1
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Karki R, Hennek JT, Chen W, Frantom PA. HDX-MS Reveals Substrate-Dependent, Localized EX1 Conformational Dynamics in the Retaining GT-B Glycosyltransferase, MshA. Biochemistry 2023; 62:2645-2657. [PMID: 37589157 DOI: 10.1021/acs.biochem.3c00338] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/18/2023]
Abstract
Glycosyltransferases (GTs) are well-characterized with respect to static 3D structures and molecular dynamics simulations, but there is a lack of reports on in-solution dynamics on time scales relevant to turnover. Here, backbone amide hydrogen/deuterium exchange followed by mass spectrometry (HDX-MS) was used to investigate the in-solution dynamics of the model retaining GT MshA from Corynebacterium glutamicum (CgMshA). CgMshA has a GT-B fold and catalyzes the transfer of N-acetyl-glucosamine (GlcNAc) from UDP-GlcNAc to l-myo-inositol-1-phosphate in the first step in mycothiol biosynthesis. HDX-MS results identify several key regions of conformational changes in response to UDP-GlcNAc binding, including residues 159-198 in the N-terminal domain and residues 323-354 in the C-terminal domain. These regions also exhibited substrate-dependent EX1 exchange kinetics consistent with conformational tension on the milliseconds to seconds time scale. A potential source of this conformational change is the flexible β4/α5 loop in the C-terminal domain, which sits at the interface of the two domains and likely interacts with the GlcNAc ring of UDP-GlcNAc. In contrast to UDP-GlcNAc, the UDP-CgMshA product complex exhibited severe decreases in deuterium incorporation, suggesting a less dynamic conformation. The HDX-MS results are complemented by solvent viscosity effects of 1.8-2.3 on the CgMshA kcat value, which are consistent with product release as a rate-determining step and possibly a direct role for protein dynamics in catalysis. The identification of in-solution dynamics that are sensitive to substrate binding allows for the proposal of a more detailed mechanism in CgMshA including conformation tension between the donor sugar and the flexible C-terminal domain β4/α5 loop providing sufficient conformational sampling for substrate-assisted catalysis to occur.
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Affiliation(s)
- Ramesh Karki
- Department of Chemistry & Biochemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
| | - Jacquelyn T Hennek
- Department of Chemistry & Biochemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
| | - Wen Chen
- Department of Chemistry & Biochemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
| | - Patrick A Frantom
- Department of Chemistry & Biochemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
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2
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Lisitsa AE, Sukovatyi LA, Deeva AA, Gulnov DV, Esimbekova EN, Kratasyuk VA, Nemtseva EV. The Role of Cosolvent-Water Interactions in Effects of the Media on Functionality of Enzymes: A Case Study of Photobacterium leiognathi Luciferase. Life (Basel) 2023; 13:1384. [PMID: 37374166 DOI: 10.3390/life13061384] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2023] [Revised: 06/08/2023] [Accepted: 06/12/2023] [Indexed: 06/29/2023] Open
Abstract
A complex heterogeneous intracellular environment seems to affect enzymatic catalysis by changing the mobility of biomolecules, their stability, and their conformational states, as well as by facilitating or hindering continuously occurring interactions. The evaluation and description of the influence of the cytoplasmic matrix components on enzymatic activity are problems that remain unsolved. In this work, we aimed to determine the mechanisms of action of two-component media with cosolvents of various molecular sizes on the complex multi-stage bioluminescent reaction catalyzed by bacterial luciferase. Kinetic and structural effects of ethylene glycol, glycerol, sorbitol, glucose, sucrose, dextran, and polyethylene glycol on bacterial luciferase were studied using stopped-flow and fluorescence spectroscopy techniques and molecular dynamics simulations. We have found that diffusion limitations in the presence of cosolvents promote the stabilization of flavin substrate and peroxyflavin intermediate of the reaction, but do not provide any advantages in bioluminescence quantum yield, because substrate binding is slowed down as well. The catalytic constant of bacterial luciferase has been found to be viscosity-independent and correlated with parameters of water-cosolvent interactions (Norrish constant, van der Waals interaction energies). Crowding agents, in contrast to low-molecular-weight cosolvents, had little effect on peroxyflavin intermediate decay and enzyme catalytic constant. We attributed specific kinetic effects to the preferential interaction of the cosolvents with enzyme surface and their penetration into the active site.
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Affiliation(s)
- Albert E Lisitsa
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
| | - Lev A Sukovatyi
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
| | - Anna A Deeva
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
| | - Dmitry V Gulnov
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
| | - Elena N Esimbekova
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
- Institute of Biophysics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk, Russia
| | - Valentina A Kratasyuk
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
- Institute of Biophysics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk, Russia
| | - Elena V Nemtseva
- Biophysics Department, Siberian Federal University, 660041 Krasnoyarsk, Russia
- Institute of Biophysics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk, Russia
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3
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Glockzin K, Kostomiris D, Minnow YVT, Suthagar K, Clinch K, Gai S, Buckler JN, Schramm VL, Tyler PC, Meek TD, Katzfuss A. Kinetic Characterization and Inhibition of Trypanosoma cruzi Hypoxanthine–Guanine Phosphoribosyltransferases. Biochemistry 2022; 61:2088-2105. [PMID: 36193631 PMCID: PMC9536471 DOI: 10.1021/acs.biochem.2c00312] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
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Chagas disease, caused by the parasitic protozoan Trypanosoma cruzi, affects over 8 million people
worldwide. Current antiparasitic treatments for Chagas disease are
ineffective in treating advanced, chronic stages of the disease, and
are noted for their toxicity. Like most parasitic protozoa, T. cruzi is unable to synthesize purines de novo, and relies on the salvage of preformed purines
from the host. Hypoxanthine–guanine phosphoribosyltransferases
(HGPRTs) are enzymes that are critical for the salvage of preformed
purines, catalyzing the formation of inosine monophosphate (IMP) and
guanosine monophosphate (GMP) from the nucleobases hypoxanthine and
guanine, respectively. Due to the central role of HGPRTs in purine
salvage, these enzymes are promising targets for the development of
new treatment methods for Chagas disease. In this study, we characterized
two gene products in the T. cruzi CL
Brener strain that encodes enzymes with functionally identical HGPRT
activities in vitro: TcA (TcCLB.509693.70) and TcC
(TcCLB.506457.30). The TcC isozyme was kinetically characterized to
reveal mechanistic details on catalysis, including identification
of the rate-limiting step(s) of catalysis. Furthermore, we identified
and characterized inhibitors of T. cruzi HGPRTs originally developed as transition-state analogue inhibitors
(TSAIs) of Plasmodium falciparum hypoxanthine–guanine–xanthine
phosphoribosyltransferase (PfHGXPRT), where the most
potent compound bound to T. cruzi HGPRT
with low nanomolar affinity. Our results validated the repurposing
of TSAIs to serve as selective inhibitors for orthologous molecular
targets, where primary and secondary structures as well as putatively
common chemical mechanisms are conserved.
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Affiliation(s)
- Kayla Glockzin
- Department of Biochemistry and Biophysics, Texas A&M University, 300 Olsen Boulevard, College Station, Texas 77843-2128, United States
| | - Demetrios Kostomiris
- Department of Biochemistry and Biophysics, Texas A&M University, 300 Olsen Boulevard, College Station, Texas 77843-2128, United States
| | - Yacoba V. T. Minnow
- Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461-1602, United States
| | - Kajitha Suthagar
- Ferrier Research Institute, Victoria University of Wellington, Gracefield Research Centre, 69 Gracefield Road, Gracefield, Lower Hutt 5010, New Zealand
| | - Keith Clinch
- Ferrier Research Institute, Victoria University of Wellington, Gracefield Research Centre, 69 Gracefield Road, Gracefield, Lower Hutt 5010, New Zealand
| | - Sinan Gai
- Ferrier Research Institute, Victoria University of Wellington, Gracefield Research Centre, 69 Gracefield Road, Gracefield, Lower Hutt 5010, New Zealand
| | - Joshua N. Buckler
- Ferrier Research Institute, Victoria University of Wellington, Gracefield Research Centre, 69 Gracefield Road, Gracefield, Lower Hutt 5010, New Zealand
| | - Vern L. Schramm
- Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461-1602, United States
| | - Peter C. Tyler
- Ferrier Research Institute, Victoria University of Wellington, Gracefield Research Centre, 69 Gracefield Road, Gracefield, Lower Hutt 5010, New Zealand
| | - Thomas D. Meek
- Department of Biochemistry and Biophysics, Texas A&M University, 300 Olsen Boulevard, College Station, Texas 77843-2128, United States
| | - Ardala Katzfuss
- Department of Biochemistry and Biophysics, Texas A&M University, 300 Olsen Boulevard, College Station, Texas 77843-2128, United States
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4
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Shrestha R, Jia K, Khadka S, Eltis LD, Li P. Mechanistic Insights into DyPB from Rhodococcus jostii RHA1 Via Kinetic Characterization. ACS Catal 2021. [DOI: 10.1021/acscatal.1c00703] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Ruben Shrestha
- Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States
| | - Kaimin Jia
- Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States
| | - Samiksha Khadka
- Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States
| | - Lindsay D. Eltis
- Department of Microbiology and Immunology, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, British Columbia V6T 1Z3, Canada
| | - Ping Li
- Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States
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5
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Inverse Solvent Isotope Effects in Enzyme-Catalyzed Reactions. Molecules 2020; 25:molecules25081933. [PMID: 32326332 PMCID: PMC7221790 DOI: 10.3390/molecules25081933] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2020] [Revised: 04/14/2020] [Accepted: 04/20/2020] [Indexed: 11/17/2022] Open
Abstract
Solvent isotope effects have long been used as a mechanistic tool for determining enzyme mechanisms. Most commonly, macroscopic rate constants such as kcat and kcat/Km are found to decrease when the reaction is performed in D2O for a variety of reasons including the transfer of protons. Under certain circumstances, these constants are found to increase, in what is termed an inverse solvent kinetic isotope effect (SKIE), which can be a diagnostic mechanistic feature. Generally, these phenomena can be attributed to an inverse solvent equilibrium isotope effect on a rapid equilibrium preceding the rate-limiting step(s). This review surveys inverse SKIEs in enzyme-catalyzed reactions by assessing their underlying origins in common mechanistic themes. Case studies for each category are presented, and the mechanistic implications are put into context. It is hoped that readers may find the illustrative examples valuable in planning and interpreting solvent isotope effect experiments.
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6
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Gadda G, Sobrado P. Kinetic Solvent Viscosity Effects as Probes for Studying the Mechanisms of Enzyme Action. Biochemistry 2018; 57:3445-3453. [DOI: 10.1021/acs.biochem.8b00232] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Affiliation(s)
| | - Pablo Sobrado
- Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061, United States
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7
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Shrestha R, Huang G, Meekins DA, Geisbrecht BV, Li P. Mechanistic Insights into Dye-Decolorizing Peroxidase Revealed by Solvent Isotope and Viscosity Effects. ACS Catal 2017; 7:6352-6364. [PMID: 29308295 PMCID: PMC5751952 DOI: 10.1021/acscatal.7b01861] [Citation(s) in RCA: 52] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Dye-decolorizing peroxidases (DyPs) are a family of H2O2-dependent heme peroxidases, which have shown potential applications in lignin degradation and valorization. However, the DyP kinetic mechanism remains underexplored. Using structural biology and solvent isotope (sKIE) and viscosity effects, many mechanistic characteristics have been uncovered for the B-class ElDyP from Enterobacter lignolyticus. Its structure revealed that a water molecule acts as the sixth axial ligand with two channels at diameters of ~3.0 and 8.0 Å leading to the heme center. A conformational change of ERS* to ERS, which have identical spectral characteristics, was proposed as the final step in DyPs' bisubstrate Ping-Pong mechanism. This step is also the rate-determining step in ABTS oxidation. The normal KIE of wild-type ElDyP with D2O2 at pH 3.5 suggested that cmpd 0 deprotonation by the distal aspartate is rate-limiting in the formation of cmpd I, which is more reactive under acidic pH than under neutral or alkaline pH. The viscosity effects and other biochemical methods implied that the reducing substrate binds with cmpd I instead of the free enzyme. The significant inverse sKIEs of kcat/KM and kERS* suggested that the aquo release in DyPs is mechanistically important and may explain the enzyme's adoption of two-electron reduction for cmpd I. The distal aspartate is catalytically more important than the distal arginine and plays key roles in determining DyPs' acidic pH optimum. The kinetic mechanism of D143H-ElDyP was also briefly studied. The results obtained will pave the way for future protein engineering to improve DyPs' lignolytic activity.
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Affiliation(s)
- Ruben Shrestha
- Department of Chemistry, Kansas State University, Manhattan, KS, 66506, USA
| | - Gaochao Huang
- Department of Chemistry, Kansas State University, Manhattan, KS, 66506, USA
| | - David A. Meekins
- Department of Chemistry, Kansas State University, Manhattan, KS, 66506, USA
| | - Brian V. Geisbrecht
- Department of Biochemistry and Molecular Biophysics, Kansas State University, Manhattan, KS, 66506, USA
| | - Ping Li
- Department of Chemistry, Kansas State University, Manhattan, KS, 66506, USA
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8
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Masschelein J, Jenner M, Challis GL. Antibiotics from Gram-negative bacteria: a comprehensive overview and selected biosynthetic highlights. Nat Prod Rep 2017. [PMID: 28650032 DOI: 10.1039/c7np00010c] [Citation(s) in RCA: 64] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Covering: up to 2017The overwhelming majority of antibiotics in clinical use originate from Gram-positive Actinobacteria. In recent years, however, Gram-negative bacteria have become increasingly recognised as a rich yet underexplored source of novel antimicrobials, with the potential to combat the looming health threat posed by antibiotic resistance. In this article, we have compiled a comprehensive list of natural products with antimicrobial activity from Gram-negative bacteria, including information on their biosynthetic origin(s) and molecular target(s), where known. We also provide a detailed discussion of several unusual pathways for antibiotic biosynthesis in Gram-negative bacteria, serving to highlight the exceptional biocatalytic repertoire of this group of microorganisms.
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Affiliation(s)
- J Masschelein
- Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, UK.
| | - M Jenner
- Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, UK.
| | - G L Challis
- Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, UK.
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9
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Buller AR, Freeman MF, Schildbach JF, Townsend CA. Exploring the role of conformational heterogeneity in cis-autoproteolytic activation of ThnT. Biochemistry 2014; 53:4273-81. [PMID: 24933323 PMCID: PMC4095933 DOI: 10.1021/bi500385d] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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In
the past decade, there have been major achievements in understanding
the relationship between enzyme catalysis and protein structural plasticity.
In autoprocessing systems, however, there is a sparsity of direct
evidence of the role of conformational dynamics, which are complicated
by their intrinsic chemical reactivity. ThnT is an autoproteolytically
activated enzyme involved in the biosynthesis of the β-lactam
antibiotic thienamycin. Conservative mutation of ThnT results in multiple
conformational states that can be observed via X-ray crystallography,
establishing ThnT as a representative and revealing system for studing
how conformational dynamics control autoactivation at a molecular
level. Removal of the nucleophile by mutation to Ala disrupts the
population of a reactive state and causes widespread structural changes
from a conformation that promotes autoproteolysis to one associated
with substrate catalysis. Finer probing of the active site polysterism
was achieved by EtHg derivatization of the nucleophile, which indicates
the active site and a neighboring loop have coupled dynamics. Disruption
of these interactions by mutagenesis precludes the ability to observe
a reactive state through X-ray crystallography, and application of
this insight to other autoproteolytically activated enzymes offers
an explanation for the widespread crystallization of inactive states.
We suggest that the N → O(S) acyl shift in cis-autoproteolysis might occur through a si-face attack,
thereby unifying the fundamental chemistry of these enzymes through
a common mechanism.
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Affiliation(s)
- Andrew R Buller
- Department of Biophysics, Johns Hopkins University , Baltimore, Maryland 21218, United States
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10
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Smyrniotis CJ, Barbour SR, Xia Z, Hixon MS, Holman TR. ATP allosterically activates the human 5-lipoxygenase molecular mechanism of arachidonic acid and 5(S)-hydroperoxy-6(E),8(Z),11(Z),14(Z)-eicosatetraenoic acid. Biochemistry 2014; 53:4407-19. [PMID: 24893149 PMCID: PMC4215895 DOI: 10.1021/bi401621d] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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5-Lipoxygenase
(5-LOX) reacts with arachidonic acid (AA) to first
generate 5(S)-hydroperoxy-6(E),8(Z),11(Z),14(Z)-eicosatetraenoic
acid [5(S)-HpETE] and then an epoxide from 5(S)-HpETE to form leukotriene A4, from a single
polyunsaturated fatty acid. This work investigates the kinetic mechanism
of these two processes and the role of ATP in their activation. Specifically,
it was determined that epoxidation of 5(S)-HpETE
(dehydration of the hydroperoxide) has a rate of substrate capture
(Vmax/Km)
significantly lower than that of AA hydroperoxidation (oxidation of
AA to form the hydroperoxide); however, hyperbolic kinetic parameters
for ATP activation indicate a similar activation for AA and 5(S)-HpETE. Solvent isotope effect results for both hydroperoxidation
and epoxidation indicate that a specific step in its molecular mechanism
is changed, possibly because of a lowering of the dependence of the
rate-limiting step on hydrogen atom abstraction and an increase in
the dependency on hydrogen bond rearrangement. Therefore, changes
in ATP concentration in the cell could affect the production of 5-LOX
products, such as leukotrienes and lipoxins, and thus have wide implications
for the regulation of cellular inflammation.
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Affiliation(s)
- Christopher J Smyrniotis
- Department of Chemistry and Biochemistry, University of California , Santa Cruz, California 95064, United States
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11
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Larrouy-Maumus G, Kelly G, de Carvalho LPS. Chemical mechanism of glycerol 3-phosphate phosphatase: pH-dependent changes in the rate-limiting step. Biochemistry 2014; 53:143-51. [PMID: 24359335 DOI: 10.1021/bi400856y] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The halo-acid dehalogenase (HAD) superfamily comprises a large number of enzymes that share a conserved core domain responsible for a diverse array of chemical transformations (e.g., phosphonatase, dehalogenase, phosphohexomutase, and phosphatase) and a cap domain that controls substrate specificity. Phosphate hydrolysis is thought to proceed via an aspartyl-phosphate intermediate, and X-ray crystallography has shown that protein active site conformational changes are required for catalytic competency. Using a combination of steady-state and pre-steady-state kinetics, pL-rate studies, solvent kinetic isotope effects, (18)O molecular isotope exchange, and partition experiments, we provide a detailed description of the chemical mechanism of a glycerol 3-phosphate phosphatase. This phosphatase has been recently recognized as a rate-limiting factor in lipid polar head recycling in Mycobacterium tuberculosis [Larrouy-Maumus, G., et al. (2013) Proc. Natl. Acad. Sci. 110 (28), 11320-11325]. Our results clearly establish the existence of an aspartyl-phosphate intermediate in this newly discovered member of the HAD superfamily. No ionizable groups are rate-limiting from pH 5.5 to 9.5, consistent with the pK values of the catalytic aspartate residues. The formation and decay of this intermediate are partially rate-limiting below pH 7.0, and a conformational change preceding catalysis is rate-limiting above pH 7.0.
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Affiliation(s)
- Gérald Larrouy-Maumus
- Mycobacterial Research Division, MRC, National Institute for Medical Research , and ‡MRC Biomolecular NMR Centre, The Ridgeway, Mill Hill, London NW7 1AA, U.K
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12
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Robbins JM, Ellis HR. Steady-state kinetic isotope effects support a complex role of Arg226 in the proposed desulfonation mechanism of alkanesulfonate monooxygenase. Biochemistry 2013; 53:161-8. [PMID: 24321058 DOI: 10.1021/bi401234e] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The alkanesulfonate monooxygenase system catalyzes the desulfonation of alkanesulfonates through proposed acid-base mechanistic steps that involves the abstraction of a proton from the alkane peroxyflavin intermediate and protonation of the FMN-O(-) intermediate. Both solvent and kinetic isotope studies were performed to define the proton transfer steps involved in the SsuD reaction. Substitution of the protium at the C1 position of octanesulfonate with deuterium resulted in an observed primary isotope effect of 3.0 ± 0.2 on the kcat parameter, supporting abstraction of the α-proton from the alkane peroxyflavin as the rate-limiting step in catalysis. Previous studies implicated Arg226 as the acid involved in the reprotonation of the hydroxyflavin intermediate. Solvent isotope kinetic studies gave an inverse isotope effect on (D2O)kcat of 0.75 ± 0.04 with no observable effect on (D2O)kcat/Km. This resulted in equivalent solvent isotope effects on (D2O)kcat and (D2O)(kcat)D, suggesting a solvent equilibrium isotope effect on a step occurring after the first irreversible step through product release. Data from proton inventory studies on kcat were best fit to a dome-shaped curve consistent with a conformational change to an open conformation during product release. The solvent isotope effect data coupled with the corresponding proton inventory results support and extend our previous observations that Arg226 donates a proton to the FMN-O(-) intermediate, triggering a conformational change that opens the enzyme to solvation and promotes product release.
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Affiliation(s)
- John M Robbins
- Department of Chemistry and Biochemistry, Auburn University , Auburn, Alabama 36849, United States
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13
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Czyzyk DJ, Sawant SS, Ramirez-Mondragon CA, Hingorani MM, Taylor EA. Escherichia coli heptosyltransferase I: investigation of protein dynamics of a GT-B structural enzyme. Biochemistry 2013; 52:5158-60. [PMID: 23865375 DOI: 10.1021/bi400807r] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Heptosyltransferase I (HepI), the enzyme responsible for the transfer of l-glycero-d-manno-heptose to a 3-deoxy-α-d-manno-oct-2-ulopyranosonic acid (Kdo) of the growing core region of lipopolysaccharide, is a member of the GT-B structural class of enzymes. Crystal structures have revealed open and closed conformations of apo and ligand-bound GT-B enzymes, implying that large-scale protein conformational dynamics play a role in their reaction mechanism. Here we report transient kinetic analysis of conformational changes in HepI reported by intrinsic tryptophan fluorescence and present the first real-time evidence of a GT-B enzyme undergoing a substrate binding-induced transition from an open to closed state prior to catalysis.
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Affiliation(s)
- Daniel J Czyzyk
- Department of Chemistry, Wesleyan University , Middletown, Connecticut 06459, United States
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14
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Origins of the β-lactam rings in natural products. J Antibiot (Tokyo) 2013; 66:401-10. [DOI: 10.1038/ja.2013.24] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2013] [Revised: 02/26/2013] [Accepted: 03/05/2013] [Indexed: 11/08/2022]
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15
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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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16
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Flagg SC, Giri N, Pektas S, Maroney MJ, Knapp MJ. Inverse solvent isotope effects demonstrate slow aquo release from hypoxia inducible factor-prolyl hydroxylase (PHD2). Biochemistry 2012; 51:6654-66. [PMID: 22747465 PMCID: PMC3525350 DOI: 10.1021/bi300229y] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Prolyl hydroxylase domain 2 (PHD2) is deemed a primary oxygen sensor in humans, yet many details of its underlying mechanism are still not fully understood. (Fe(2+) + αKG)PHD2 is 6-coordinate, with a 2His/1Asp facial triad occupying three coordination sites, a bidentate α-ketoglutarate occupying two sites, and an aquo ligand in the final site. Turnover is thought to be initiated upon release of the aquo ligand, creating a site for O(2) to bind at the iron. Herein we show that steady-state turnover is faster under acidic conditions, with k(cat) exhibiting a kinetic pK(a) = 7.22. A variety of spectroscopic probes were employed to identify the active-site acid, through comparison of (Fe(2+) + αKG)PHD2 at pH 6.50 with pH 8.50. The near-UV circular dichroism spectrum was virtually unchanged at elevated pH, indicating that the secondary structure did not change as a function of pH. UV-visible and Fe X-ray absorption spectroscopy indicated that the primary coordination sphere of Fe(2+) changed upon increasing the pH; extended X-ray absorption fine structure analysis found a short Fe-(O/N) bond length of 1.96 Å at pH 8.50, strongly suggesting that the aquo ligand was deprotonated at this pH. Solvent isotope effects were measured during steady-sate turnover over a wide pH-range, with an inverse solvent isotope effect (SIE) of k(cat) observed ((D(2)O)k(cat) = 0.91 ± 0.03) for the acid form; a similar SIE was observed for the basic form of the enzyme ((D(2)O)k(cat) = 0.9 ± 0.1), with an acid equilibrium offset of ΔpK(a) = 0.67 ± 0.04. The inverse SIE indicated that aquo release from the active site Fe(2+) immediately precedes a rate-limiting step, suggesting that turnover in this enzyme may be partially limited by the rate of O(2) binding or activation, and suggesting that aquo release is relatively slow. The unusual kinetic pK(a) further suggested that PHD2 might function physiologically to sense both intracellular pO(2) as well as pH, which could provide for feedback between anaerobic metabolism and hypoxia sensing.
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Affiliation(s)
- Shannon C. Flagg
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
| | - Nitai Giri
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
| | - Serap Pektas
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
| | - Michael J. Maroney
- Department of Chemistry, University of Massachusetts, Amherst, MA, 01003
- Program in Molecular and Cellular Biology, 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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17
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Huang X, Hernick M. Examination of mechanism of N-acetyl-1-D-myo-inosityl-2-amino-2-deoxy-α-D-glucopyranoside deacetylase (MshB) reveals unexpected role for dynamic tyrosine. J Biol Chem 2012; 287:10424-10434. [PMID: 22315231 DOI: 10.1074/jbc.m111.320184] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Actinomycetes are a group of gram-positive bacteria that includes pathogenic mycobacterial species, such as Mycobacterium tuberculosis. These organisms do not have glutathione and instead utilize the small molecule mycothiol (MSH) as their primary reducing agent and for the detoxification of xenobiotics. Due to these important functions, enzymes involved in MSH biosynthesis and MSH-dependent detoxification are targets for drug development. The metal-dependent deacetylase N-acetyl-1-D-myo-inosityl-2-amino-2-deoxy-α-D-glucopyranoside deacetylase (MshB) catalyzes the hydrolysis of N-acetyl-1-D-myo-inosityl-2-amino-2-deoxy-α-D-glucopyranoside to form 1-D-myo-inosityl-2-amino-2-deoxy-α-D-glucopyranoside and acetate in MSH biosynthesis. Herein we examine the chemical mechanism of MshB. We demonstrate that the side chains of Asp-15, Tyr-142, His-144, and Asp-146 are important for catalytic activity. We show that NaF is an uncompetitive inhibitor of MshB, consistent with a metal-water/hydroxide functioning as the reactive nucleophile in the catalytic mechanism. We have previously shown that MshB activity has a bell-shaped dependence on pH with pK(a) values of ∼7.3 and 10.5 (Huang, X., Kocabas, E. and Hernick, M. (2011) J. Biol. Chem. 286, 20275-20282). Mutagenesis experiments indicate that the observed pK(a) values reflect ionization of Asp-15 and Tyr-142, respectively. Together, findings from our studies suggest that MshB functions through a general acid-base pair mechanism with the side chain of Asp-15 functioning as the general base catalyst and His-144 serving as the general acid catalyst, whereas the side chain of Tyr-142 probably assists in polarizing substrate/stabilizing the oxyanion intermediate. Additionally, our results indicate that Tyr-142 is a dynamic side chain that plays key roles in catalysis, modulating substrate binding, chemistry, and product release.
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Affiliation(s)
- Xinyi Huang
- Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061
| | - Marcy Hernick
- Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061.
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18
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Labonte JW, Kudo F, Freeman MF, Raber ML, Townsend CA. Engineering the synthetic potential of β-lactam synthetase and the importance of catalytic loop dynamics. MEDCHEMCOMM 2012; 3:960-966. [PMID: 23616913 DOI: 10.1039/c2md00305h] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
The 2-azetidinone ring of the Class A and D β-lactamase inhibitor clavulanic acid (1) is synthesized by the ATP-utilizing enzyme β-lactam synthetase (βLS). A hydroxyethyl group attached to C-6 of 1 in the (S) configuration markedly enhances the efficacy of this compound against Class C β-lactamases. Guided by a series of X-ray structures of βLS, we have engineered this enzyme to act upon a methylated substrate analogue to give selectively the (3S)-methyl β-lactam core, which, upon closure of the second ring of the bicyclic system of 1, would lead to the (6S)-methylated clavulanic acid derivative.
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Affiliation(s)
- Jason W Labonte
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD, USA. ; Tel: 410 516 7444
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19
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Bodner MJ, Li R, Phelan RM, Freeman MF, Moshos KA, Lloyd EP, Townsend CA. Definition of the common and divergent steps in carbapenem β-lactam antibiotic biosynthesis. Chembiochem 2011; 12:2159-65. [PMID: 21913298 PMCID: PMC3281309 DOI: 10.1002/cbic.201100366] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2011] [Indexed: 11/11/2022]
Abstract
Approximately 50 naturally occurring carbapenem β-lactam antibiotics are known. All but one of these have been isolated from Streptomyces species and are disubstituted structural variants of a simple core that is synthesized by Pectobacterium carotovorum (Erwinia carotovora), a phylogenetically distant plant pathogen. While the biosynthesis of the simple carbapenem, (5R)-carbapen-2-em-3-carboxylic acid, is impressively efficient requiring only three enzymes, CarA, CarB and CarC, the formation of thienamycin, one of the former group of metabolites from Streptomyces, is markedly more complex. Despite their phylogenetic separation, bioinformatic analysis of the encoding gene clusters suggests that the two pathways could be related. Here we demonstrate with gene swapping, stereochemical and kinetics experiments that CarB and CarA and their S. cattleya orthologues, ThnE and ThnM, respectively, are functionally and stereochemically equivalent, although their catalytic efficiencies differ. The biosynthetic pathways, therefore, to thienamycin, and likely to the other disubstituted carbapenems, and to the simplest carbapenem, (5R)-carbapen-2-em-3-carboxylic acid, are initiated in the same manner, but share only two common steps before diverging.
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Affiliation(s)
- Micah J. Bodner
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Rongfeng Li
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Ryan M. Phelan
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Michael F. Freeman
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Kristos A. Moshos
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Evan P. Lloyd
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Craig A. Townsend
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
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20
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Thibodeaux CJ, Liu HW. Mechanistic studies of 1-aminocyclopropane-1-carboxylate deaminase: characterization of an unusual pyridoxal 5'-phosphate-dependent reaction. Biochemistry 2011; 50:1950-62. [PMID: 21244019 DOI: 10.1021/bi101927s] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
1-Aminocyclopropane-1-carboxylic acid (ACC) deaminase (ACCD) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that cleaves the cyclopropane ring of ACC, to give α-ketobutyric acid and ammonia as products. The cleavage of the C(α)-C(β) bond of an amino acid substrate is a rare event in PLP-dependent enzyme catalysis. Potential chemical mechanisms involving nucleophile- or acid-catalyzed cyclopropane ring opening have been proposed for the unusual transformation catalyzed by ACCD, but the actual mode of cyclopropane ring cleavage remains obscure. In this report, we aim to elucidate the mechanistic features of ACCD catalysis by investigating the kinetic properties of ACCD from Pseudomonas sp. ACP and several of its mutant enzymes. Our studies suggest that the pK(a) of the conserved active site residue, Tyr294, is lowered by a hydrogen bonding interaction with a second conserved residue, Tyr268. This allows Tyr294 to deprotonate the incoming amino group of ACC to initiate the aldimine exchange reaction between ACC and the PLP coenzyme and also likely helps to activate Tyr294 for a role as a nucleophile to attack and cleave the cyclopropane ring of the substrate. In addition, solvent kinetic isotope effect (KIE), proton inventory, and (13)C KIE studies of the wild type enzyme suggest that the C(α)-C(β) bond cleavage step in the chemical mechanism is at least partially rate-limiting under k(cat)/K(m) conditions and is likely preceded in the mechanism by a partially rate-limiting step involving the conversion of a stable gem-diamine intermediate into a reactive external aldimine intermediate that is poised for cyclopropane ring cleavage. When viewed within the context of previous mechanistic and structural studies of ACCD enzymes, our studies are most consistent with a mode of cyclopropane ring cleavage involving nucleophilic catalysis by Tyr294.
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Affiliation(s)
- Christopher J Thibodeaux
- Division of Medicinal Chemistry, College of Pharmacy, Department of Chemistry and Biochemistry, and Institute for Cellular and Molecular Biology, University of Texas, Austin, Texas 78712, United States
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21
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Ma B, Nussinov R. Enzyme dynamics point to stepwise conformational selection in catalysis. Curr Opin Chem Biol 2010; 14:652-9. [PMID: 20822947 PMCID: PMC6407632 DOI: 10.1016/j.cbpa.2010.08.012] [Citation(s) in RCA: 171] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2010] [Revised: 08/10/2010] [Accepted: 08/10/2010] [Indexed: 12/13/2022]
Abstract
Recent data increasingly reveal that conformational dynamics are indispensable to enzyme function throughout the catalytic cycle, in substrate recruiting, chemical transformation, and product release. Conformational transitions may involve conformational selection and induced fit, which can be viewed as a special case in the catalytic network. NMR, X-ray crystallography, single-molecule FRET, and simulations clearly demonstrate that the free enzyme dynamics already encompass all the conformations necessary for substrate binding, preorganization, transition-state stabilization, and product release. Conformational selection and substate population shift at each step of the catalytic turnover can accommodate enzyme specificity and efficiency. Within such a framework, entropy can have a larger role in conformational dynamics than in direct energy transfer in dynamically promoted catalysis.
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Affiliation(s)
- Buyong Ma
- Basic Science Program, SAIC-Frederick, Inc., Center for Cancer Research Nanobiology Program, NCI-Frederick, Frederick, MD 21702, USA.
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22
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Lendrihas T, Hunter GA, Ferreira GC. Targeting the active site gate to yield hyperactive variants of 5-aminolevulinate synthase. J Biol Chem 2010; 285:13704-11. [PMID: 20194506 DOI: 10.1074/jbc.m109.074237] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The rate of porphyrin biosynthesis in mammals is controlled by the activity of the pyridoxal 5'-phosphate-dependent enzyme 5-aminolevulinate synthase (EC 2.3.1.37). Based on the postulate that turnover in this enzyme is controlled by conformational dynamics associated with a highly conserved active site loop, we constructed a variant library by targeting imperfectly conserved noncatalytic loop residues and examined the effects on product and porphyrin production. Functional loop variants of the enzyme were isolated via genetic complementation in Escherichia coli strain HU227. Colony porphyrin fluorescence varied widely when bacterial cells harboring the loop variants were grown on inductive media; this facilitated identification of clones encoding unusually active enzyme variants. Nine loop variants leading to high in vivo porphyrin production were purified and characterized kinetically. Steady state catalytic efficiencies for the two substrates were increased by up to 100-fold. Presteady state single turnover reaction data indicated that the second step of quinonoid intermediate decay, previously assigned as reaction rate-limiting, was specifically accelerated such that in three of the variants this step was no longer kinetically significant. Overall, our data support the postulate that the active site loop controls the rate of product and porphyrin production in vivo and suggest the possibility of an as yet undiscovered means of allosteric regulation.
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Affiliation(s)
- Thomas Lendrihas
- Department of Molecular Medicine, College of Medicine, University of South Florida, Tampa, Florida 33612, USA
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23
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Raber ML, Castillo A, Greer A, Townsend CA. A conserved lysine in beta-lactam synthetase assists ring cyclization: Implications for clavam and carbapenem biosynthesis. Chembiochem 2010; 10:2904-12. [PMID: 19882698 DOI: 10.1002/cbic.200900389] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
beta-Lactam synthetase (beta-LS) is the paradigm of a growing class of enzymes that form the critical beta-lactam ring in the clavam and carbapenem antibiotics. beta-LS catalyzes a two-stage reaction in which N(2)-(2-carboxyethyl)-L-arginine is first adenylated, and then undergoes intramolecular ring closure. It was previously shown that the forward kinetic commitment to beta-lactam formation is high, and that the overall rate of reaction is partially limited to a protein conformational change rather than to the chemical step alone of closing the strained ring. beta-Lactam formation was evaluated on the basis of X-ray crystal structures, site-specific mutation, and kinetic and computational studies. The combined evidence clearly points to a reaction coordinate involving the formation of a tetrahedral transition state/intermediate stabilized by a conserved Lys. The combination of substrate preorganization, a well-stabilized transition state and an excellent leaving group facilitates this acyl substitution to account for the strong forward commitment to catalysis and to lower the barrier of four-membered ring formation to the magnitude of a protein conformational change.
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Affiliation(s)
- Mary L Raber
- Department of Chemistry, The Johns Hopkins University, Baltimore, MD 21218, USA
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24
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Raber ML, Arnett SO, Townsend CA. A conserved tyrosyl-glutamyl catalytic dyad in evolutionarily linked enzymes: carbapenam synthetase and beta-lactam synthetase. Biochemistry 2009; 48:4959-71. [PMID: 19371088 DOI: 10.1021/bi900432n] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Beta-lactam-synthesizing enzymes carbapenam synthetase (CPS) and beta-lactam synthetase (beta-LS) are evolutionarily linked to a common ancestor, asparagine synthetase B (AS-B). These three relatives catalyze substrate acyl-adenylation and nucleophilic acyl substitution by either an external (AS-B) or internal (CPS, beta-LS) nitrogen source. Unlike AS-B, crystal structures of CPS and beta-LS revealed a putative Tyr-Glu dyad (CPS, Y345/E380; beta-LS, Y348/E382) proposed to deprotonate the respective internal nucleophile. CPS and beta-LS site-directed mutagenesis (Y345/8A, Y345/8F, E380/2D, E380/2Q, E380A) resulted in the reduction of their catalytic efficiency, with Y345A, E380A, and E382Q producing undetectable amounts of beta-lactam product. However, [(32)P]PP(i)-ATP exchange assays demonstrated Y345A and E380A undergo the first half-reaction, with the remaining active mutants showing decreased forward commitment to beta-lactam cyclization. pH-rate profiles of CPS and beta-LS supported the importance of a Tyr-Glu dyad in beta-lactam formation and suggested its reverse protonation in beta-LS. The kinetics of CPS double-site mutants reinforced the synergism of Tyr-Glu in catalysis. Furthermore, significant solvent isotope effects on k(cat) ((D)k(cat)) for Y345F (1.9) and Y348F (1.7) maintained the assignment of Y345/8 in proton transfer. A proton inventory on Y348F determined its (D)(k(cat)/K(m)) = 0.2 to arise from multiple reactant-state fractionation factors, presumably from water molecule(s) replacing the missing Tyr hydroxyl. The role of a CPS and beta-LS Tyr-Glu catalytic dyad was solidified by a significant decrease in mutant k(cat) viscosity dependence with respect to the wild-type enzymes. The evolutionary relation and potential for engineered biosynthesis were demonstrated by beta-LS acting as a carbapenam synthetase.
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Affiliation(s)
- Mary L Raber
- Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, USA
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