Why might a point mutation in dna make a difference in the level of a protein’s activity?

Why might a point mutation in DNA make a difference in the level
of protein’s activity substitute an amino acid in the active site??

It seems that the mutation would be
a missense mutation, in which the mutated triplet codon codes for a
different amino acid from the actual that alters the active site
amino acids interaction to bind with a suitable substrate. The
change from charged to uncharged or neutral amino acid or +ve to
-ve amino acids or vice versa strictly affects the electrostatic
interaction among active site amino acids that affects the active
site conformation. So that substrate may bind or may not bind and
how much efficiently interacting with this modified active site
will determine the protein activity.
For example, in the sickle cell
anemia beta-hemoglobin activity altered due to a mutation in a
single codon GAG to GUG that codes for valine instead of glutamic
The gain o function also appears in
some proteins, an example that the mutation alters a valine to
glutamic acid in the braf gene which activates the RAF
protein results in unlimited proliferative signal leads to

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