Question

which statement about posttranslational modifications of proteins is false?
they are not directed by the sequence of the protein.
they can include the cleavage of signal sequences.
they can affect the activity of an enzyme.
they can include the addition of polysaccharides to proteins.
they may operate in signal transduction.

Explanation:

Posttranslational modifications (PTMs) are directed by the protein's amino acid sequence (e.g., specific residues for phosphorylation/glycosylation), so the claim they are not directed by the sequence is false. Other options are true: PTMs include signal sequence cleavage, polysaccharide addition (glycosylation), affect enzyme activity, and operate in signal transduction.

Answer:

A. They are not directed by the sequence of the protein.