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
-What type of reaction does porphobilinogen synthase catalyze?

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
-What type of reaction does porphobilinogen synthase catalyze?

Quizplus · Accepted Answer

11ecba2d_0f01_70d5_a3bf_81805fec3902_MD0008_00 Enzymes are biological catalysts that increase the rate of a reaction. They are broadly divided into six classes based on the general reaction types they catalyze. Each class is further divided into subclasses based on specific reaction types. Lyases break bonds by forming double bonds or rings (double-bond equivalents) elsewhere. Porphobilinogen synthase is a lyase that combines two δ-aminolevulinate molecules by breaking C=O bonds and forming C=C bonds and an aromatic ring. Reaction 1 shows that two water molecules are released during bond formation in PBGN synthesis. Reactions in which molecules release water during bond formation are known as condensation reactions. Therefore, porphobilinogen synthase catalyzes a condensation reaction. (Choice A) Hydrolysis reactions consume water to break bonds. Reaction 1 shows that porphobilinogen synthase catalyzes a reaction that produces water (a condensation reaction). (Choice C) Deamination is the removal of one or more amino groups from a molecule, typically in the form of ammonia (NH₄⁺). Deamination occurs in Reaction 2, catalyzed by HMBS, but does not occur in the reaction catalyzed by porphobilinogen synthase. (Choice D) Methylation is the addition of a methyl group (-CH₃) to a molecule. None of the reactions shown in the passage are methylation reactions. Educational objective: Enzymes are biological catalysts that are classified according to the reactions they catalyze. Condensation reactions join two molecules together, and release one or more water molecules in the process.

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