Passage
Synthesis of coenzyme A (CoA) requires vitamin B₅ as a substrate. In bacteria, vitamin B₅ is synthesized by condensation of β-alanine and pantoic acid. β-alanine is produced by decarboxylation of L-aspartate, mediated by the enzyme aspartate α-decarboxylase (ADC) . Active ADC is formed when its inactive precursor, PanD, binds to a regulatory complex composed of acetyl coenzyme A (AcCoA) and a protein known as PanZ. Formation of the PanD-PanZ-AcCoA complex causes the activation loop of PanD to fold into a β-pleated sheet, resulting in PanD autocleavage. Researchers hypothesize that this conformational change allows the Thr57 residue to cleave the N-terminal end of PanD to form ADC.The limited information on the role of PanZ in the synthesis of CoA has stimulated an investigation of its effect on ADC activity. Escherichia coli cells were provided with varying concentrations of PanZ, and the rate of β-alanine production was measured in three different conditions (Figure 1) . Reaction vessels were maintained at pH 6 to ensure optimal decarboxylation activity.
Figure 1 Catalytic activity of ADC in E. coli with increasing concentrations of L-aspartate
Adapted from Monteiro DC, Patel V, Bartlett CP, et al. The structure of the PanD/PanZ protein complex reveals negative feedback regulation of pantothenate biosynthesis by coenzyme A. Chem Biol. 2015;22(4) :492-503.
-Which of the following graphs accurately predicts the aspartate decarboxylation activity of PanD and ADC between pH 3 and pH 9?

Question

Passage
Synthesis of coenzyme A (CoA) requires vitamin B₅ as a substrate. In bacteria, vitamin B₅ is synthesized by condensation of β-alanine and pantoic acid. β-alanine is produced by decarboxylation of L-aspartate, mediated by the enzyme aspartate α-decarboxylase (ADC) . Active ADC is formed when its inactive precursor, PanD, binds to a regulatory complex composed of acetyl coenzyme A (AcCoA) and a protein known as PanZ. Formation of the PanD-PanZ-AcCoA complex causes the activation loop of PanD to fold into a β-pleated sheet, resulting in PanD autocleavage. Researchers hypothesize that this conformational change allows the Thr57 residue to cleave the N-terminal end of PanD to form ADC.The limited information on the role of PanZ in the synthesis of CoA has stimulated an investigation of its effect on ADC activity. Escherichia coli cells were provided with varying concentrations of PanZ, and the rate of β-alanine production was measured in three different conditions (Figure 1) . Reaction vessels were maintained at pH 6 to ensure optimal decarboxylation activity.

Passage Synthesis of coenzyme A (CoA) requires vitamin B<sub>5</sub> as a substrate. In bacteria, vitamin B<sub>5</sub> is synthesized by condensation of β-alanine and pantoic acid. β-alanine is produced by decarboxylation of L-aspartate, mediated by the enzyme aspartate α-decarboxylase (ADC) . Active ADC is formed when its inactive precursor, PanD, binds to a regulatory complex composed of acetyl coenzyme A (AcCoA) and a protein known as PanZ. Formation of the PanD-PanZ-AcCoA complex causes the activation loop of PanD to fold into a β-pleated sheet, resulting in PanD autocleavage. Researchers hypothesize that this conformational change allows the Thr57 residue to cleave the N-terminal end of PanD to form ADC.The limited information on the role of PanZ in the synthesis of CoA has stimulated an investigation of its effect on ADC activity. Escherichia coli cells were provided with varying concentrations of PanZ, and the rate of β-alanine production was measured in three different conditions (Figure 1) . Reaction vessels were maintained at pH 6 to ensure optimal decarboxylation activity. <strong>Figure 1</strong> Catalytic activity of ADC in E. coli with increasing concentrations of L-aspartate Adapted from Monteiro DC, Patel V, Bartlett CP, et al. The structure of the PanD/PanZ protein complex reveals negative feedback regulation of pantothenate biosynthesis by coenzyme A. Chem Biol. 2015;22(4) :492-503. -Which of the following graphs accurately predicts the aspartate decarboxylation activity of PanD and ADC between pH 3 and pH 9? A) B) C) D)

Figure 1 Catalytic activity of ADC in E. coli with increasing concentrations of L-aspartate
Adapted from Monteiro DC, Patel V, Bartlett CP, et al. The structure of the PanD/PanZ protein complex reveals negative feedback regulation of pantothenate biosynthesis by coenzyme A. Chem Biol. 2015;22(4) :492-503.
-Which of the following graphs accurately predicts the aspartate decarboxylation activity of PanD and ADC between pH 3 and pH 9?

Quizplus · Accepted Answer

11ecba2d_0e29_11f2_a3bf_b1a442a20b82_MD0008_00 Changes in the pH of the aqueous environment can alter the charge and folding of proteins. Enzymes in particular require a specific pH range to maintain their activity and structure. The highest enzymatic activity is achieved at a particular pH that is specific to the enzyme (ie, optimal pH). Activity is greatly reduced at pH values that are significantly lower or higher than the optimal value. The passage states that reactions were maintained at pH 6 to ensure that ADC reached its highest enzymatic activity. Therefore, a graph of ADC activity ranging from pH 3 to pH 9 would appear as a bell curve with a peak at pH 6. All other pH levels should show lower activity. The passage also states that PanD is the inactive form of the enzyme ADC and cannot decarboxylate L-aspartate. Enzymes in their inactive forms, referred to as zymogens or proenzymes, must be altered by covalent modifications to become active. In the case of PanD, proteolytic cleavage (proteolysis) is used to activate the proenzyme. Because PanD is inactive, it displays no aspartate decarboxylation activity at any pH. Only Choice D illustrates both the correct optimal pH of ADC and the inactivity of PanD. (Choice A, B, and C) These graphs incorrectly show aspartate decarboxylation activity for PanD. Graphs B and C also incorrectly show optimal ADC activity at a pH other than 6. Educational objective: The activity of an enzyme is greatest at the optimal pH of that enzyme, and gradually decreases at pH values that are significantly lower or higher. Proenzymes or zymogens are the inactive forms of enzymes that require post-translational modifications to become activated. Proteolytic cleavage (proteolysis) is one such post-translational modification that involves the breakdown of proteins into shorter polypeptides or amino acids by enzymes known as proteases.