🧬AP Biology
6. Polyphenol oxidase (PPO) is an enzyme found in many plants that catalyzes the oxidation of phenolic compounds, leading to the browning of tissues upon injury. The function of PPO depends on its specific three-dimensional protein structure.
Scientists investigated the effect of mutations in the PPO gene on the enzyme's function. They studied four groups of plants with different genotypes: homozygous wild-type (WT/WT), heterozygous for a deletion mutation (WT/del), homozygous for the deletion mutation (del/del), and homozygous for a substitution mutation (sub/sub).
The scientists extracted proteins from the leaves of each group. They measured the relative concentration of PPO protein (Figure 1A) and the specific enzyme activity, defined as the rate of product formation per minute (Figure 1B), for each genotype.
Figure 1. (A) Relative concentration of PPO protein detected in leaf extracts and (B) relative PPO enzyme activity measured in the same extracts for four plant genotypes.
Based on Figure 1A, identify the genotype that produces the lowest amount of PPO protein.
Based on Figure 1B, describe the difference in enzyme activity between the WT/del plants and the sub/sub plants.
Scientists hypothesize that the PPO enzyme is produced in excess of what is needed for a wild-type response, such that a reduction in protein amount does not necessarily reduce total activity. Use the data in Figures 1A and 1B to support the scientists' hypothesis.
The sub/sub mutation results in the substitution of a polar histidine amino acid with a nonpolar leucine amino acid in the active site of the enzyme. Explain why this change in the amino acid sequence results in the phenotype observed in Figure 1B.
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