Mapping hydrophobic residues of the interaction surface of charybdotoxin

association rates are all within a factor oftwo to threeofthe wild-type value and the block times are voltagedependent. This strongly supports the assumption thatthe mutants are all similar to wild-typeCTX in overallstructure.Atposition 29 even a very conservative mutation (Metto Leu) produces a drastic decrease in affinity. Thetyrosine at position 36 represents a special case where itis possible to produce a series of mutations with dif­ferent effects. Phe, which is the most conservativesubstitute, has noeffectonthechannelblocking; replace­ment ofthe large hydrophobic Tyr with the small polarAsn decreases the affinity 20-fold.Substituting by Ala,His or Pro totally abolishes blocking activity. The 7-and