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.
-ADC isolated from the Gram-negative bacteria Alcaligenes faecalis showed a sigmoidal dependence on L-aspartate concentration. Which of the following could be the Hill coefficient for this form of ADC?

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. -ADC isolated from the Gram-negative bacteria Alcaligenes faecalis showed a sigmoidal dependence on L-aspartate concentration. Which of the following could be the Hill coefficient for this form of ADC? A) 0 B) 0.5 C) 1 D) 2

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.
-ADC isolated from the Gram-negative bacteria Alcaligenes faecalis showed a sigmoidal dependence on L-aspartate concentration. Which of the following could be the Hill coefficient for this form of ADC?

Quizplus · Accepted Answer

11ecba2d_0e29_fc53_a3bf_81635728acc0_MD0008_00 An enzyme or protein with multiple subunits is described as cooperative if a ligand or substrate binding to one subunit changes the binding affinity of other subunits. The Hill coefficient n is a quantitative measure of cooperativity obtained through kinetic analysis. It can be determined by fitting the data to a modified form of the Michaelis-Menten equation: 11ecba2d_0e29_fc54_a3bf_bfa67859ae82_MD0008_00 V0=Vmax [S]n Kmn+[S]n Enzymes that exhibit increased binding affinity when the first subunit binds its substrate are described as positively cooperative (n > 1); these enzymes display sigmoidal (s-shaped) dependence on substrate concentration. In contrast, some enzymes show decreased binding affinity after the first subunit binds its substrate. These enzymes are said to be negatively cooperative (n < 1) and show hyperbolic dependence on substrate concentration. In addition, many enzymes exhibit no cooperativity (n = 1) and also display hyperbolic dependence on substrate concentration. The ADC obtained from Alcaligenes faecalis displays sigmoidal dependence on the concentration of its substrate, L-aspartate. Therefore, it exhibits positive cooperativity and must have a Hill coefficient that is greater than 1. (Choice A) A Hill coefficient of 0 would indicate infinitely negative cooperativity. All enzymes have a Hill coefficient greater than 0. (Choice B) A Hill coefficient of 0.5 would indicate negative cooperativity and would show hyperbolic (not sigmoidal) dependence on substrate concentration. (Choice C) A Hill coefficient of 1 would indicate no cooperativity and would appear as a hyperbolic curve rather than a sigmoidal curve. Educational objective: The Hill coefficient n is a quantitative measure of cooperativity. Enzymes in which n > 1 display positive cooperativity and have sigmoidal dependence on substrate concentration. Enzymes where n = 1 or n < 1 exhibit no cooperativity or negative cooperativity, respectively, and exhibit hyperbolic kinetics.