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.
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.
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 V_max measured in nmol/hr and [PBGN] and K_m measured in μM. (Note: All reactions were carried out with 1 μM HMBS.)
Figure 1 Thermal stability of several HMBS variants
-Which HMBS mutant variant is LEAST likely to cause acute intermittent porphyria?

Question

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.

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.

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 V_max measured in nmol/hr and [PBGN] and K_m measured in μM. (Note: All reactions were carried out with 1 μM HMBS.)

Figure 1 Thermal stability of several HMBS variants
-Which HMBS mutant variant is LEAST likely to cause acute intermittent porphyria?

Quizplus · Accepted Answer

11ecba2d_0f06_a101_a3bf_e1fcb530ba4a_MD0008_00 Genetic disorders commonly occur when one or more mutations in a gene cause the protein expressed by that gene to have impaired function. Different genetic mutations may impair function to varying degrees. Some mutations completely inactivate the protein, whereas others essentially have no effect. To determine how likely a mutation is to cause a genetic disorder, the activity of the mutant protein should be compared to the wild-type form. The passage states that defects in HMBS can lead to accumulation of δ-aminolevulinate and porphobilinogen, which can cause acute intermittent porphyria. Accumulation most likely occurs because HMBS fails to catalyze conversion of PBGN to hydroxymethylbilane. The Michaelis-Menten equations in Table 1 indicate how quickly each HMBS variant catalyzes the reaction. The numerator of each equation shows the maximum possible reaction velocity (V_max) for each variant. According to the table, the V_max of the K132N variant (1160 nmol/h) is the closest to the V_max of the wild-type form (1250 nmol/h). The K_m of the K132N variant (found in the denominator) is also similar to the K_m of the wild-type form. Therefore, of the mutant variants shown, K132N exhibits activity that is most similar to wild-type HMBS and would be least likely to cause acute intermittent porphyria. (Choices A, C, and D) Table 1 shows that the C73R, R167W, and V215E mutants each have significantly lower V_max values than wild-type HMBS, and therefore they catalyze the reaction much more slowly. A decreased reaction rate would lead to accumulation of reactants (ie, PBGN), so these variants are likely to cause acute intermittent porphyria. Educational objective: Genetic disorders are commonly caused by mutations to genes that reduce the activity of the proteins they express. The impact of a mutation can be assessed by comparing the activities of mutant forms of the protein to the activity of the wild-type form.

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