1
|
Di Paolo ML, Cervelli M, Mariottini P, Leonetti A, Polticelli F, Rosini M, Milelli A, Basagni F, Venerando R, Agostinelli E, Minarini A. Exploring the activity of polyamine analogues on polyamine and spermine oxidase: methoctramine, a potent and selective inhibitor of polyamine oxidase. J Enzyme Inhib Med Chem 2019; 34:740-752. [PMID: 30829081 PMCID: PMC6407594 DOI: 10.1080/14756366.2019.1584620] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/27/2018] [Revised: 02/06/2019] [Accepted: 02/14/2019] [Indexed: 01/16/2023] Open
Abstract
Fourteen polyamine analogues, asymmetric or symmetric substituted spermine (1-9) or methoctramine (10-14) analogues, were evaluated as potential inhibitors or substrates of two enzymes of the polyamine catabolic pathway, spermine oxidase (SMOX) and acetylpolyamine oxidase (PAOX). Compound 2 turned out to be the best substrate for PAOX, having the highest affinity and catalytic efficiency with respect to its physiological substrates. Methoctramine (10), a well-known muscarinic M2 receptor antagonist, emerged as the most potent competitive PAOX inhibitor known so far (Ki = 10 nM), endowed with very good selectivity compared with SMOX (Ki=1.2 μM vs SMOX). The efficacy of methoctramine in inhibiting PAOX activity was confirmed in the HT22 cell line. Methoctramine is a very promising tool in the design of drugs targeting the polyamine catabolism pathway, both to understand the physio-pathological role of PAOX vs SMOX and for pharmacological applications, being the polyamine pathway involved in various pathologies.
Collapse
Affiliation(s)
| | | | | | | | - Fabio Polticelli
- Department of Sciences, University of Roma Tre, Roma, Italy
- Roma Tre Section, National Institute of Nuclear Physics, Roma, Italy
| | - Michela Rosini
- Department of Pharmacy and Biotechnology, Alma Mater Studiorum-University of Bologna, Bologna, Italy
| | - Andrea Milelli
- Department for Life Quality Studies, Alma Mater Studiorum-University of Bologna, Rimini, Italy
| | - Filippo Basagni
- Department of Pharmacy and Biotechnology, Alma Mater Studiorum-University of Bologna, Bologna, Italy
| | - Rina Venerando
- Department of Molecular Medicine, University of Padova, Padova, Italy
| | - Enzo Agostinelli
- Department of Biochemical Science "A. Rossi Fanelli", University of Rome "La Sapienza", Rome, Italy
- International Polyamines Foundation – ONLUS –Via del Forte Tiburtino 98, Rome, Italy
| | - Anna Minarini
- Department of Pharmacy and Biotechnology, Alma Mater Studiorum-University of Bologna, Bologna, Italy
| |
Collapse
|
2
|
Roberts KM, Fitzpatrick PF. Measurement of Kinetic Isotope Effects in an Enzyme-Catalyzed Reaction by Continuous-Flow Mass Spectrometry. Methods Enzymol 2017; 596:149-161. [PMID: 28911769 PMCID: PMC6767924 DOI: 10.1016/bs.mie.2017.07.001] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
Abstract
Kinetic isotope effects (KIEs) provide powerful probes of the mechanisms of enzyme-catalyzed reactions. In this chapter, we describe the use of continuous-flow mass spectrometry to determine the deuterium KIE for the enzyme N-acetylpolyamine oxidase based on the ratio of labeled and unlabeled products in mass spectra of whole reaction mixtures.
Collapse
Affiliation(s)
| | - Paul F Fitzpatrick
- University of Texas Health Science Center, San Antonio, TX, United States.
| |
Collapse
|
3
|
Tormos JR, Suarez MB, Fitzpatrick PF. 13C kinetic isotope effects on the reaction of a flavin amine oxidase determined from whole molecule isotope effects. Arch Biochem Biophys 2016; 612:115-119. [PMID: 27815088 PMCID: PMC5257176 DOI: 10.1016/j.abb.2016.10.018] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2016] [Revised: 10/26/2016] [Accepted: 10/27/2016] [Indexed: 12/14/2022]
Abstract
A large number of flavoproteins catalyze the oxidation of amines. Because of the importance of these enzymes in metabolism, their mechanisms have previously been studied using deuterium, nitrogen, and solvent isotope effects. While these results have been valuable for computational studies to distinguish among proposed mechanisms, a measure of the change at the reacting carbon has been lacking. We describe here the measurement of a 13C kinetic isotope effect for a representative amine oxidase, polyamine oxidase. The isotope effect was determined by analysis of the isotopic composition of the unlabeled substrate, N, N'-dibenzyl-1,4-diaminopropane, to obtain a pH-independent value of 1.025. The availability of a 13C isotope effect for flavoprotein-catalyzed amine oxidation provides the first measure of the change in bond order at the carbon involved in this carbon-hydrogen bond cleavage and will be of value to understanding the transition state structure for this class of enzymes.
Collapse
Affiliation(s)
- José R Tormos
- Department of Chemistry and Biochemistry, St. Mary's University, San Antonio, TX 78228, United States
| | - Marina B Suarez
- Department of Geological Sciences, University of Texas-San Antonio, San Antonio, TX 78249, United States
| | - Paul F Fitzpatrick
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229, United States.
| |
Collapse
|
4
|
Abstract
Polyamines are ubiquitous and essential components of mammalian cells. They have multiple functions including critical roles in nucleic acid and protein synthesis, gene expression, protein function, protection from oxidative damage, the regulation of ion channels, and maintenance of the structure of cellular macromolecules. It is essential to maintain a correct level of polyamines, and this amount is tightly regulated at the levels of transport, synthesis, and degradation. Catabolic pathways generate reactive aldehydes including acrolein and hydrogen peroxide via a number of oxidases. These metabolites, particularly those from spermine, can cause significant toxicity with damage to proteins, DNA, and other cellular components. Their production can be increased as a result of infection or cell damage that releases free polyamines and activates the oxidative catabolic pathways. Since polyamines also have an important physiological role in protection from oxidative damage, the reduction in polyamine content may exacerbate the toxic potential of these agents. Increases in polyamine catabolism have been implicated in the development of diseases including stroke, other neurological diseases, renal failure, liver disease, and cancer. These results provide new opportunities for the early diagnosis, prevention, and treatment of disease.
Collapse
Affiliation(s)
- Anthony E Pegg
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine , Hershey, Pennsylvania 17033, United States
| |
Collapse
|
5
|
Tormos JR, Pozzi MH, Fitzpatrick PF. Mechanistic studies of the role of a conserved histidine in a mammalian polyamine oxidase. Arch Biochem Biophys 2012; 528:45-9. [PMID: 22959971 PMCID: PMC3483376 DOI: 10.1016/j.abb.2012.08.007] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2012] [Revised: 08/13/2012] [Accepted: 08/14/2012] [Indexed: 11/21/2022]
Abstract
Polyamine oxidases are peroxisomal flavoproteins that catalyze the oxidation of an endo carbon nitrogen bond of N1-acetylspermine in the catabolism of polyamines. While no structure has been reported for a mammalian polyamine oxidase, sequence alignments of polyamine oxidizing flavoproteins identify a conserved histidine residue. Based on the structure of a yeast polyamine oxidase, Saccharomyces cerevisiae Fms1, this residue has been proposed to hydrogen bond to the reactive nitrogen in the polyamine substrate. The corresponding histidine in mouse polyamine oxidase, His64, has been mutated to glutamine, asparagine, and alanine to determine if this residue plays a similar role in the mammalian enzymes. The kinetics of the mutant enzymes were examined with N1-acetylspermine and the slow substrates spermine and N,N'-dibenzyl-1,4-diaminobutane. On average the mutations result in a decrease of ~15-fold in the rate constant for amine oxidation. Rapid-reaction kinetic analyses established that amine oxidation is rate-limiting with spermine as substrate for the wild-type and mutant enzymes and for the H64N enzyme with N1-acetylspermine as substrate. The k(cat)/K(O(2)) value was unaffected by the mutations with N1-acetylspermine as substrate, but decreased ~55-fold with the two slower substrates. The results are consistent with this residue assisting in properly positioning the amine substrate for oxidation.
Collapse
Affiliation(s)
- José R. Tormos
- Department of Biochemistry, University of Texas Health Science Center, San Antonio TX 78229
| | | | - Paul F. Fitzpatrick
- Department of Biochemistry, University of Texas Health Science Center, San Antonio TX 78229
| |
Collapse
|
6
|
Adachi MS, Taylor AB, Hart PJ, Fitzpatrick PF. Mechanistic and structural analyses of the roles of active site residues in yeast polyamine oxidase Fms1: characterization of the N195A and D94N enzymes. Biochemistry 2012; 51:8690-7. [PMID: 23034052 PMCID: PMC3548949 DOI: 10.1021/bi3011434] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Flavoprotein Fms1 from Saccharomyces cerevisiae catalyzes the oxidation of spermine in the biosynthetic pathway for pantothenic acid. The same reaction is catalyzed by the mammalian polyamine and spermine oxidases. The active site of Fms1 contains three amino acid residues positioned to interact with the polyamine substrate, His67, Asn195, and Asp94. These three residues form a hydrogen-bonding triad with Asn195 being the central residue. Previous studies of the effects of mutating His67 are consistent with that residue being important both for interacting with the substrate and for maintaining the hydrogen bonds in the triad [Adachi, M. S., Taylor, A. B., Hart, P. J., and Fitzpatrick, P. F. (2012) Biochemistry 51, 4888-4897]. The N195A and D94N enzymes have now been characterized to evaluate their roles in catalysis. Both mutations primarily affect the reductive half-reaction. With N(1)-acetylspermine as the substrate, the rate constant for flavin reduction decreases ~450-fold for both mutations; the effects with spermine as the substrate are smaller, 20-40-fold. The k(cat)/K(amine)- and k(cat)-pH profiles with N(1)-acetylspermine are only slightly changed from the profiles for the wild-type enzyme, consistent with the pK(a) values arising from the amine substrate or product and not from active site residues. The structure of the N195A enzyme was determined at a resolution of 2.0 Å. The structure shows a molecule of tetraethylene glycol in the active site and establishes that the mutation has no effect on the protein structure. Overall, the results are consistent with the role of Asn195 and Asp94 being to properly position the polyamine substrate for oxidation.
Collapse
Affiliation(s)
- Mariya S. Adachi
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| | - Alexander B. Taylor
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| | - P. John Hart
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
- Department of Veterans Affairs, Audie Murphy Division, Geriatric Research, Education, and Clinical Center, South Texas Veterans Health Care System, San Antonio, TX 78229
| | - Paul F. Fitzpatrick
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| |
Collapse
|
7
|
Gaweska H, Fitzpatrick PF. Structures and Mechanism of the Monoamine Oxidase Family. Biomol Concepts 2011; 2:365-377. [PMID: 22022344 DOI: 10.1515/bmc.2011.030] [Citation(s) in RCA: 147] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
Members of the monoamine oxidase family of flavoproteins catalyze the oxidation of primary and secondary amines, polyamines, amino acids, and methylated lysine side chains in proteins. The enzymes have similar overall structures, with conserved FAD-binding domains and varied substrate-binding sites. Multiple mechanisms have been proposed for the catalytic reactions of these enzymes. The present review compares the structures of different members of the family and the various mechanistic proposals.
Collapse
Affiliation(s)
- Helena Gaweska
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| | | |
Collapse
|
8
|
Kong X, Ouyang S, Liang Z, Lu J, Chen L, Shen B, Li D, Zheng M, Li KK, Luo C, Jiang H. Catalytic mechanism investigation of lysine-specific demethylase 1 (LSD1): a computational study. PLoS One 2011; 6:e25444. [PMID: 21984927 PMCID: PMC3184146 DOI: 10.1371/journal.pone.0025444] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2011] [Accepted: 09/05/2011] [Indexed: 12/13/2022] Open
Abstract
Lysine-specific demethylase 1 (LSD1), the first identified histone demethylase, is a flavin-dependent amine oxidase which specifically demethylates mono- or dimethylated H3K4 and H3K9 via a redox process. It participates in a broad spectrum of biological processes and is of high importance in cell proliferation, adipogenesis, spermatogenesis, chromosome segregation and embryonic development. To date, as a potential drug target for discovering anti-tumor drugs, the medical significance of LSD1 has been greatly appreciated. However, the catalytic mechanism for the rate-limiting reductive half-reaction in demethylation remains controversial. By employing a combined computational approach including molecular modeling, molecular dynamics (MD) simulations and quantum mechanics/molecular mechanics (QM/MM) calculations, the catalytic mechanism of dimethylated H3K4 demethylation by LSD1 was characterized in details. The three-dimensional (3D) model of the complex was composed of LSD1, CoREST, and histone substrate. A 30-ns MD simulation of the model highlights the pivotal role of the conserved Tyr761 and lysine-water-flavin motif in properly orienting flavin adenine dinucleotide (FAD) with respect to substrate. The synergy of the two factors effectively stabilizes the catalytic environment and facilitated the demethylation reaction. On the basis of the reasonable consistence between simulation results and available mutagenesis data, QM/MM strategy was further employed to probe the catalytic mechanism of the reductive half-reaction in demethylation. The characteristics of the demethylation pathway determined by the potential energy surface and charge distribution analysis indicates that this reaction belongs to the direct hydride transfer mechanism. Our study provides insights into the LSD1 mechanism of reductive half-reaction in demethylation and has important implications for the discovery of regulators against LSD1 enzymes.
Collapse
Affiliation(s)
- Xiangqian Kong
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Sisheng Ouyang
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Zhongjie Liang
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Junyan Lu
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Liang Chen
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Bairong Shen
- Center for Systems Biology, Soochow University, Jiangsu, China
| | - Donghai Li
- State Key Laboratory of Pharmaceutical Biotechnology, School of Life Sciences, Jiangsu Diabetes Research Center, Nanjing University, Nanjing, China
| | - Mingyue Zheng
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Keqin Kathy Li
- State Key Laboratory of Medical Genomics, Shanghai Institute of Hematology, Rui Jin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
- * E-mail: (CL); (KKL)
| | - Cheng Luo
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
- Center for Systems Biology, Soochow University, Jiangsu, China
- * E-mail: (CL); (KKL)
| | - Hualiang Jiang
- State Key Laboratory of Drug Research, Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
- School of Pharmacy, East China University of Science and Technology, Shanghai, China
| |
Collapse
|
9
|
Wang J, Edmondson DE. ²H kinetic isotope effects and pH dependence of catalysis as mechanistic probes of rat monoamine oxidase A: comparisons with the human enzyme. Biochemistry 2011; 50:7710-7. [PMID: 21819071 DOI: 10.1021/bi200951z] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Monoamine oxidase A (MAO A) is a mitochondrial outer membrane-bound flavoenzyme important in the regulation of serotonin and dopamine levels. Because the rat is extensively used as an animal model in drug studies, it is important to understand how rat MAO A behaves in comparison with the more extensively studied human enzyme. For many reversible inhibitors, rat MAO A exhibits K(i) values similar to those of human MAO A. The pH profile of k(cat) for rat MAO A shows a pK(a) of 8.2 ± 0.1 for the benzylamine ES complex and pK(a) values of 7.5 ± 0.1 and 7.6 ± 0.1 for the ES complexes with p-CF(3)-(1)H- and p-CF(3)-(2)H-benzylamine, respectively. In contrast to the human enzyme, the rat enzyme exhibits a single pK(a) value (8.3 ± 0.1) with k(cat)/K(m) for benzylamine versus pH and pK(a) values of 7.8 ± 0.1 and 8.1 ± 0.2 for the ascending limbs, respectively, of k(cat)/K(m) versus pH profiles for p-CF(3)-(1)H- and p-CF(3)-(2)H-benzylamine and 9.3 ± 0.1 and 9.1 ± 0.2 for the descending limbs, respectively. The oxidation of para-substituted benzylamine substrate analogues by rat MAO A has large deuterium kinetic isotope effects on k(cat) and on k(cat)/K(m). These effects are pH-independent and range from 7 to 14, demonstrating a rate-limiting α-C-H bond cleavage step in catalysis. Quantitative structure-activity correlations of log k(cat) with the electronic substituent parameter (σ) at pH 7.5 and 9.0 show a dominant contribution with positive ρ values (1.2-1.3) and a pH-independent negative contribution from the steric term. Quantitative structure-activity relationship analysis of the binding affinities of the para-substituted benzylamine analogues for rat MAO A shows an increased van der Waals volume (V(w)) increases the affinity of the deprotonated amine for the enzyme. These results demonstrate that rat MAO A exhibits functional properties similar but not identical with those of the human enzyme and provide additional support for C-H bond cleavage via a polar nucleophilic mechanism.
Collapse
Affiliation(s)
- Jin Wang
- Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia 30322, United States
| | | |
Collapse
|
10
|
Adachi MS, Torres JM, Fitzpatrick PF. Mechanistic studies of the yeast polyamine oxidase Fms1: kinetic mechanism, substrate specificity, and pH dependence. Biochemistry 2010; 49:10440-8. [PMID: 21067138 PMCID: PMC2999662 DOI: 10.1021/bi1016099] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
The flavoprotein oxidase Fms1 from Saccharomyces cerevisiae catalyzes the oxidation of spermine and N(1)-acetylspermine to yield spermidine and 3-aminopropanal or N-acetyl-3-aminopropanal. The kinetic mechanism of the enzyme has been determined with both substrates. The initial velocity patterns are ping-pong, consistent with reduction being kinetically irreversible. Reduction of Fms1 by either substrate is biphasic. The rate constant for the rapid phase varies with the substrate concentration, with limiting rates for reduction of the enzyme of 126 and 1410 s(-1) and apparent K(d) values of 24.3 and 484 μM for spermine and N(1)-acetylspermine, respectively. The rapid phase is followed by a concentration-independent phase that is slower than turnover. The reaction of the reduced enzyme with oxygen is monophasic, with a rate constant of 402 mM(-1) s(-1) with spermine at 25 °C and 204 mM(-1) s(-1) with N(1)-acetylspermine at 4 °C and pH 9.0. This step is followed by rate-limiting product dissociation. The k(cat)/K(amine)-pH profiles are bell-shaped, with an average pK(a) value of 9.3 with spermine and pK(a) values of 8.3 and 9.6 with N(1)-acetylspermine. Both profiles are consistent with the active forms of substrates having two charged nitrogens. The pH profiles for the rate constant for flavin reduction show pK(a) values of 8.3 and 7.2 for spermine and N(1)-acetylspermine, respectively, for groups that must be unprotonated; these pK(a) values are assigned to the substrate N4. The k(cat)/K(O(2))-pH profiles show pK(a) values of 7.5 for spermine and 6.8 for N(1)-acetylspermine. With both substrates, the k(cat) value decreases when a single residue is protonated.
Collapse
Affiliation(s)
- Mariya S. Adachi
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| | - Jason M. Torres
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| | - Paul F. Fitzpatrick
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229
| |
Collapse
|
11
|
Pozzi MH, Fitzpatrick PF. A lysine conserved in the monoamine oxidase family is involved in oxidation of the reduced flavin in mouse polyamine oxidase. Arch Biochem Biophys 2010; 498:83-8. [PMID: 20417173 PMCID: PMC2880204 DOI: 10.1016/j.abb.2010.04.015] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2010] [Revised: 04/19/2010] [Accepted: 04/20/2010] [Indexed: 10/19/2022]
Abstract
Lysine 315 of mouse polyamine amine oxidase corresponds to a lysine residue that is conserved in the flavoprotein amine oxidases of the monoamine oxidase structural family. In several structures, this lysine residue forms a hydrogen bond to a water molecule that is hydrogen-bonded to the flavin N(5). Mutation of Lys315 in polyamine oxidase to methionine was previously shown to have no effect on the kinetics of the reductive half-reaction of the enzyme (M. Henderson Pozzi, V. Gawandi, P.F. Fitzpatrick, Biochemistry 48 (2009) 1508-1516). In contrast, the mutation does affect steps in the oxidative half-reaction. The k(cat) value is unaffected by the mutation; this kinetic parameter likely reflects product release. At pH 10, the k(cat)/K(m) value for oxygen is 25-fold lower in the mutant enzyme. The k(cat)/K(O2) value is pH-dependent for the wild-type enzyme, decreasing below a pK(a) of 7.0, while this kinetic parameter for the mutant enzyme is pH-independent. This is consistent with the neutral form of Lys315 being required for more rapid flavin oxidation. The solvent isotope effect on the k(cat)/K(O2) value increases from 1.4 in the wild-type enzyme to 1.9 in the mutant protein, and the solvent inventory changes from linear to bowed. The effects of the mutation can be explained by the lysine orienting the bridging water so that it can accept the proton from the flavin N(5) during flavin oxidation. In the mutant enzyme the lysine amine would be replaced by a water chain.
Collapse
Affiliation(s)
| | - Paul F. Fitzpatrick
- Department of Biochemistry and Center for Biomedical Neuroscience, University of Texas Health Science Center at San Antonio, San Antonio TX 78229-3900
| |
Collapse
|