🧬AP Biology
1. Proteins are essential macromolecules that perform a diverse range of functions in living organisms, including catalyzing metabolic reactions. The function of a protein is directly determined by its specific three-dimensional shape, which is established by interactions between its constituent amino acids.
Researchers are investigating the enzyme dihydrofolate reductase (DHFR) in a species of bacteria, Thermophilus aquaticus, which lives in hot springs, and a related species, Escherichia coli, which lives in the mammalian gut. DHFR is an enzyme critical for DNA synthesis. The researchers purified DHFR from both species to study their thermal stability.
They incubated the purified enzymes from both species at temperatures ranging from 30°C to 90°C for 10 minutes. After incubation, they cooled the samples to 37°C and measured the remaining enzyme activity by adding substrate and monitoring product formation. The activity was calculated as a percentage relative to the activity of the enzyme kept at 37°C (Figure 1).
To understand the molecular basis for the difference in thermal stability, the researchers analyzed the protein structures. They focused on a specific region of the protein surface where the amino acid sequence differed between the two species. Figure 2 shows a simplified model of the interactions between amino acid side chains in this region for both the E. coli and T. aquaticus enzymes.
Describe how the R-groups of amino acids contribute to the tertiary structure of a protein.
Figure 1. Effect of 10-minute incubation temperature on DHFR activity. Relative enzyme activity is measured at 37°C after cooling and is expressed as a percent of the 37°C control for each species. Error bars show ±SE.
Identify the independent variable in the experiment shown in Figure 1.
Justify the researchers' decision to cool the enzyme samples to 37°C before measuring their activity, rather than measuring activity at the incubation temperatures.
Based on Figure 1, describe the effect of increasing temperature from 40°C to 60°C on the activity of the E. coli enzyme.
Figure 2. Amino acid side-chain interactions in a structural domain of DHFR from E. coli versus T. aquaticus. Panel A shows nonionic hydrophobic side chains without an inter-side-chain bond; Panel B shows an ionic interaction (salt bridge) between oppositely charged side chains.
Based on Figure 2, identify the specific type of chemical interaction present in the T. aquaticus enzyme that is absent in the E. coli enzyme.
Identify the level of protein structure (primary, secondary, tertiary, or quaternary) that is stabilized by the interaction between the side chains of Glutamic Acid and Lysine shown in Figure 2.
The molecular mass of the amino acid Glutamic Acid is 147 Daltons, and the molecular mass of Valine is 117 Daltons. Calculate the difference in mass (in Daltons) of the protein if a single Glutamic Acid residue is substituted with a Valine residue.
Researchers claim that the T. aquaticus enzyme is evolutionarily adapted to function in high-temperature environments. Using data from Figure 1, support the researchers' claim.
Researchers claim that replacing the Glutamic Acid in the T. aquaticus enzyme with Valine would decrease the thermal stability of the enzyme. Justify the researchers' claim based on the information in Figure 2 and your understanding of protein structure.
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