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What is the typical effect of phosphorylating serine and threonine residues, and what does phosphorylating tyrosine residues typically do?
ASerine or threonine phosphorylation always destroys the protein, and tyrosine phosphorylation has no known effect
BSerine or threonine phosphorylation often activates enzymes, tyrosine phosphorylation can affect activity or create a binding site
CTyrosine phosphorylation always activates enzymes, and serine or threonine phosphorylation creates a binding site
DBoth types of phosphorylation permanently and irreversibly switch off every protein they touch
Answer & Solution
Correct answer: B. Serine or threonine phosphorylation often activates enzymes, tyrosine phosphorylation can affect activity or create a binding site
1. Phosphorylation of serine and threonine residues often activates enzymes.
2. Phosphorylation of tyrosine residues can either affect enzyme activity or create a binding site for downstream signaling components.
3. Dephosphorylation, carried out by a phosphatase, reverses these effects, showing they are not permanent or irreversible.
4. The option claiming serine and threonine phosphorylation always destroys the protein contradicts the activation role described for these residues.
5. The option that swaps the two roles, giving tyrosine the activation role and serine or threonine the binding-site role, reverses the actual roles described for these residues.
6. The option describing an irreversible switch off for every protein ignores that phosphatases can reverse phosphorylation.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 9 "Cell Communication", section Phosphorylation_
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