How might reduced enzyme function lead to disease?

When there is a mutation in a gene that produces an enzyme, it can cause disease. Phenylketonuria or PKU is a rare inherited disorder that causes the build up of phenylalanine. This can lead to brain damage in individuals with this disorder if they don't eat a very protein-- restricted diet. The gene mutation doesn't allow the creation of the enzymeneeded to break down Phenylalanine, an amino acid in proteins.   

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1.989×10^30 kg is the weight of the sun.