🧬AP Biology
6. The function of an enzyme depends on its specific three-dimensional shape, or tertiary structure, which is maintained by chemical interactions between amino acids in the polypeptide chain. Changes in environmental conditions, such as temperature, can disrupt these interactions, leading to protein unfolding (denaturation) and loss of function.
Scientists studied the thermal stability of a specific bacterial enzyme, Enzyme X. They compared the wild-type (WT) enzyme to a mutant version (Mutant M) that contains a single amino acid substitution. To determine the effect of the mutation, the scientists incubated samples of both enzymes at temperatures ranging from 30°C to 70°C for 15 minutes.
After incubation, the scientists measured the enzymatic activity of each sample (Figure 1A) and the fraction of the protein that remained in its folded, functional conformation (Figure 1B). The wild-type enzyme contains a lysine amino acid at position 105, which forms a stabilizing ionic bond within the protein's tertiary structure. In Mutant M, this lysine is replaced by a leucine amino acid.
Figure 1. Thermal stability comparison of Enzyme X after a 15-minute incubation. Panel A shows enzymatic activity as a percent of each enzyme’s maximum activity. Panel B shows the fraction of protein remaining folded (functional conformation). Wild-Type (WT) is compared with Mutant M (single amino acid substitution).
Based on Figure 1A, identify the temperature at which the Mutant M enzyme exhibits approximately 25% of its maximum activity.
Based on Figure 1B, describe the difference in the fraction of folded protein between the Wild-Type and Mutant M enzymes at 50°C.
Scientists hypothesize that the loss of enzymatic activity in Mutant M at higher temperatures is caused by the destabilization of the protein's tertiary structure. Use the data in Figures 1A and 1B to support the scientists' hypothesis.
The mutation in Mutant M substitutes lysine (a positively charged amino acid) with leucine (a nonpolar amino acid). Explain why this substitution results in the decreased stability observed in Figure 1B.
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