanine nucleotides.

C. They generally contain seven membrane-spanning α helices.

Explanation:

The G protein-coupled receptors are components of signal transduction through G protein. Here, a plasma membrane receptor with seven transmembrane helical segments serves as a G-protein coupled receptor. These receptors are the integral membrane proteins and have seven hydrophobic regions of 20 to 28 amino acid residues.

These hydrophobic alpha-helices span the membrane back and forth seven times. Binding of a signaling molecule to the G-protein coupled receptor promoters a conformational change in the receptor’s intracellular domain. The activated receptor then triggers the replacement of bound GDP to the G protein by GTP and thereby activating it.

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well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."