Article

Molecular enzymology of 5-Aminolevulinate synthase, the gatekeeper of heme biosynthesis

Department of Molecular Medicine, College of Medicine, University of South Florida, Tampa, Florida 33612-4799, USA.
Biochimica et Biophysica Acta (Impact Factor: 4.66). 11/2011; 1814(11):1467-73. DOI: 10.1016/j.bbapap.2010.12.015
Source: PubMed

ABSTRACT

Pyridoxal-5'-phosphate (PLP) is an obligatory cofactor for the homodimeric mitochondrial enzyme 5-aminolevulinate synthase (ALAS), which controls metabolic flux into the porphyrin biosynthetic pathway in animals, fungi, and the α-subclass of proteobacteria. Recent work has provided an explanation for how this enzyme can utilize PLP to catalyze the mechanistically unusual cleavage of not one but two substrate amino acid α-carbon bonds, without violating the theory of stereoelectronic control of PLP reaction-type specificity. Ironically, the complex chemistry is kinetically insignificant, and it is the movement of an active site loop that defines k(cat) and ultimately, the rate of porphyrin biosynthesis. The kinetic behavior of the enzyme is consistent with an equilibrium ordered induced-fit mechanism wherein glycine must bind first and a portion of the intrinsic binding energy with succinyl-Coenzyme A is then utilized to perturb the enzyme conformational equilibrium towards a closed state wherein catalysis occurs. Return to the open conformation, coincident with ALA dissociation, is the slowest step of the reaction cycle. A diverse variety of loop mutations have been associated with hyperactivity, suggesting the enzyme has evolved to be purposefully slow, perhaps as a means to allow for rapid up-regulation of activity in response to an as yet undiscovered allosteric type effector. Recently it was discovered that human erythroid ALAS mutations can be associated with two very different diseases. Mutations that down-regulate activity can lead to X-linked sideroblastic anemia, which is characterized by abnormally high iron levels in mitochondria, while mutations that up-regulate activity are associated with X-linked dominant protoporphyria, which in contrast is phenotypically identified by abnormally high porphyrin levels. This article is part of a Special Issue entitled: Pyridoxal Phosphate Enzymology.

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    • "5-Aminolevulinate (ALA) represents the first committed precursor of heme biosynthesis that is produced by all heme-synthesizing organisms [1]. In non-plant eukaryotes and certain bacteria, ALA [with coenzymeA (CoA) and CO 2 as byproducts] is synthesized from glycine and succinyl-CoA, during a condensation reaction catalyzed by the pyridoxal 5 0 -phosphate (PLP)-dependent enzyme, 5-aminolevulinate synthase (ALAS) [1] [2]. Disturbances in the rate of ALA biosynthesis due to loss-of-function and gain-of-function mutations in the erythroid-specific ALAS gene (ALAS2) result in the erythropoietic disorders X-linked sideroblastic anemia (XLSA) and X-linked protoporphyria (XLPP), respectively [3] [4] [5]. "
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    ABSTRACT: 5-Aminolevulinate synthase (ALAS) catalyzes the initial step of mammalian heme biosynthesis, the condensation between glycine and succinyl-CoA to produce CoA, CO2, and 5-aminolevulinate. The crystal structure of Rhodobacter capsulatus ALAS indicates that the adenosyl moiety of succinyl-CoA is positioned in a mainly hydrophobic pocket, where the ribose group forms a putative hydrogen bond with Lys156. Loss-of-function mutations in the analogous lysine of human erythroid ALAS (ALAS2) cause X-linked sideroblastic anemia. To characterize the contribution of this residue toward catalysis, the equivalent lysine in murine ALAS2 was substituted with valine, eliminating the possibility of a hydrogen bond. The K221V substitution produced a 23-fold increase in the KmSCoA and a 97% decrease in kcat/KmSCoA. This reduction in the specificity constant does not stem from lower affinity toward succinyl-CoA, since the KdSCoA of K221V is lower than that of wild-type ALAS. For both enzymes, the KdSCoA value is significantly different from the KmSCoA. That K221V has stronger binding affinity for succinyl-CoA was further deduced from substrate protection studies, as K221V achieved maximal protection at lower succinyl-CoA concentration than wild-type ALAS. Moreover, it is the CoA, rather than the succinyl moiety, that facilitates binding of succinyl-CoA to wild-type ALAS, as evident from identical KdSCoA and KdCoA values. Transient kinetic analyses of the K221V-catalyzed reaction revealed that the mutation reduced the rates of quinonoid intermediate II formation and decay. Altogether, the results imply that the adenosyl-binding site Lys221 contributes to binding and orientation of succinyl-CoA for effective catalysis.
    Full-text · Article · Oct 2015 · FEBS Open Bio
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    • "The complex outer membrane of Gram-negative bacteria acts as a barrier that hinders the PS to transport through the cell membranes; hence, Gram-negative bacteria appear to be less sensitive to the lethal action of PDI with exogenously supplied porphyrins [7e9]. 5-Aminolevulinic acid (ALA) is a naturally occurring intermediate in the hemesynthesis pathway [10]. It is a precursor of porphyrins that can be biosynthesized in nearly all aerobic cells in mammals. "
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    ABSTRACT: The aim of the present study was to develop a simple and fast screening technique to directly evaluate the bactericidal effects of 5-aminolevulinic acid (ALA)-mediated photodynamic inactivation (PDI) and to determine the optimal antibacterial conditions of ALA concentrations and the total dosage of light in vitro. The effects of PDI on Staphylococcus aureus and Pseudomonas aeruginosa in the presence of various concentrations of ALA (1.0 mM, 2.5 mM, 5.0 mM, 10.0 mM) were examined. All bacterial strains were exponentially grown in the culture medium at room temperature in the dark for 60 minutes and subsequently irradiated with 630 ± 5 nm using a light-emitting diode (LED) red light device for accumulating the light doses up to 216 J/cm2. Both bacterial species were susceptible to the ALA-induced PDI. Photosensitization using 1.0 mM ALA with 162 J/cm2 light dose was able to completely reduce the viable counts of S. aureus. A significant decrease in the bacterial viabilities was observed for P. aeruginosa, where 5.0 mM ALA was photosensitized by accumulating the light dose of 162 J/cm2. We demonstrated that the use of microplate-based assays—by measuring the apparent optical density of bacterial colonies at 595 nm—was able to provide a simple and reliable approach for quickly choosing the parameters of ALA-mediated PDI in the cell suspensions.
    Full-text · Article · Sep 2014 · Journal of Food and Drug Analysis
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    • "The first committed step of heme biosynthesis in non-plant eukaryotes and some prokaryotes, the pyridoxal 5′-phosphate (PLP)-dependent condensation of glycine and succinyl-coenzyme A to generate 5-aminolevulinate (ALA), coenzyme A (CoA), and CO2, is catalyzed by 5-aminolevulinate synthase (ALAS) [1], [2]. This reaction is directly coupled to the citric acid cycle via the substrate succinyl-CoA and is the key regulatory step of heme biosynthesis [3]. "
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    ABSTRACT: 5-Aminolevulinate synthase (ALAS; EC 2.3.1.37) catalyzes the first committed step of heme biosynthesis in animals. The erythroid-specific ALAS isozyme (ALAS2) is negatively regulated by heme at the level of mitochondrial import and, in its mature form, certain mutations of the murine ALAS2 active site loop result in increased production of protoporphyrin IX (PPIX), the precursor for heme. Importantly, generation of PPIX is a crucial component in the widely used photodynamic therapies (PDT) of cancer and other dysplasias. ALAS2 variants that cause high levels of PPIX accumulation provide a new means of targeted, and potentially enhanced, photosensitization. In order to assess the prospective utility of ALAS2 variants in PPIX production for PDT, K562 human erythroleukemia cells and HeLa human cervical carcinoma cells were transfected with expression plasmids for ALAS2 variants with greater enzymatic activity than the wild-type enzyme. The levels of accumulated PPIX in ALAS2-expressing cells were analyzed using flow cytometry with fluorescence detection. Further, cells expressing ALAS2 variants were subjected to white light treatments (21-22 kLux) for 10 minutes after which cell viability was determined. Transfection of HeLa cells with expression plasmids for murine ALAS2 variants, specifically for those with mutated mitochondrial presequences and a mutation in the active site loop, caused significant cellular accumulation of PPIX, particularly in the membrane. Light treatments revealed that ALAS2 expression results in an increase in cell death in comparison to aminolevulinic acid (ALA) treatment producing a similar amount of PPIX. The delivery of stable and highly active ALAS2 variants has the potential to expand and improve upon current PDT regimes.
    Full-text · Article · Apr 2014 · PLoS ONE
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