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Passage Porphyrin Biosynthesis Is a Multistep Process That Is Required for for Production

Question 7

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Passage
Porphyrin biosynthesis is a multistep process that is required for production of porphyrin derivatives such as heme, which is a crucial component of hemoglobin, myoglobin, and the cytochrome cofactors of the electron transport chain.  In the first step of this pathway, the enzyme porphobilinogen synthase catalyzes the production of porphobilinogen (PBGN) from two δ-aminolevulinate molecules, as shown in Reaction 1.
Passage Porphyrin biosynthesis is a multistep process that is required for production of porphyrin derivatives such as heme, which is a crucial component of hemoglobin, myoglobin, and the cytochrome cofactors of the electron transport chain.  In the first step of this pathway, the enzyme porphobilinogen synthase catalyzes the production of porphobilinogen (PBGN) from two δ-aminolevulinate molecules, as shown in Reaction 1. <strong>Reaction 1</strong>In the next step, the monomeric enzyme hydroxymethylbilane synthase (HMBS) combines four PBGN molecules to form hydroxymethylbilane (Reaction 2) .  Several additional steps are required to convert hydroxymethylbilane to heme. <strong>Reaction 2</strong>Defects in HMBS activity can lead to accumulation of δ-aminolevulinate and porphobilinogen in the liver and blood plasma.  The accumulation of these molecules can cause acute intermittent porphyria, a disorder characterized by episodes of severe abdominal pain.  Researchers expressed, purified, and assessed several variants of HMBS for enzymatic activity (Table 1) and thermal stability (Figure 1) .<strong>Table 1</strong>  Michaelis-Menten equations generated from several HMBS variants, with V<sub>0</sub> and V<sub>max</sub> measured in nmol/hr and [PBGN] and K<sub>m</sub> measured in μM.  (Note: All reactions were carried out with 1 μM HMBS.) <strong>Figure 1</strong>  Thermal stability of several HMBS variants -Which statement most accurately describes the binding of PBGN to HMBS?  (Note: Assume binding is significantly faster than catalysis.) A) PBGN has a higher affinity for the wild-type enzyme than for any other enzyme form. B) PBGN has a higher affinity for the K132N variant than for the V215E variant. C) PBGN has a lower affinity for the R167W variant than for any other enzyme form. D) PBGN binds all forms of the enzyme with approximately equal affinity. Reaction 1In the next step, the monomeric enzyme hydroxymethylbilane synthase (HMBS) combines four PBGN molecules to form hydroxymethylbilane (Reaction 2) .  Several additional steps are required to convert hydroxymethylbilane to heme.
Passage Porphyrin biosynthesis is a multistep process that is required for production of porphyrin derivatives such as heme, which is a crucial component of hemoglobin, myoglobin, and the cytochrome cofactors of the electron transport chain.  In the first step of this pathway, the enzyme porphobilinogen synthase catalyzes the production of porphobilinogen (PBGN) from two δ-aminolevulinate molecules, as shown in Reaction 1. <strong>Reaction 1</strong>In the next step, the monomeric enzyme hydroxymethylbilane synthase (HMBS) combines four PBGN molecules to form hydroxymethylbilane (Reaction 2) .  Several additional steps are required to convert hydroxymethylbilane to heme. <strong>Reaction 2</strong>Defects in HMBS activity can lead to accumulation of δ-aminolevulinate and porphobilinogen in the liver and blood plasma.  The accumulation of these molecules can cause acute intermittent porphyria, a disorder characterized by episodes of severe abdominal pain.  Researchers expressed, purified, and assessed several variants of HMBS for enzymatic activity (Table 1) and thermal stability (Figure 1) .<strong>Table 1</strong>  Michaelis-Menten equations generated from several HMBS variants, with V<sub>0</sub> and V<sub>max</sub> measured in nmol/hr and [PBGN] and K<sub>m</sub> measured in μM.  (Note: All reactions were carried out with 1 μM HMBS.) <strong>Figure 1</strong>  Thermal stability of several HMBS variants -Which statement most accurately describes the binding of PBGN to HMBS?  (Note: Assume binding is significantly faster than catalysis.) A) PBGN has a higher affinity for the wild-type enzyme than for any other enzyme form. B) PBGN has a higher affinity for the K132N variant than for the V215E variant. C) PBGN has a lower affinity for the R167W variant than for any other enzyme form. D) PBGN binds all forms of the enzyme with approximately equal affinity. Reaction 2Defects in HMBS activity can lead to accumulation of δ-aminolevulinate and porphobilinogen in the liver and blood plasma.  The accumulation of these molecules can cause acute intermittent porphyria, a disorder characterized by episodes of severe abdominal pain.  Researchers expressed, purified, and assessed several variants of HMBS for enzymatic activity (Table 1) and thermal stability (Figure 1) .Table 1  Michaelis-Menten equations generated from several HMBS variants, with V0 and Vmax measured in nmol/hr and [PBGN] and Km measured in μM.  (Note: All reactions were carried out with 1 μM HMBS.)
Passage Porphyrin biosynthesis is a multistep process that is required for production of porphyrin derivatives such as heme, which is a crucial component of hemoglobin, myoglobin, and the cytochrome cofactors of the electron transport chain.  In the first step of this pathway, the enzyme porphobilinogen synthase catalyzes the production of porphobilinogen (PBGN) from two δ-aminolevulinate molecules, as shown in Reaction 1. <strong>Reaction 1</strong>In the next step, the monomeric enzyme hydroxymethylbilane synthase (HMBS) combines four PBGN molecules to form hydroxymethylbilane (Reaction 2) .  Several additional steps are required to convert hydroxymethylbilane to heme. <strong>Reaction 2</strong>Defects in HMBS activity can lead to accumulation of δ-aminolevulinate and porphobilinogen in the liver and blood plasma.  The accumulation of these molecules can cause acute intermittent porphyria, a disorder characterized by episodes of severe abdominal pain.  Researchers expressed, purified, and assessed several variants of HMBS for enzymatic activity (Table 1) and thermal stability (Figure 1) .<strong>Table 1</strong>  Michaelis-Menten equations generated from several HMBS variants, with V<sub>0</sub> and V<sub>max</sub> measured in nmol/hr and [PBGN] and K<sub>m</sub> measured in μM.  (Note: All reactions were carried out with 1 μM HMBS.) <strong>Figure 1</strong>  Thermal stability of several HMBS variants -Which statement most accurately describes the binding of PBGN to HMBS?  (Note: Assume binding is significantly faster than catalysis.) A) PBGN has a higher affinity for the wild-type enzyme than for any other enzyme form. B) PBGN has a higher affinity for the K132N variant than for the V215E variant. C) PBGN has a lower affinity for the R167W variant than for any other enzyme form. D) PBGN binds all forms of the enzyme with approximately equal affinity. Passage Porphyrin biosynthesis is a multistep process that is required for production of porphyrin derivatives such as heme, which is a crucial component of hemoglobin, myoglobin, and the cytochrome cofactors of the electron transport chain.  In the first step of this pathway, the enzyme porphobilinogen synthase catalyzes the production of porphobilinogen (PBGN) from two δ-aminolevulinate molecules, as shown in Reaction 1. <strong>Reaction 1</strong>In the next step, the monomeric enzyme hydroxymethylbilane synthase (HMBS) combines four PBGN molecules to form hydroxymethylbilane (Reaction 2) .  Several additional steps are required to convert hydroxymethylbilane to heme. <strong>Reaction 2</strong>Defects in HMBS activity can lead to accumulation of δ-aminolevulinate and porphobilinogen in the liver and blood plasma.  The accumulation of these molecules can cause acute intermittent porphyria, a disorder characterized by episodes of severe abdominal pain.  Researchers expressed, purified, and assessed several variants of HMBS for enzymatic activity (Table 1) and thermal stability (Figure 1) .<strong>Table 1</strong>  Michaelis-Menten equations generated from several HMBS variants, with V<sub>0</sub> and V<sub>max</sub> measured in nmol/hr and [PBGN] and K<sub>m</sub> measured in μM.  (Note: All reactions were carried out with 1 μM HMBS.) <strong>Figure 1</strong>  Thermal stability of several HMBS variants -Which statement most accurately describes the binding of PBGN to HMBS?  (Note: Assume binding is significantly faster than catalysis.) A) PBGN has a higher affinity for the wild-type enzyme than for any other enzyme form. B) PBGN has a higher affinity for the K132N variant than for the V215E variant. C) PBGN has a lower affinity for the R167W variant than for any other enzyme form. D) PBGN binds all forms of the enzyme with approximately equal affinity. Figure 1  Thermal stability of several HMBS variants
-Which statement most accurately describes the binding of PBGN to HMBS?  (Note: Assume binding is significantly faster than catalysis.)


A) PBGN has a higher affinity for the wild-type enzyme than for any other enzyme form.
B) PBGN has a higher affinity for the K132N variant than for the V215E variant.
C) PBGN has a lower affinity for the R167W variant than for any other enzyme form.
D) PBGN binds all forms of the enzyme with approximately equal affinity.

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