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

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 Michaelis-Menten plot represents the kinetics of wild-type HMBS?

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