5-Aminolevulinate synthase (ALAS) catalyzes the pyridoxal 5'-phosphate (PLP)-dependent condensation between glycine and succinyl-CoA to generate coenzyme A (CoA), CO2, and 5-aminolevulinate (ALA). The chemical mechanism of this reaction, which represents the first and regulated step of heme biosynthesis in mammals, involves the formation of a short-lived glycine quinonoid intermediate and an unstable 2-amino-3-ketoadipate intermediate. Using liquid chromatography coupled with tandem mass spectrometry to analyze the products from the reaction of murine erythroid ALAS (mALAS2) with O-methylglycine and succinyl-CoA, we directly identified the chemical nature of the inherently unstable 2-amino-3-ketoadipate intermediate, which predicates the glycine quinonoid species as its precursor. With stopped-flow absorption spectroscopy, we detected and confirmed the formation of the quinonoid intermediate upon reacting glycine with ALAS. Significantly, in the absence of the succinyl-CoA substrate, the external aldimine predominates over the glycine quinonoid intermediate. When instead of glycine, L-serine was reacted with ALAS, a lag phase was observed in the progress curve for the L-serine external aldimine formation, indicating a hysteretic behavior in ALAS. Hysteresis was not detected in the T148A-catalyzed L-serine external aldimine formation. These results with T148A, a mALAS2 variant, which, in contrast to the wild-type enzyme, is active with L-serine, suggest that the active site T148 modulates the strict amino acid substrate specificity of ALAS. The rate of ALA release is also controlled by a hysteretic kinetic mechanism (observed as a lag in the ALA external aldimine formation progress curve), consistent with conformational changes governing the dissociation of ALA from ALAS.
In Rhodobacter capsulatus ALAS, apart from coordinating the positioning of succinyl-CoA, N85 has an important role in regulating the opening of an active site channel. Here, we have mutated the analogous asparagine of murine erythroid ALAS to a histidine (N150H) and assessed its effects on catalysis through steady-state and pre-steady-state kinetic studies. Quinonoid intermediate formation occurred with a significantly reduced rate for the N150H-catalyzed condensation of glycine with succinyl-CoA during a single turnover. When the same forward reaction was examined under multiple turnovers, the progress curve of the N150H reaction displayed a prolonged decay of the quinonoid intermediate into the steady-state, distinct from the steep decay in the wild-type ALAS reaction. This prolonged decay results from an accelerated transformation of the product, ALA, into the quinonoid intermediate during the reverse N150H-catalyzed reaction. In fact, while wild-type ALAS catalyzes the conversion of ALA into the quinonoid intermediate at a rate 6.3-fold lower than the formation of the same quinonoid intermediate from glycine and succinyl-CoA, the rate for the N150H-catalyzed reverse reaction is 1.7-fold higher than that of the forward reaction. We conclude that N150 is important in establishing a catalytic balance between the forward and reverse reactions, by favoring ALA synthesis over its non-productive transformation into the quinonoid intermediate. Mutations at this position could perturb the delicate heme biosynthetic equilibrium.
Circular dichroism (CD) and fluorescence spectroscopies were used to examine the effects of pH (1.0-3.0 and 7.5-10.5) and temperature (20 and 37 °C) on the structural integrity of ALAS. The secondary structure, as deduced from far-UV CD, is mostly resilient to pH and temperature changes. Partial unfolding was observed at pH 2.0, but further decreasing pH resulted in acid-induced refolding of the secondary structure to nearly native levels. The tertiary structure rigidity, monitored by near-UV CD, is lost under acidic and specific alkaline conditions (pH 10.5 and pH 9.5/37 °C), where ALAS populates a molten globule state. As the enzyme becomes less structured with increased alkalinity, the chiral environment of the internal aldimine is also modified, with a shift from a 420 nm to 330 nm dichroic band. Under acidic conditions, the PLP cofactor dissociates from ALAS. Reaction with 8-anilino-1-naphtalenesulfonic acid corroborates increased exposure of hydrophobic clusters in the alkaline and acidic molten globules, although the reaction is more pronounced with the latter. Furthermore, quenching the intrinsic fluorescence of ALAS with acrylamide at pH 1.0 and 9.5 yielded subtly different dynamic quenching constants. The alkaline molten globule state of ALAS is catalytically active (pH 9.5/37 °C), although the kcat value is significantly decreased. Finally, the binding of 5-aminolevulinate restricts conformational fluctuations in the alkaline molten globule. Overall, our findings prove how the structural plasticity of ALAS contributes to reaching a functional enzyme.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-7075 |
| Date | 26 February 2015 |
| Creators | Stojanovski, Bosko M. |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
Page generated in 0.0018 seconds