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 best describes the ADC activation mechanism?

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 best describes the ADC activation mechanism? A) AcCoA binds to the PanD-PanZ complex to induce cleavage. B) PanD, PanZ, and AcCoA bind in a random order to form a ternary complex. C) A binary complex composed of PanD and AcCoA cleaves PanZ. D) PanZ binds AcCoA followed by PanD to form a ternary complex.

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 best describes the ADC activation mechanism?

Quizplus · Accepted Answer

11ecba2d_0e25_1b4a_a3bf_adc4fbddf201_MD0008_00 Ternary complexes form when three distinct molecules, at least one of which is a protein, bind to each other to form a single system. They can form through random or ordered mechanisms. In random reactions, the sequence of binding does not affect the progress of the reaction, and products are released in no specific order. In contrast, ordered reactions require that the components of the complex bind in a defined sequence for the reaction to proceed. The ADC activation mechanism described in the passage forms a ternary complex in an ordered fashion. Active ADC is only produced after PanD binds the PanZ-AcCoA complex, forming the ternary complex PanD-PanZ-AcCoA. Because PanZ and AcCoA must already exist as a complex before PanD can bind, the first step of ternary complex formation must be PanZ binding to AcCoA. Only then can the catalytic conversion of PanD to ADC occur. (Choice A) PanD will not bind PanZ or undergo cleavage unless AcCoA is bound to PanZ first. (Choice B) The PanD-PanZ-AcCoA ternary complex is not formed by a random binding mechanism. PanZ and AcCoA bind each other first, and then PanD binds to the resulting complex. (Choice C) The binary complex that forms prior to ternary complex formation is composed of PanZ and AcCoA rather than PanD and AcCoA. PanZ-AcCoA then binds to PanD, inducing a conformational shift in PanD that allows it to cleave its own N-terminal region. Educational objective: For ternary enzyme complexes (one enzyme and two substrates), an ordered reaction requires that substrates and products be bound and released in a specific sequence. For random reactions, the order in which substrates bind is not important and does not affect the progress of the reaction.