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==='''Introduction'''=== | ==='''Introduction'''=== | ||
---- | ---- | ||
[[Image:Arginases_homotrimer.jpg]] | [[Image:Arginases_homotrimer.jpg|thumb|right|300px|Figure 1: Liver arginase illustrating that the general homotrimeric strucutre of arginase<ref name="Homotrimer">accessed April 3,2011: http://en.wikipedia.org/wiki/Arginase.</ref>.]] | ||
Arginase is a 105 kD homotrimeric metallo-protein, as shown in figure 1, and catalysis the hydrolysis of arginine to ornithine and urea by means of a binuclear spin-coupled Mn<sup>2+</sup> cluster in the active site<ref name="a">PMID: 19456858 </ref>. Many organisms contain the enzyme arginase, for example ''Homo sapiens'' and [http://en.wikipedia.org/wiki/Plasmodium_falciparum ''Plasmodium falciparum''], a parasite that causes cerebral malaria<ref name="b">PMID: 20527960 </ref>. In humans there are two forms of arginases that have evolved with differing tissue distributions and sub-cellular locations in mammals<ref name="c">PMID: 15766238 </ref>. | |||
The two types of arginase is found in mammalian, are arginase I (hAI) and arginases II (hAII)<ref name="c"/>. Arginase I is found predominantly in the liver, where it catalyzes the final cytosolic step of the urea cycle<ref name="c"/>. Arginases II is a mitochondrial enzyme that does not appear to function in the urea cycle and is more widely disturbed in numerous tissues, for example kidney, brains, skeletal muscle, mammary gland and penile corpus cavernosum<ref name="c"/>. Recent studies show that ''Plasmodium falciparum'' arginase (PFA) plays a role in systemic depletion of arginine levels, which in turn has been associated with human cerebral malaria pathogenesis<ref name="a"/>. In addition the arginase fold is part of the [http://en.wikipedia.org/wiki/Ureohydrolase ureohydrolase] superfamily, which also includes agmatinase, histone de-acetylase and acetylpolyamine amidohydrolase<ref name="a"/>. | |||
==='''Structure and Function'''=== | ==='''Structure and Function'''=== | ||
---- | ---- | ||
[[Image:Arginine. | [[Image:Arginine.png|thumb|right|300px|Figure 2: General reaction of arginase hydrolyzing L-arginine to urea and L-ornithine adopted from Christianson<ref name="c"/>.]] | ||
In general arginase is a homotrimeric enzyme, which is present in the fifth and final step of the urea cycle for mammals. In humans, hAI converts L-arginine into L-orinithine and urea as shown in figure 2. Human arginase II plays a role in L-arginine homeostasis, by regulating L-arginine concentrations from cellular biosynthetic reactions such as nitric oxide (NO) biosynthesis<ref name="c"/>. Additionally Plasmodium falciparum arginase is comparable to human arginase, due to the fact that it is 27% identical with human aginase I and II<ref name="b"/>. | In general arginase is a homotrimeric enzyme, which is present in the fifth and final step of the urea cycle for mammals. In humans, hAI converts L-arginine into L-orinithine and urea as shown in figure 2. Human arginase II plays a role in L-arginine homeostasis, by regulating L-arginine concentrations from cellular biosynthetic reactions such as nitric oxide (NO) biosynthesis<ref name="c"/>. Additionally ''Plasmodium falciparum'' arginase is comparable to human arginase, due to the fact that it is 27% identical with human aginase I and II<ref name="b"/>. | ||
Overall arginase is a homotrimeric metallo-enzyme with a binuclear manganese <scene name='Sandbox_Reserved_322/Mn/5'>MN</scene> cluster in each monomer as shown in the PDB identifier 3mmr<ref name="c"/>. The overall fold of the arginase monomer belongs to the α/β protein class with a globular structure<ref name="d">PMID: 8849731 </ref>. | Overall arginase is a homotrimeric metallo-enzyme with a binuclear manganese <scene name='Sandbox_Reserved_322/Mn/5'>MN</scene> cluster in each monomer as shown in the PDB identifier 3mmr<ref name="c"/>. The overall fold of the arginase monomer belongs to the α/β protein class with a globular structure<ref name="d">PMID: 8849731 </ref>. | ||
One site of the active-site cleft is partially defined by the central 8-stranded <scene name='Sandbox_Reserved_322/8-stranded_beta-sheet/1'>β-sheet</scene>, and the <scene name='Sandbox_Reserved_322/Metal_binding_sites/1'>metal binding sites</scene> is located on the edge of the β-sheet<ref name="d"/>. The metal ion that is more deeply situated in the active-site cleft is designated | One site of the active-site cleft is partially defined by the central 8-stranded <scene name='Sandbox_Reserved_322/8-stranded_beta-sheet/1'>β-sheet</scene>, and the <scene name='Sandbox_Reserved_322/Metal_binding_sites/1'>metal binding sites</scene> is located on the edge of the β-sheet<ref name="d"/>. The metal ion that is more deeply situated in the active-site cleft is designated <scene name='Sandbox_Reserved_322/Mn2a/1'>Mn2+A</scene> while the other metal ion is designated <scene name='Sandbox_Reserved_322/Mn_b/1'>Mn2+B</scene>. In ''Plasmodium falciparum'' arginase Mn<sup>2+</sup><sub>A</sub> is coordinated by <scene name='Sandbox_Reserved_322/Mna_bonds/1'>His 193, Asp 216, Asp 220, Asp 323</scene> and a solvent molecule, with a square pyramidal geometry<ref name="b"/><ref name="d"/>. The solvent molecule bridges both metal ions and also donates a hydrogen bond to Asp 220<ref name="b"/><ref name="d"/>. Mn<sup>2+</sup><sub>B</sub> is coordinated by <scene name='Sandbox_Reserved_322/Mnb_ligand/1'>His 218, Asp 216, Asp, 323, Asp 325</scene> and the bridging solvent molecule in a distorted octahedral fashion<ref name="d"/>. All metal ligands except for Asp 220 make hydrogen-bond interactions with other protein residues, and these interactions contribute to the stability of the metal binding site<ref name="b"/><ref name="d"/>. | ||
There are three different types of bridging metal ligands that facilitate the observed spin coupling between Mn<sup>2+</sup><sub>A</sub> and Mn<sup>2+</sup><sub>B</sub><ref name="d"/>. For the first ligand, the carboxylate side chain of Asp 216 is a syn-syn bidentate bridging ligand, with Oδ1 coordinated to Mn<sup>2+</sup><sub>A</sub> and Oδ2 coordinated to Mn<sup>2+</sup><sub>B</sub><ref name="d"/>. For the second ligand, the carboxylate side chain of Asp 323 is a monodentate bridging ligand, with Oδ1 coordinated to both Mn<sup>2+</sup><sub>A</sub> and Mn<sup>2+</sup><sub>B</sub> with anti- and syn-coordination stereo-chemistry, respectively<ref name="b"/><ref name="d"/>. And finally the third ligand, is the solvent molecule bridges both manganese ion symmetrically<ref name="d"/>. Also the Mn<sup>2+</sup> ions coordinate with water, orienting and stabilizing the molecule and allowing water to act as a nucleophile and attack L-arginine, hydrolyzing it into orinithine and urea<ref name="c"/>. Overall the two manganese metal ion in arginase maintain the proper function of the enzyme<ref name="b"/>. | |||
==='''Mechanism'''=== | ==='''Mechanism'''=== | ||
---- | ---- | ||
[[Image: | [[Image:Arginase_mechanism_(2).png|thumb|right|300px|Figure 3: Proposed mechanism of arginase hydrolyzing L-arginine to urea and L-ornithine adopted from Kanyo and colleagues<ref name="d"/>.]] | ||
In general arginase is known to convert L-arginine into urea and L-ornithine, via hydrolysis, the proposed mechanism is adopted from Kanyo and colleagues as shown in figure 3<ref name="d"/>. In the first step of the hydrolytic mechanism, Asp 220 stabilizes the metal-bridging hydroxide ion with a hydrogen bond during a nucleophilic attack at the guanidinium carbon of arginine<ref name="b"/><ref name="d"/>. The resulting tetrahedral intermediate fall apart once a proton is transferred to the amino group of ornithine, and the proton transfer is mediated by Asp 220<ref name="b"/><ref name="d"/>. It is proposed that His 233 shuttles a proton from bulk solvent to the ε-amino group of ornithine, before the product dissociation, as well a water molecule displaces urea<ref name="b"/><ref name="d"/>. In addition, the metal coordination facilitates the ionization of this water molecule to regenerate a nucleophilic hydroxide ion<ref name="d"/>. During this process a proton transfer occurs to the bulk solvent and is mediated by shuttle-group His 233<ref name="b"/><ref name="d"/>. | |||
==='''Arginase and the Physiology of Sexual Arousal'''=== | ==='''Arginase and the Physiology of Sexual Arousal'''=== | ||
---- | ---- | ||
Female sexual arousal disorder | [[Image:Arginine_catabloism_by_arginase_and_NO_synthase.png|thumb|left|300px|Figure 4: Chemical reaction of arginine, illustrating how arginase and NO synthase compete for arginine<ref name="c"/>.]] | ||
Female sexual arousal disorder is defined as an inability to achieve or maintain sufficient sexual excitement, including clitoral erection and genital engorgement, and it is a physiologically analogous to male erectile dysfunction, which is defined as a deficiency in genital blood circulation which compromises the hemodynamic of erectons<ref name="c"/>. Nitric oxide (NO) is the principle mediator of erectile functions and governs nonadrenergic, noncholinergic neurotransmission in penile corpus cavernosum smooth muscle<ref name="c"/>. NO cause’s rapid relaxation of smooth muscle tissue and thereby facilitates the engorgement of the corpus cavernosum<ref name="c"/>. Thus, NO synthase is a critical enzyme in the physiology of sexual arousal<ref name="c"/>. Also, human arginase II is a critical enzyme in the physiology of sexual arousal, due to the fact it coexpressed with NO synthase in smooth muscle tissue<ref name="c"/>. Given that hAII and NO synthase compete for the same substrate L-arginine as shown in figure 4, arginase appears to attenuate NO synthase activity and NO-dependent smooth muscle relaxation by depleting the substrate pool of L-arginine that would be available to NO synthase<ref name="c"/>. In addition arginase is inhibited by the boronic acid inhibitor (<scene name='Sandbox_Reserved_322/Abh/1'>ABH</scene>), which maintains L-arginine concentrations, which in turn enhances NO synthase activity and NO-dependent smooth muscle relaxation in tissue<ref name="c"/>. Thus over expression of human arginase II contributes to erectile dysfunction, and human penile arginase is a potential target for the treatment of male sexual dysfunction<ref name="c"/>. | |||
==='''Reference'''=== | ==='''Reference'''=== | ||
---- | |||
<references/> | <references/> | ||
Latest revision as of 03:56, 13 April 2011
| This Sandbox is Reserved from January 10, 2010, through April 10, 2011 for use in BCMB 307-Proteins course taught by Andrea Gorrell at the University of Northern British Columbia, Prince George, BC, Canada. |
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| |||||||
| 3mmr, resolution 2.14Å () | |||||||
|---|---|---|---|---|---|---|---|
| Ligands: | , , | ||||||
| Gene: | PFI0320w (Plasmodium falciparum 3D7) | ||||||
| Activity: | Arginase, with EC number 3.5.3.1 | ||||||
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| Resources: | FirstGlance, OCA, RCSB, PDBsum | ||||||
| Coordinates: | save as pdb, mmCIF, xml | ||||||
ArginaseArginase
IntroductionIntroduction
Arginase is a 105 kD homotrimeric metallo-protein, as shown in figure 1, and catalysis the hydrolysis of arginine to ornithine and urea by means of a binuclear spin-coupled Mn2+ cluster in the active site[2]. Many organisms contain the enzyme arginase, for example Homo sapiens and Plasmodium falciparum, a parasite that causes cerebral malaria[3]. In humans there are two forms of arginases that have evolved with differing tissue distributions and sub-cellular locations in mammals[4].
The two types of arginase is found in mammalian, are arginase I (hAI) and arginases II (hAII)[4]. Arginase I is found predominantly in the liver, where it catalyzes the final cytosolic step of the urea cycle[4]. Arginases II is a mitochondrial enzyme that does not appear to function in the urea cycle and is more widely disturbed in numerous tissues, for example kidney, brains, skeletal muscle, mammary gland and penile corpus cavernosum[4]. Recent studies show that Plasmodium falciparum arginase (PFA) plays a role in systemic depletion of arginine levels, which in turn has been associated with human cerebral malaria pathogenesis[2]. In addition the arginase fold is part of the ureohydrolase superfamily, which also includes agmatinase, histone de-acetylase and acetylpolyamine amidohydrolase[2].
Structure and FunctionStructure and Function
In general arginase is a homotrimeric enzyme, which is present in the fifth and final step of the urea cycle for mammals. In humans, hAI converts L-arginine into L-orinithine and urea as shown in figure 2. Human arginase II plays a role in L-arginine homeostasis, by regulating L-arginine concentrations from cellular biosynthetic reactions such as nitric oxide (NO) biosynthesis[4]. Additionally Plasmodium falciparum arginase is comparable to human arginase, due to the fact that it is 27% identical with human aginase I and II[3].
Overall arginase is a homotrimeric metallo-enzyme with a binuclear manganese cluster in each monomer as shown in the PDB identifier 3mmr[4]. The overall fold of the arginase monomer belongs to the α/β protein class with a globular structure[5]. One site of the active-site cleft is partially defined by the central 8-stranded , and the is located on the edge of the β-sheet[5]. The metal ion that is more deeply situated in the active-site cleft is designated while the other metal ion is designated . In Plasmodium falciparum arginase Mn2+A is coordinated by and a solvent molecule, with a square pyramidal geometry[3][5]. The solvent molecule bridges both metal ions and also donates a hydrogen bond to Asp 220[3][5]. Mn2+B is coordinated by and the bridging solvent molecule in a distorted octahedral fashion[5]. All metal ligands except for Asp 220 make hydrogen-bond interactions with other protein residues, and these interactions contribute to the stability of the metal binding site[3][5].
There are three different types of bridging metal ligands that facilitate the observed spin coupling between Mn2+A and Mn2+B[5]. For the first ligand, the carboxylate side chain of Asp 216 is a syn-syn bidentate bridging ligand, with Oδ1 coordinated to Mn2+A and Oδ2 coordinated to Mn2+B[5]. For the second ligand, the carboxylate side chain of Asp 323 is a monodentate bridging ligand, with Oδ1 coordinated to both Mn2+A and Mn2+B with anti- and syn-coordination stereo-chemistry, respectively[3][5]. And finally the third ligand, is the solvent molecule bridges both manganese ion symmetrically[5]. Also the Mn2+ ions coordinate with water, orienting and stabilizing the molecule and allowing water to act as a nucleophile and attack L-arginine, hydrolyzing it into orinithine and urea[4]. Overall the two manganese metal ion in arginase maintain the proper function of the enzyme[3].
MechanismMechanism
.png)
In general arginase is known to convert L-arginine into urea and L-ornithine, via hydrolysis, the proposed mechanism is adopted from Kanyo and colleagues as shown in figure 3[5]. In the first step of the hydrolytic mechanism, Asp 220 stabilizes the metal-bridging hydroxide ion with a hydrogen bond during a nucleophilic attack at the guanidinium carbon of arginine[3][5]. The resulting tetrahedral intermediate fall apart once a proton is transferred to the amino group of ornithine, and the proton transfer is mediated by Asp 220[3][5]. It is proposed that His 233 shuttles a proton from bulk solvent to the ε-amino group of ornithine, before the product dissociation, as well a water molecule displaces urea[3][5]. In addition, the metal coordination facilitates the ionization of this water molecule to regenerate a nucleophilic hydroxide ion[5]. During this process a proton transfer occurs to the bulk solvent and is mediated by shuttle-group His 233[3][5].
Arginase and the Physiology of Sexual ArousalArginase and the Physiology of Sexual Arousal
Female sexual arousal disorder is defined as an inability to achieve or maintain sufficient sexual excitement, including clitoral erection and genital engorgement, and it is a physiologically analogous to male erectile dysfunction, which is defined as a deficiency in genital blood circulation which compromises the hemodynamic of erectons[4]. Nitric oxide (NO) is the principle mediator of erectile functions and governs nonadrenergic, noncholinergic neurotransmission in penile corpus cavernosum smooth muscle[4]. NO cause’s rapid relaxation of smooth muscle tissue and thereby facilitates the engorgement of the corpus cavernosum[4]. Thus, NO synthase is a critical enzyme in the physiology of sexual arousal[4]. Also, human arginase II is a critical enzyme in the physiology of sexual arousal, due to the fact it coexpressed with NO synthase in smooth muscle tissue[4]. Given that hAII and NO synthase compete for the same substrate L-arginine as shown in figure 4, arginase appears to attenuate NO synthase activity and NO-dependent smooth muscle relaxation by depleting the substrate pool of L-arginine that would be available to NO synthase[4]. In addition arginase is inhibited by the boronic acid inhibitor (), which maintains L-arginine concentrations, which in turn enhances NO synthase activity and NO-dependent smooth muscle relaxation in tissue[4]. Thus over expression of human arginase II contributes to erectile dysfunction, and human penile arginase is a potential target for the treatment of male sexual dysfunction[4].
ReferenceReference
- ↑ accessed April 3,2011: http://en.wikipedia.org/wiki/Arginase.
- ↑ 2.0 2.1 2.2 Wells GA, Muller IB, Wrenger C, Louw AI. The activity of Plasmodium falciparum arginase is mediated by a novel inter-monomer salt-bridge between Glu295-Arg404. FEBS J. 2009 Jul;276(13):3517-30. Epub 2009 May 18. PMID:19456858 doi:10.1111/j.1742-4658.2009.07073.x
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 Dowling DP, Ilies M, Olszewski KL, Portugal S, Mota MM, Llinas M, Christianson DW. Crystal Structure of Arginase from Plasmodium falciparum and Implications for l-Arginine Depletion in Malarial Infection . Biochemistry. 2010 Jun 9. PMID:20527960 doi:10.1021/bi100390z
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 Christianson DW. Arginase: structure, mechanism, and physiological role in male and female sexual arousal. Acc Chem Res. 2005 Mar;38(3):191-201. PMID:15766238 doi:10.1021/ar040183k
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14 5.15 5.16 Kanyo ZF, Scolnick LR, Ash DE, Christianson DW. Structure of a unique binuclear manganese cluster in arginase. Nature. 1996 Oct 10;383(6600):554-7. PMID:8849731 doi:10.1038/383554a0