How can a mutation decrease the activity of a protein?

We need to explain how mutation decrease the activity of a protein.

Some mutations do not cause a significant change in the primary structure of a protein and the protein is able to maintain biological activity. Following is explanation of decrease in activity of a protein due to mutation:

The protein loses biological activity if the mutation changes an amino acid that is important for protein structure or function. If the protein is an enzyme, it may lose its ability to bind to its substrate or to react with it at the active site. When a changed enzyme can't catalyze a process, certain compounds can build up in the cell until they become poisonous, or essential substances can't be created. A mutation that arises in a major metabolic pathway or is involved in the construction of a cell membrane can be fatal. A genetic illness is defined as a protein deficit that is inherited.