Inositol oxygenase

Last updated
myo-inositol oxygenase
Mouse miox.png
Structure of the mouse myo-inositol oxygenase monomer, generated from 2HUO, colored by secondary structure element.
Identifiers
SymbolMIOX
Alt. symbolsALDRL6
NCBI gene 55586
HGNC 14522
OMIM 606774
PDB 2IBN
RefSeq NM_017584
UniProt Q9UGB7
Other data
EC number 1.13.99.1
Locus Chr. 22 q
Search for
Structures Swiss-model
Domains InterPro

Inositol oxygenase, also commonly referred to as myo-inositol oxygenase (MIOX), is a non-heme di-iron enzyme that oxidizes myo-inositol to glucuronic acid. [1] The enzyme employs a unique four-electron transfer at its Fe(II)/Fe(III) coordination sites and the reaction proceeds through the direct binding of myo -inositol followed by attack of the iron center by diatomic oxygen. This enzyme is part of the only known pathway for the catabolism of inositol in humans [2] and is expressed primarily in the kidneys. [3] [4] Recent medical research regarding MIOX has focused on understanding its role in metabolic and kidney diseases such as diabetes, obesity and acute kidney injury. Industrially-focused engineering efforts are centered on improving MIOX activity in order to produce glucaric acid in heterologous hosts.

Contents

Structure

The active site of the mouse MIOX enzyme highlighting the di-iron active site along with the coordinated amino acids. The Fe atom binds to the oxygens of the C1 and C6 of myo-inositol. Lys 127 helps to promote the abstraction of the hydrogen atom from the C1 carbon. Miox final 1.png
The active site of the mouse MIOX enzyme highlighting the di-iron active site along with the coordinated amino acids. The Fe atom binds to the oxygens of the C1 and C6 of myo-inositol. Lys 127 helps to promote the abstraction of the hydrogen atom from the C1 carbon.

Myo-inositol oxygenase is a monomeric 33 kDa protein in both solution and crystal. [5] This enzyme possesses a Fe(II)/Fe(III) atomic pair at the catalytic active site which enables its unique four-electron transfer mechanism. Recent crystallization studies have elucidated the structures of the mouse MIOX [5] in 2006 followed by the human MIOX [6] in 2008.

The overall structure of the mouse MIOX is primarily helical with five alpha helices forming the core of the protein. [5] Like other di-iron oxygenases, the iron coordination centers are buried deep inside the protein presumably to protect the cell from the superoxide and radical reaction intermediates that are formed. [7] The two iron centers are coordinated by various amino acids and water molecules as shown in complex with the myo-inositol substrate. The human MIOX structure superimposes closely onto the mouse MIOX structure, sharing 86% sequence identity over the structural alignment but with some differences in the residues surrounding the active site. [6] The human enzyme is characterized by eight alpha helices and a small anti-parallel two-stranded beta sheet. [6]

the MIOX protein fold diverges from that of other non-heme di-iron oxygenases including ribonucleotide reductase and soluble methane monooxygenase. [8] Instead, MIOX closely resembles proteins in the HD-domain superfamily based on its highly conserved metal binding strategy and the presence of the four His ligands on the iron center. [5]

Mechanism

MIOX can accept D- myo- inosito l as well as the less abundant chiro isomer of inositol as substrates. [9] A series of crystallization, spectroscopy and density functional theory experiments have revealed a putative mechanism (shown right) for the oxidation of myo- inositol. [10] [11] [12] ENDOR spectroscopy was used to determine that the substrate directly binds to the Fe(II)/Fe(III) di-iron center of MIOX most likely through the O1 atom of myo- inositol. [7] In the mouse MIOX, this binding process was shown to be dependent on proximal amino acid residues as alanine mutants D85A and K127A were unable to turnover substrate. [5] This binding step positions the myo-inositol prior to the catalytic steps which involve attack of an iron center by diatomic oxygen followed by abstraction of a myo-inositol hydrogen atom.

A superoxide Fe(III)/Fe(III) species is formed as diatomic oxygen displaces water as a coordinating ligand on one of the Fe atoms. Next, the hydrogen atom from C1 of myo- inositol is abstracted to generate a radical that can be attacked by an oxygen radical. Release of D-glucuronic acid is achieved in the fourth step.

Biological Function

Myo -inositol can be ingested from fruits and vegetables and actively transported into cells or instead directly synthesized from glucose. [13] In the kidney, MIOX converts myo-inositol to glucuronic acid which is then able to enter the glucuronate-xylulose pathway for conversion to xylulose-5-phosphate. [13] This product can then easily enter the pentose phosphate pathway. Hence, MIOX enables the conversion and catabolism of inositol to generate NADPH and other pentose sugars.

Disease Relevance.

Myo-inositol is a component of the inositol phosphates and phosphoinositides that serve as secondary messengers in many cellular processes including insulin action. Due to its exclusive expression in the kidney, research has focused on understanding the potential role of both myo-inositol levels and MIOX activity on metabolic diseases like diabetes mellitus and obesity. Depletion of MIOX and accumulation of polyols, such as inositol and xylitol, have been cited as contributing factors in complications associated with diabetes. [14] Additionally, a recent study has shown that MIOX is upregulated in the diabetic state with its transcription heavily regulated by osmolarity, glucose levels and oxidative stress. [15] This upregulation is associated with the formation of reactive oxidative species that lead to interstitial injury in the kidney. [15]

There is also interest in evaluating MIOX expression as a potential biomarker of acute kidney injury. MIOX expression was shown to increase in the serum of animals and plasma of critically ill patients within 24 hours of acute kidney injury specifically. [16] An immunoassay of MIOX expression may potentially predict these life-threatening injuries earlier than the current diagnostic—detection of plasma creatinine.

Industrial Relevance

The MIOX enzyme has been the object of intense metabolic engineering efforts to produce glucaric acid through biosynthetic pathways. In 2004, the U.S. Department of Energy released a list of the top value-added chemicals from biomass which included glucaric acid—the direct product of the oxidation of glucuronic acid. The first biosynthetic production of glucaric acid was achieved in 2009 with use of the uronate dehydrogenase (UDH) enzyme. [17] Since then, the MIOX enzyme has been engineered for improved glucaric acid production through numerous strategies including appendage of an N-terminal SUMO-tag, directed evolution [18] and also the use of modular, synthetic scaffolds to increase its effective local concentration.

The conversion of myo-inositol to glucaric acid--a top value-added chemical from biomass--can be achieved with a combination of MIOX and UDH enabling heterologous production of glucaric acid. MIOX Udh.tif
The conversion of myo-inositol to glucaric acid--a top value-added chemical from biomass--can be achieved with a combination of MIOX and UDH enabling heterologous production of glucaric acid.

See also

Related Research Articles

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<span class="mw-page-title-main">Inositol</span> Carbocyclic sugar

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<span class="mw-page-title-main">Phytic acid</span> Chemical compound

Phytic acid is a six-fold dihydrogenphosphate ester of inositol, also called inositol hexakisphosphate (IP6) or inositol polyphosphate. At physiological pH, the phosphates are partially ionized, resulting in the phytate anion.

<span class="mw-page-title-main">Glucuronic acid</span> Chemical compound

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<span class="mw-page-title-main">Phytase</span> Class of enzymes

A phytase is any type of phosphatase enzyme that catalyzes the hydrolysis of phytic acid – an indigestible, organic form of phosphorus that is found in many plant tissues, especially in grains and oil seeds – and releases a usable form of inorganic phosphorus. While phytases have been found to occur in animals, plants, fungi and bacteria, phytases have been most commonly detected and characterized from fungi.

<span class="mw-page-title-main">4-Hydroxyphenylpyruvate dioxygenase</span> Fe(II)-containing non-heme oxygenase

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<span class="mw-page-title-main">Phytanoyl-CoA dioxygenase</span> Class of enzymes

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<span class="mw-page-title-main">Oxygen rebound mechanism</span> Catalytic pathway

In biochemistry, the oxygen rebound mechanism is the pathway for hydroxylation of organic compounds by iron-containing oxygenases. Many enzymes effect the hydroxylation of hydrocarbons as a means for biosynthesis, detoxification, gene regulation, and other functions. These enzymes often utilize Fe-O centers that convert C-H bonds into C-OH groups. The oxygen rebound mechanism starts with abstraction of H from the hydrocarbon, giving an organic radical and an iron hydroxide. In the rebound step, the organic radical attacks the Fe-OH center to give an alcohol group, which is bound to Fe as a ligand. Dissociation of the alcohol from the metal allows the cycle to start anew. This mechanistic scenario is an alternative to the direct insertion of an O center into a C-H bond. The pathway is an example of C-H activation.

<span class="mw-page-title-main">Beta-propeller phytase</span> Group of enzymes

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<span class="mw-page-title-main">Aldehyde deformylating oxygenase</span> Enzyme family

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References

  1. ↑ Bollinger JM, Diao Y, Matthews ML, Xing G, Krebs C (February 2009). "myo-Inositol oxygenase: a radical new pathway for O(2) and C-H activation at a nonheme diiron cluster". Dalton Transactions (6): 905–14. doi:10.1039/b811885j. PMC   2788986 . PMID   19173070.
  2. ↑ Hankes LV, Politzer WM, Touster O, Anderson L (October 1969). "Myo-inositol catabolism in human pentosurics: the predominant role of the glucuronate-xylulose-pentose phosphate pathway". Annals of the New York Academy of Sciences. 165 (2): 564–76. doi:10.1111/j.1749-6632.1970.tb56424.x. PMID   5259614. S2CID   33916229.
  3. ↑ Reddy CC, Swan JS, Hamilton GA (August 1981). "myo-Inositol oxygenase from hog kidney. I. Purification and characterization of the oxygenase and of an enzyme complex containing the oxygenase and D-glucuronate reductase". The Journal of Biological Chemistry. 256 (16): 8510–8. doi: 10.1016/S0021-9258(19)68873-3 . PMID   7263666.
  4. ↑ Charalampous FC (February 1959). "Biochemical studies on inositol. V. Purification and properties of the enzyme that cleaves inositol to D-glucuronic acid". The Journal of Biological Chemistry. 234 (2): 220–7. doi: 10.1016/S0021-9258(18)70276-7 . PMID   13630882.
  5. 1 2 3 4 5 Brown PM, Caradoc-Davies TT, Dickson JM, Cooper GJ, Loomes KM, Baker EN (October 2006). "Crystal structure of a substrate complex of myo-inositol oxygenase, a di-iron oxygenase with a key role in inositol metabolism". Proceedings of the National Academy of Sciences of the United States of America. 103 (41): 15032–7. Bibcode:2006PNAS..10315032B. doi: 10.1073/pnas.0605143103 . PMC   1622774 . PMID   17012379.
  6. 1 2 3 Thorsell AG, Persson C, Voevodskaya N, Busam RD, Hammarström M, Gräslund S, Gräslund A, Hallberg BM (May 2008). "Structural and biophysical characterization of human myo-inositol oxygenase". The Journal of Biological Chemistry. 283 (22): 15209–16. doi: 10.1074/jbc.M800348200 . PMC   3258897 . PMID   18364358.
  7. 1 2 Kim SH, Xing G, Bollinger JM, Krebs C, Hoffman BM (August 2006). "Demonstration by 2H ENDOR spectroscopy that myo-inositol binds via an alkoxide bridge to the mixed-valent diiron center of myo-inositol oxygenase". Journal of the American Chemical Society. 128 (32): 10374–5. doi:10.1021/ja063602c. PMID   16895396.
  8. ↑ Hirao H, Morokuma K (December 2009). "Insights into the (superoxo)Fe(III)Fe(III) intermediate and reaction mechanism of myo-inositol oxygenase: DFT and ONIOM(DFT:MM) study". Journal of the American Chemical Society. 131 (47): 17206–14. doi:10.1021/ja905296w. PMID   19929019.
  9. ↑ Arner RJ, Prabhu KS, Thompson JT, Hildenbrandt GR, Liken AD, Reddy CC (December 2001). "myo-Inositol oxygenase: molecular cloning and expression of a unique enzyme that oxidizes myo-inositol and D-chiro-inositol". The Biochemical Journal. 360 (Pt 2): 313–20. doi:10.1042/0264-6021:3600313. PMC   1222231 . PMID   11716759.
  10. ↑ Xing G, Barr EW, Diao Y, Hoffart LM, Prabhu KS, Arner RJ, Reddy CC, Krebs C, Bollinger JM (May 2006). "Oxygen activation by a mixed-valent, diiron(II/III) cluster in the glycol cleavage reaction catalyzed by myo-inositol oxygenase". Biochemistry. 45 (17): 5402–12. doi:10.1021/bi0526276. PMID   16634621.
  11. ↑ Xing G, Hoffart LM, Diao Y, Prabhu KS, Arner RJ, Reddy CC, Krebs C, Bollinger JM (May 2006). "A coupled dinuclear iron cluster that is perturbed by substrate binding in myo-inositol oxygenase". Biochemistry. 45 (17): 5393–401. doi:10.1021/bi0519607. PMID   16634620.
  12. ↑ Xing G, Diao Y, Hoffart LM, Barr EW, Prabhu KS, Arner RJ, Reddy CC, Krebs C, Bollinger JM (April 2006). "Evidence for C-H cleavage by an iron-superoxide complex in the glycol cleavage reaction catalyzed by myo-inositol oxygenase". Proceedings of the National Academy of Sciences of the United States of America. 103 (16): 6130–5. Bibcode:2006PNAS..103.6130X. doi: 10.1073/pnas.0508473103 . PMC   1458843 . PMID   16606846.
  13. 1 2 Croze ML, Soulage CO (October 2013). "Potential role and therapeutic interests of myo-inositol in metabolic diseases". Biochimie. 95 (10): 1811–27. doi:10.1016/j.biochi.2013.05.011. PMID   23764390.
  14. ↑ Cohen RA, MacGregor LC, Spokes KC, Silva P, Epstein FH (October 1990). "Effect of myo-inositol on renal Na-K-ATPase in experimental diabetes". Metabolism. 39 (10): 1026–32. doi:10.1016/0026-0495(90)90161-5. PMID   2170818.
  15. 1 2 Tominaga T, Dutta RK, Joladarashi D, Doi T, Reddy JK, Kanwar YS (January 2016). "Transcriptional and Translational Modulation of myo-Inositol Oxygenase (Miox) by Fatty Acids: Implications in Renal Tubular Injury Induced in Obesity and Diabetes". The Journal of Biological Chemistry. 291 (3): 1348–67. doi: 10.1074/jbc.M115.698191 . PMC   4714220 . PMID   26578517.
  16. ↑ Gaut JP, Crimmins DL, Ohlendorf MF, Lockwood CM, Griest TA, Brada NA, Hoshi M, Sato B, Hotchkiss RS, Jain S, Ladenson JH (May 2014). "Development of an immunoassay for the kidney-specific protein myo-inositol oxygenase, a potential biomarker of acute kidney injury". Clinical Chemistry. 60 (5): 747–57. doi:10.1373/clinchem.2013.212993. PMC   4128578 . PMID   24486646.
  17. ↑ Moon TS, Yoon SH, Lanza AM, Roy-Mayhew JD, Prather KL (February 2009). "Production of glucaric acid from a synthetic pathway in recombinant Escherichia coli". Applied and Environmental Microbiology. 75 (3): 589–95. Bibcode:2009ApEnM..75..589M. doi:10.1128/AEM.00973-08. PMC   2632142 . PMID   19060162.
  18. ↑ Shiue E, Prather KL (March 2014). "Improving D-glucaric acid production from myo-inositol in E. coli by increasing MIOX stability and myo-inositol transport" (PDF). Metabolic Engineering. 22: 22–31. doi:10.1016/j.ymben.2013.12.002. hdl: 1721.1/101389 . PMID   24333274.
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