Why might a point mutation in DNA make a difference in the level of proteins activity?
It might substitute an amino acid in the active site For example the codon TTT codes for phenylalanine, TCT is serine, TAT is tyrosine and TGT is cystene. So, a point mutation (one base) in the second position of the sequence TTT would cause a protein that normally has a tyrosine in a particular location to have a tyrosine, for example, which could alter the tertiary (3D) structure and the charge distribution of the molecule, either or both of which could in the case of an enyme, change its ability to grasp and hold its substrate, or, in the case of a cellular membrane embedded protein in an ion channel, alter its ability to distinguish and permit passage of the ion it is supposed to. In hemoglobin, a single base mutation can cause the molecule to stack abnormally and allow the red cells that contain it to collapse into a sickle shape.
