A type of muscular dystrophy, called severe childhood autosomal recessive muscular dystrophy (SCARMD), results from a mutation in the gene for a 50-kD muscle protein. The defective protein leads to muscle necrosis. Detailed studies of this protein have revealed that an arginine residue at position 98 has been mutated to a histadine. Why might replacing an arginine with a histadine result in a defective protein?

Difference in pKa and PI values of Arginine and histadine

Explanation:

There is a huge difference between the pK values of the two amino acid groups for different groups  

For instance – for alpha CO2H group, PK value of arginine and histidine  is 2.18 and 1.78 respectively

Similarly, for NH3 group, PK value of arginine and histidine  is 9.09 and 8.97 respectively

And for R –group, PK value of arginine and histidine  is 13.2 and 5.97  respectively

Also there are difference in the isoelectronic point or isoionic point  of the two groups at a given temperature due to which the two amino acids migrate differently in an electric field.  

This give rise to a non-conservative replacement where replacement of one group by the other makes significant differences in the protein structure and function.