1
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Volkova YL, Pickel C, Jucht AE, Wenger RH, Scholz CC. The Asparagine Hydroxylase FIH: A Unique Oxygen Sensor. Antioxid Redox Signal 2022; 37:913-935. [PMID: 35166119 DOI: 10.1089/ars.2022.0003] [Citation(s) in RCA: 22] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Significance: Limited oxygen availability (hypoxia) commonly occurs in a range of physiological and pathophysiological conditions, including embryonic development, physical exercise, inflammation, and ischemia. It is thus vital for cells and tissues to monitor their local oxygen availability to be able to adjust in case the oxygen supply is decreased. The cellular oxygen sensor factor inhibiting hypoxia-inducible factor (FIH) is the only known asparagine hydroxylase with hypoxia sensitivity. FIH uniquely combines oxygen and peroxide sensitivity, serving as an oxygen and oxidant sensor. Recent Advances: FIH was first discovered in the hypoxia-inducible factor (HIF) pathway as a modulator of HIF transactivation activity. Several other FIH substrates have now been identified outside the HIF pathway. Moreover, FIH enzymatic activity is highly promiscuous and not limited to asparagine hydroxylation. This includes the FIH-mediated catalysis of an oxygen-dependent stable (likely covalent) bond formation between FIH and selected substrate proteins (called oxomers [oxygen-dependent stable protein oligomers]). Critical Issues: The (patho-)physiological function of FIH is only beginning to be understood and appears to be complex. Selective pharmacologic inhibition of FIH over other oxygen sensors is possible, opening new avenues for therapeutic targeting of hypoxia-associated diseases, increasing the interest in its (patho-)physiological relevance. Future Directions: The contribution of FIH enzymatic activity to disease development and progression should be analyzed in more detail, including the assessment of underlying molecular mechanisms and relevant FIH substrate proteins. Also, the molecular mechanism(s) involved in the physiological functions of FIH remain(s) to be determined. Furthermore, the therapeutic potential of recently developed FIH-selective pharmacologic inhibitors will need detailed assessment. Antioxid. Redox Signal. 37, 913-935.
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Affiliation(s)
- Yulia L Volkova
- Institute of Physiology, University of Zurich, Zurich, Switzerland
| | - Christina Pickel
- Institute of Physiology, University of Zurich, Zurich, Switzerland
| | | | - Roland H Wenger
- Institute of Physiology, University of Zurich, Zurich, Switzerland
| | - Carsten C Scholz
- Institute of Physiology, University of Zurich, Zurich, Switzerland
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2
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Mingroni MA, Knapp MJ. Kinetic Studies of the Hydrogen Atom Transfer in a Hypoxia-Sensing Enzyme, FIH-1: KIE and O 2 Reactivity. Biochemistry 2021; 60:3315-3322. [PMID: 34714626 DOI: 10.1021/acs.biochem.1c00476] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Cellular hypoxia plays a crucial role in tissue development and adaptation to pO2. Central to cellular oxygen sensing is factor-inhibiting HIF-1α (FIH), an α-ketoglutarate (αKG)/non-heme iron(II)-dependent dioxygenase that hydroxylates a specific asparagine residue of hypoxia inducible factor-1α (HIF-1α). The high KM(O2) and rate-limiting decarboxylation step upon O2 activation are key features of the enzyme that classify it as an oxygen sensor and set it apart from other αKG/Fe(II)-dependent dioxygenases. Although the chemical intermediates following decarboxylation are presumed to follow the consensus mechanism of other αKG/Fe(II)-dependent dioxygenases, experiments have not previously demonstrated these canonical steps in FIH. In this work, a deuterated peptide substrate was used as a mechanistic probe for the canonical hydrogen atom transfer (HAT). Our data show a large kinetic isotope effect (KIE) in steady-state kinetics (Dkcat = 10 ± 1), revealing that the HAT occurs and is partially rate limiting on kcat. Kinetic studies showed that the deuterated peptide led FIH to uncouple O2 activation and provided the opportunity to spectroscopically observe the ferryl intermediate. This enzyme uncoupling was used as an internal competition with respect to the fate of the ferryl intermediate, demonstrating a large observed KIE on the uncoupling (Dk5 = 1.147 ± 0.005) and an intrinsic KIE on the HAT step (Dk > 15). The close energy barrier between αKG decarboxylation and HAT distinguishes FIH as an O2-sensing enzyme and is crucial for ensuring substrate specificity in the regulation of cellular O2 homeostasis.
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Affiliation(s)
- Michael A Mingroni
- Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
| | - Michael J Knapp
- Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
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3
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Chaplin VD, Hangasky JA, Huang HT, Duan R, Maroney MJ, Knapp MJ. Chloride Supports O 2 Activation in the D201G Facial Triad Variant of Factor-Inhibiting Hypoxia Inducible Factor, an α-Ketoglutarate Dependent Oxygenase. Inorg Chem 2018; 57:12588-12595. [PMID: 30252455 DOI: 10.1021/acs.inorgchem.8b01736] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
α-Ketoglutarate (αKG) dependent oxygenases comprise a large superfamily of enzymes that activate O2 for varied reactions. While most of these enzymes contain a nonheme Fe bound by a His2(Asp/Glu) facial triad, a small number of αKG-dependent halogenases require only the two His ligands to bind Fe and activate O2. The enzyme "factor inhibiting HIF" (FIH) contains a His2Asp facial triad and selectively hydroxylates polypeptides; however, removal of the Asp ligand in the Asp201→Gly variant leads to a highly active enzyme, seemingly without a complete facial triad. Herein, we report on the formation of an Fe-Cl cofactor structure for the Asp201→Gly FIH variant using X-ray absorption spectroscopy (XAS), which provides insight into the structure of the His2Cl facial triad found in halogenases. The Asp201→Gly variant supports anion dependent peptide hydroxylation, demonstrating the requirement for a complete His2X facial triad to support O2 reactivity. Our results indicated that exogenous ligand binding to form a complete His2X facial triad was essential for O2 activation and provides a structural model for the His2Cl-bound nonheme Fe found in halogenases.
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Affiliation(s)
- Vanessa D Chaplin
- Department of Chemistry , University of Massachusetts at Amherst , Amherst , Massachusetts 01003 , United States
| | - John A Hangasky
- Department of Chemistry , University of Massachusetts at Amherst , Amherst , Massachusetts 01003 , United States
| | - Hsin-Ting Huang
- Department of Chemistry , University of Massachusetts at Amherst , Amherst , Massachusetts 01003 , United States
| | - Ran Duan
- Department of Chemistry , University of Massachusetts at Amherst , Amherst , Massachusetts 01003 , United States
| | - Michael J Maroney
- Department of Chemistry , University of Massachusetts at Amherst , Amherst , Massachusetts 01003 , United States
| | - Michael J Knapp
- Department of Chemistry , University of Massachusetts at Amherst , Amherst , Massachusetts 01003 , United States
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4
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Iyer SR, Chaplin VD, Knapp MJ, Solomon EI. O 2 Activation by Nonheme Fe II α-Ketoglutarate-Dependent Enzyme Variants: Elucidating the Role of the Facial Triad Carboxylate in FIH. J Am Chem Soc 2018; 140:11777-11783. [PMID: 30148961 PMCID: PMC6146021 DOI: 10.1021/jacs.8b07277] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
FIH [factor inhibiting HIF (hypoxia inducible factor)] is an α-ketoglutarate (αKG)-dependent nonheme iron enzyme that catalyzes the hydroxylation of the C-terminal transactivation domain (CAD) asparagine residue in HIF-1α to regulate cellular oxygen levels. The role of the facial triad carboxylate ligand in O2 activation and catalysis was evaluated by replacing the Asp201 residue with Gly (D201G), Ala (D201A), and Glu (D201E). Magnetic circular dichroism (MCD) spectroscopy showed that the (FeII)FIH variants were all 6-coordinate (6C) and the αKG plus CAD bound FIH variants were all 5-coordinate (5C), mirroring the behavior of the wild-type ( wt) enzyme. When only αKG is bound, all FIH variants exhibited weaker FeII-OH2 bonds for the sixth ligand compared to wt, and for αKG-bound D201E this is either extremely weak or the site is 5C, demonstrating that the Asp201 residue plays an important role in the wt enzyme in ensuring that the (FeII/αKG)FIH site remains 6C. Variable-temperature, variable-field (VTVH) MCD spectroscopy showed that all of the αKG- and CAD-bound FIH variants, though 5C, have different ground-state geometric and electronic structures, which impair their oxygen activation rates. Comparison of O2 consumption to substrate hydroxylation kinetics revealed uncoupling between the two half reactions in the variants. Thus, the Asp201 residue also ensures fidelity between CAD substrate binding and oxygen activation, enabling tightly coupled turnover.
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Affiliation(s)
- Shyam R. Iyer
- Department of Chemistry, Stanford University, Stanford, California 94305, United States
| | - Vanessa D. Chaplin
- Department of Chemistry, University of Massachusetts at Amherst, Amherst, Massachusetts 01003, United States
| | - Michael J. Knapp
- Department of Chemistry, University of Massachusetts at Amherst, Amherst, Massachusetts 01003, United States
| | - Edward I. Solomon
- Department of Chemistry, Stanford University, Stanford, California 94305, United States
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5
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Hangasky JA, Taabazuing CY, Martin CB, Eron SJ, Knapp MJ. The facial triad in the α-ketoglutarate dependent oxygenase FIH: A role for sterics in linking substrate binding to O 2 activation. J Inorg Biochem 2017; 166:26-33. [PMID: 27815979 PMCID: PMC5161613 DOI: 10.1016/j.jinorgbio.2016.10.007] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2016] [Revised: 10/07/2016] [Accepted: 10/13/2016] [Indexed: 02/06/2023]
Abstract
The factor inhibiting hypoxia inducible factor-1α (FIH) is a nonheme Fe(II)/αKG oxygenase using a 2-His-1-Asp facial triad. FIH activates O2 via oxidative decarboxylation of α-ketoglutarate (αKG) to generate an enzyme-based oxidant which hydroxylates the Asn803 residue within the C-terminal transactivation domain (CTAD) of HIF-1α. Tight coupling of these two sequential reactions requires a structural linkage between the Fe(II) and the substrate binding site to ensure that O2 activation occurs after substrate binds. We tested the hypothesis that the facial triad carboxylate (Asp201) of FIH linked substrate binding and O2 binding sites. Asp201 variants of FIH were constructed and thoroughly characterized in vitro using steady-state kinetics, crystallography, autohydroxylation, and coupling measurements. Our studies revealed each variant activated O2 with a catalytic efficiency similar to that of wild-type (WT) FIH (kcataKM(O2)=0.17μM-1min-1), but led to defects in the coupling of O2 activation to substrate hydroxylation. Steady-state kinetics showed similar catalytic efficiencies for hydroxylation by WT-FIH (kcat/KM(CTAD)=0.42μM-1min-1) and D201G (kcat/KM(CTAD)=0.34μM-1min-1); hydroxylation by D201E was greatly impaired, while hydroxylation by D201A was undetectable. Analysis of the crystal structure of the D201E variant revealed steric crowding near the diffusible ligand site supporting a role for sterics from the facial triad carboxylate in the O2 binding order. Our data support a model in which the facial triad carboxylate Asp201 provides both steric and polar contacts to favor O2 access to the Fe(II) only after substrate binds, leading to coupled turnover in FIH and other αKG oxygenases.
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Affiliation(s)
- John A Hangasky
- Department of Chemistry, University of Massachusetts, Amherst, United States
| | | | - Cristina B Martin
- Department of Chemistry, University of Massachusetts, Amherst, United States
| | - Scott J Eron
- Department of Chemistry, University of Massachusetts, Amherst, United States
| | - Michael J Knapp
- Department of Chemistry, University of Massachusetts, Amherst, United States.
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6
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Electron flow through biological molecules: does hole hopping protect proteins from oxidative damage? Q Rev Biophys 2016; 48:411-20. [PMID: 26537399 DOI: 10.1017/s0033583515000062] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Biological electron transfers often occur between metal-containing cofactors that are separated by very large molecular distances. Employing photosensitizer-modified iron and copper proteins, we have shown that single-step electron tunneling can occur on nanosecond to microsecond timescales at distances between 15 and 20 Å. We also have shown that charge transport can occur over even longer distances by hole hopping (multistep tunneling) through intervening tyrosines and tryptophans. In this perspective, we advance the hypothesis that such hole hopping through Tyr/Trp chains could protect oxygenase, dioxygenase, and peroxidase enzymes from oxidative damage. In support of this view, by examining the structures of P450 (CYP102A) and 2OG-Fe (TauD) enzymes, we have identified candidate Tyr/Trp chains that could transfer holes from uncoupled high-potential intermediates to reductants in contact with protein surface sites.
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7
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Taabazuing CY, Fermann J, Garman S, Knapp MJ. Substrate Promotes Productive Gas Binding in the α-Ketoglutarate-Dependent Oxygenase FIH. Biochemistry 2016; 55:277-86. [PMID: 26727884 PMCID: PMC4793777 DOI: 10.1021/acs.biochem.5b01003] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The Fe(2+)/α-ketoglutarate (αKG)-dependent oxygenases use molecular oxygen to conduct a wide variety of reactions with important biological implications, such as DNA base excision repair, histone demethylation, and the cellular hypoxia response. These enzymes follow a sequential mechanism in which O2 binds and reacts after the primary substrate binds, making those structural factors that promote productive O2 binding central to their chemistry. A large challenge in this field is to identify strategies that engender productive turnover. Factor inhibiting HIF (FIH) is a Fe(2+)/αKG-dependent oxygenase that forms part of the O2 sensing machinery in human cells by hydroxylating the C-terminal transactivation domain (CTAD) found within the HIF-1α protein. The structure of FIH was determined with the O2 analogue NO bound to Fe, offering the first direct insight into the gas binding geometry in this enzyme. Through a combination of density functional theory calculations, {FeNO}(7) electron paramagnetic resonance spectroscopy, and ultraviolet-visible absorption spectroscopy, we demonstrate that CTAD binding stimulates O2 reactivity by altering the orientation of the bound gas molecule. Although unliganded FIH binds NO with moderate affinity, the bound gas can adopt either of two orientations with similar stability; upon CTAD binding, NO adopts a single preferred orientation that is appropriate for supporting oxidative decarboxylation. Combined with other studies of related enzymes, our data suggest that substrate-induced reorientation of bound O2 is the mechanism utilized by the αKG oxygenases to tightly couple O2 activation to substrate hydroxylation.
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Affiliation(s)
| | - Justin Fermann
- Department of Chemistry, University of Massachusetts, Amherst
| | - Scott Garman
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst
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8
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Pektas S, Taabazuing CY, Knapp MJ. Increased Turnover at Limiting O2 Concentrations by the Thr(387) → Ala Variant of HIF-Prolyl Hydroxylase PHD2. Biochemistry 2015; 54:2851-7. [PMID: 25857330 DOI: 10.1021/bi501540c] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
PHD2 is a 2-oxoglutarate, non-heme Fe(2+)-dependent oxygenase that senses O2 levels in human cells by hydroxylating two prolyl residues in the oxygen-dependent degradation domain (ODD) of HIF1α. Identifying the active site contacts that determine the rate of reaction at limiting O2 concentrations is crucial for understanding how this enzyme senses pO2 and may suggest methods for chemically altering hypoxia responses. A hydrogen bonding network extends from the Fe(II) cofactor through ordered waters to the Thr(387) residue in the second coordination sphere. Here we tested the impact of the side chain of Thr(387) on the reactivity of PHD2 toward O2 through a combination of point mutagenesis, steady state kinetic experiments and {FeNO}(7) EPR spectroscopy. The steady state kinetic parameters for Thr(387) → Asn were very similar to those of wild-type (WT) PHD2, but kcat and kcat/KM(O2) for Thr(387) → Ala were increased by roughly 15-fold. X-Band electron paramagnetic resonance spectroscopy of the {FeNO}(7) centers of the (Fe+NO+2OG) enzyme forms showed the presence of a more rhombic line shape in Thr(387) → Ala than in WT PHD2, indicating an altered conformation for bound gas in this variant. Here we show that the side chain of residue Thr(387) plays a significant role in determining the rate of turnover by PHD2 at low O2 concentrations.
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Affiliation(s)
- Serap Pektas
- Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
| | - Cornelius Y Taabazuing
- Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
| | - Michael J Knapp
- Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
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9
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Hangasky JA, Ivison GT, Knapp MJ. Substrate positioning by Gln(239) stimulates turnover in factor inhibiting HIF, an αKG-dependent hydroxylase. Biochemistry 2014; 53:5750-8. [PMID: 25119663 PMCID: PMC4165446 DOI: 10.1021/bi500703s] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
![]()
Nonheme Fe(II)/αKG-dependent
oxygenases catalyze diverse
reactions, typically inserting an O atom from O2 into a
C–H bond. Although the key to their catalytic cycle is the
fact that binding and positioning of primary substrate precede O2 activation, the means by which substrate binding stimulates
turnover is not well understood. Factor Inhibiting HIF (FIH) is a
Fe(II)/αKG-dependent oxygenase that acts as a cellular oxygen
sensor in humans by hydroxylating the target residue Asn803, found in the C-terminal transactivation domain (CTAD) of hypoxia
inducible factor-1. FIH-Gln239 makes two hydrogen bonds
with CTAD-Asn803, positioning this target residue over
the Fe(II). We hypothesized the positioning of the side chain of CTAD-Asn803 by FIH-Gln239 was critical for stimulating O2 activation and subsequent substrate hydroxylation. The steady-state
characterization of five FIH-Gln239 variants (Ala, Asn,
Glu, His, and Leu) tested the role of hydrogen bonding potential and
sterics near the target residue. Each variant exhibited a 20–1200-fold
decrease in kcat and kcat/KM(CTAD), but no change
in KM(CTAD), indicating that the step
after CTAD binding was affected by point mutation. Uncoupled O2 activation was prominent in these variants, as shown by large
coupling ratios (C = [succinate]/[CTAD-OH] = 3–5)
for each of the FIH-Gln239 → X variants. The coupling
ratios decreased in D2O, indicating an isotope-sensitive
inactivation for variants, not observed in the wild type. The data
presented indicate that the proper positioning of CTAD-Asn803 by FIH-Gln239 is necessary to suppress uncoupled turnover
and to support substrate hydroxylation, suggesting substrate positioning
may be crucial for directing O2 reactivity within the broader
class of αKG hydroxylases.
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Affiliation(s)
- John A Hangasky
- Department of Chemistry, University of Massachusetts at Amherst , Amherst, Massachusetts 01003, United States
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10
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Taabazuing CY, Hangasky JA, Knapp MJ. Oxygen sensing strategies in mammals and bacteria. J Inorg Biochem 2014; 133:63-72. [PMID: 24468676 PMCID: PMC4097052 DOI: 10.1016/j.jinorgbio.2013.12.010] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2013] [Revised: 12/23/2013] [Accepted: 12/24/2013] [Indexed: 12/21/2022]
Abstract
The ability to sense and adapt to changes in pO2 is crucial for basic metabolism in most organisms, leading to elaborate pathways for sensing hypoxia (low pO2). This review focuses on the mechanisms utilized by mammals and bacteria to sense hypoxia. While responses to acute hypoxia in mammalian tissues lead to altered vascular tension, the molecular mechanism of signal transduction is not well understood. In contrast, chronic hypoxia evokes cellular responses that lead to transcriptional changes mediated by the hypoxia inducible factor (HIF), which is directly controlled by post-translational hydroxylation of HIF by the non-heme Fe(II)/αKG-dependent enzymes FIH and PHD2. Research on PHD2 and FIH is focused on developing inhibitors and understanding the links between HIF binding and the O2 reaction in these enzymes. Sulfur speciation is a putative mechanism for acute O2-sensing, with special focus on the role of H2S. This sulfur-centered model is discussed, as are some of the directions for further refinement of this model. In contrast to mammals, bacterial O2-sensing relies on protein cofactors that either bind O2 or oxidatively decompose. The sensing modality for bacterial O2-sensors is either via altered DNA binding affinity of the sensory protein, or else due to the actions of a two-component signaling cascade. Emerging data suggests that proteins containing a hemerythrin-domain, such as FBXL5, may serve to connect iron sensing to O2-sensing in both bacteria and humans. As specific molecular machinery becomes identified, these hypoxia sensing pathways present therapeutic targets for diseases including ischemia, cancer, or bacterial infection.
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Affiliation(s)
| | - John A Hangasky
- Department of Chemistry, University of Massachusetts, Amherst, MA 01003, United States
| | - Michael J Knapp
- Department of Chemistry, University of Massachusetts, Amherst, MA 01003, United States.
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11
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Pektas S, Knapp MJ. Substrate preference of the HIF-prolyl hydroxylase-2 (PHD2) and substrate-induced conformational change. J Inorg Biochem 2013; 126:55-60. [PMID: 23787140 PMCID: PMC4046702 DOI: 10.1016/j.jinorgbio.2013.05.006] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2013] [Revised: 04/04/2013] [Accepted: 05/15/2013] [Indexed: 12/22/2022]
Abstract
HIF prolyl-4-hydroxylase 2 (PHD2) is a non-heme Fe, 2-oxoglutarate (2OG) dependent dioxygenase that regulates the hypoxia inducible transcription factor (HIF) by hydroxylating two conserved prolyl residues in N-terminal oxygen degradation domain (NODD) and C-terminal oxygen degradation domain (CODD) of HIF-1α. Prior studies have suggested that the substrate preference of PHD2 arises from binding contacts with the β2β3 loop of PHD2. In this study we tested the substrate selectivity of PHD2 by kinetic competition assays, varied ionic strength, and global protein flexibility using amide H/D exchange (HDX). Our results revealed that PHD2 preferred CODD by 20-fold over NODD and that electrostatics influenced this effect. Global HDX monitored by mass spectrometry indicated that binding of Fe(II) and 2OG stabilized the overall protein structure but the saturating concentrations of either NODD or CODD caused an identical change in protein flexibility. These observations imply that both substrates stabilize the β2β3 loop to the same extent. Under unsaturated substrate conditions NODD led to a higher HDX rate than CODD due to its lower binding affinity to PHD2. Our results suggest that loop closure is the dominant contributor to substrate selectivity in PHD2.
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Affiliation(s)
- Serap Pektas
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003, Voice 413-545-4001, FAX 413-545-4490
| | - Michael J. Knapp
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003, Voice 413-545-4001, FAX 413-545-4490
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12
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Hangasky JA, Saban E, Knapp MJ. Inverse solvent isotope effects arising from substrate triggering in the factor inhibiting hypoxia inducible factor. Biochemistry 2013; 52:1594-602. [PMID: 23351038 DOI: 10.1021/bi3015482] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Oxygen homeostasis plays a critical role in angiogenesis, erythropoiesis, and cell metabolism. Oxygen homeostasis is set by the hypoxia inducible factor-1α (HIF-1α) pathway, which is controlled by factor inhibiting HIF-1α (FIH). FIH is a non-heme Fe(II), α-ketoglutarate (αKG)-dependent dioxygenase that inhibits HIF-1α by hydroxylating the C-terminal transactivation domain (CTAD) of HIF-1α at HIF-Asn(803). A tight coupling between CTAD binding and O2 activation is essential for hypoxia sensing, making changes in the coordination geometry of Fe(II) upon CTAD encounter a crucial feature of this enzyme. Although the consensus chemical mechanism for FIH proposes that CTAD binding triggers O2 activation by causing the Fe(II) cofactor to release an aquo ligand, experimental evidence of this has been absent. More broadly, this proposed coordination change at Fe(II) has not been observed during steady-state turnover in any αKG oxygenase to date. In this work, solvent isotope effects (SIEs) were used as a direct mechanistic probe of substrate-triggered aquo release in FIH, as inverse SIEs (SIE < 1) are signatures for pre-equilibrium aquo release from metal ions. Our mechanistic studies of FIH have revealed inverse solvent isotope effects in the steady-state rate constants at limiting concentrations of CTAD or αKG [(D2O)kcat/KM(CTAD) = 0.40 ± 0.07, and (D2O)kcat/KM(αKG) = 0.32 ± 0.08], providing direct evidence of aquo release during steady-state turnover. Furthermore, the SIE at saturating concentrations of CTAD and αKG was inverse ((D2O)kcat = 0.51 ± 0.07), indicating that aquo release occurs after CTAD binds. The inverse kinetic SIEs observed in the steady state for FIH can be explained by a strong Fe-OH2 bond. The stable Fe-OH2 bond plays an important part in FIH's regulatory role over O2 homeostasis in humans and points toward a strategy for tightly coupling O2 activation with CTAD hydroxylation that relies on substrate triggering.
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Affiliation(s)
- John A Hangasky
- Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, USA
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13
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Saban E, Chen YH, Hangasky J, Taabazuing C, Holmes BE, Knapp MJ. The second coordination sphere of FIH controls hydroxylation. Biochemistry 2011; 50:4733-40. [PMID: 21456582 PMCID: PMC3138472 DOI: 10.1021/bi102042t] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
The factor inhibiting HIF (FIH) is a proximate oxygen sensor for human cells, hydroxylating Asn(803) within the α-subunit of the hypoxia inducible factor (HIF). FIH is an α-ketoglutatrate (αKG)-dependent, non-heme Fe(II) dioxygenase, in which Fe(II) is coordinated by a (His(2)Asp) facial triad, αKG, and H(2)O. Hydrogen bonding among the facial triad, the HIF-Asn(803) side chain, and various second-sphere residues suggests a functional role for the second coordination sphere in tuning the chemistry of the Fe(II) center. Point mutants of FIH were prepared to test the functional role of the αKG-centered (Asn(205) and Asn(294)) or HIF-Asn(803)-centered (Arg(238) and Gln(239)) second-sphere residues. The second sphere was tested for local effects on priming Fe(II) to react with O(2), oxidative decarboxylation, and substrate positioning. Steady-sate kinetics were used to test for overall catalytic effects; autohydroxylation rates were used to test for priming and positioning, and electronic spectroscopy was used to assess the primary coordination sphere and the electrophilicity of αKG. Asn(205) → Ala and Asn(294) → Ala mutants exhibited diminished rates of steady-state turnover, while minimally affecting autohydroxylation, consistent with impaired oxidative decarboxylation. Blue-shifted metal to ligand charge transfer transitions for (Fe+αKG)FIH indicated that these point mutations destabilized the π* orbitals of αKG, further supporting a slowed rate of oxidative decarboxylation. The Arg(238) → Met mutant exhibited steady-state rates too low to measure and diminished product yields, suggesting impaired substrate positioning or priming; the Arg(238) → Met mutant was capable of O(2) activation for the autohydroxylation reaction. The Gln(239) → Asn mutant exhibited significantly slowed steady-state kinetics and diminished product yields, suggesting impaired substrate positioning or priming. As HIF binding to the Gln(239) → Asn mutant stimulated autohydroxylation, it is more likely that this point mutant simply mispositions the HIF-Asn(803) side chain. This work combines kinetics and spectroscopy to show that these second-sphere hydrogen bonds play roles in promoting oxidative decarboxylation, priming Fe(II) to bind O(2), and positioning HIF-Asn(803).
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Affiliation(s)
- Evren Saban
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
| | - Yuan-Han Chen
- Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, MA, 01003
| | - John Hangasky
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
| | | | - Breanne E. Holmes
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
| | - Michael J. Knapp
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
- Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, MA, 01003
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