Discuss how only one blocked metabolic pathway, such as the one leading from phenylalanine, has the potential to
cause many different disorders. What will be an ideal response?
ANSWER: In a metabolic pathway, the product of one reaction serves as the substrate for the next
reaction. If a mutation shuts down an enzyme that performs one step in a pathway, all the
reactions beyond that point are also shut down, because there is no substrate available for
reactions beyond the one that is blocked. If one reaction in a pathway is shut down, it can
also result in the toxic accumulation of products leading up to the block. In the case of the
phenylalanine pathway, normally, phenylalanine is converted to tyrosine, and from there into
many other compounds. A metabolic block caused by a mutation in the gene that encodes the
enzyme phenylalanine hydroxylase prevents the conversion of phenylalanine to tyrosine.
People homozygous for this mutation have the phenotype of phenylketonuria (PKU). But
other genetic disorders such as genetic goitrous cretinism, alkaptonuria, and albinism are also
produced by mutations in genes that encode other enzymes in this pathway.
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