Cocaethylene Synthesis and Pathophysiology: Difference between revisions
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In 2003, a crystal structure of hCE1 was developed using X-ray diffraction techniques to obtain a resolution of 2.80 Angrstoms. The structure was shown in complex with <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Homatropine/1'>homatropine</scene>, a cocaine analogue, and it comprised of two trimers for a total of six identical subunits. Each subunit is 548 amino acids long and each is equipped with two catalytic binding sites to process cocaine. Each trimer subunit associates with a <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Chlorine/3'>chlorine ion</scene>, and the overall protein is aptly named a glycoprotein due to ligand interactions forming with <scene name='Cocaethylene_Synthesis_and_Pathophysiology/N-acetyl-d-glucosamine/1'>n-acetyl-d-glucosamine</scene>, <scene name='Cocaethylene_Synthesis_and_Pathophysiology/2-acetylamino-2-deoxy-a-d-gl/1'>2-(acetylamino)-2-deoxy-a-d-glucopyranose</scene>, and <scene name='Cocaethylene_Synthesis_and_Pathophysiology/O-sialic_acid/1'>o-sialic acid.</scene> | In 2003, a crystal structure of hCE1 was developed using X-ray diffraction techniques to obtain a resolution of 2.80 Angrstoms. The structure was shown in complex with <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Homatropine/1'>homatropine</scene>, a cocaine analogue, and it comprised of two trimers for a total of six identical subunits. Each subunit is 548 amino acids long and each is equipped with two catalytic binding sites to process cocaine. Each trimer subunit associates with a <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Chlorine/3'>chlorine ion</scene>, and the overall protein is aptly named a glycoprotein due to ligand interactions forming with <scene name='Cocaethylene_Synthesis_and_Pathophysiology/N-acetyl-d-glucosamine/1'>n-acetyl-d-glucosamine</scene>, <scene name='Cocaethylene_Synthesis_and_Pathophysiology/2-acetylamino-2-deoxy-a-d-gl/1'>2-(acetylamino)-2-deoxy-a-d-glucopyranose</scene>, and <scene name='Cocaethylene_Synthesis_and_Pathophysiology/O-sialic_acid/1'>o-sialic acid.</scene> | ||
While there are two binding sites for homatropine on each monomer, only one is catalytically active. The homatropine molecule must first enter the catalytic site through the <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Bindinggorge/1'>binding gorge</scene>. Once the homatropine (blue) molecule has entered through the pocket, it can then bind to the <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Activesite/8'>active site</scene> of hCE1 which is characterized by a classic catalytic triad comprised of the following residues: Serine221 (orange), Glutamate354 (yellow), and Histidine468 (black). Directly adjacent to the homatropine molecule in the active site is an <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Oxyanion_red_pocket/1'>oxyanion pocket (red)</scene> which is comprised of the two amide nitrogens of two adjacent glycine residues. These nitrogens serve to stabilize the polar tetrahedral acyl-enzyme intermediate formed in an otherwise relatively hydrophobic enzyme pocket. It should be noted that the presence of the S-enantiomer of cocaine in this pocket forms a steric clash with the oxyanion pocket and thus prevents it from being hydrolyzed. | While there are two binding sites for homatropine on each monomer, only one is catalytically active. The homatropine molecule must first enter the catalytic site through the <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Bindinggorge/1'>binding gorge</scene>. Once the homatropine (blue) molecule has entered through the pocket, it can then bind to the <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Activesite/8'>active site</scene> of hCE1 which is characterized by a classic catalytic triad comprised of the following residues: Serine221 (orange), Glutamate354 (yellow), and Histidine468 (black). Directly adjacent to the homatropine molecule in the active site is an <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Oxyanion_red_pocket/1'>oxyanion pocket (red)</scene> which is comprised of the two amide nitrogens of two adjacent glycine residues. These nitrogens serve to stabilize the polar tetrahedral acyl-enzyme intermediate formed in an otherwise relatively hydrophobic enzyme pocket. It should be noted that the presence of the S-enantiomer of cocaine in this pocket forms a steric clash with the oxyanion pocket and thus prevents it from being hydrolyzed. | ||
The ability of hCE1 to catalyze the trans-esterification reaction of ethanol and the methyl ester on cocaine seems like a difficult case to make. The reason for this is that ethanol is ostensibly hindered sterically by the benzoyl ring of cocaine jutting out of the binding gorge which would arguably prevent ethanol from reaching the bottom of the active site to attack the tetrahedral intermediate. However, a mechanism has been proposed that would explain ethanol's ability to still attack the acyl-enzyme intermediate based on the presence of what is known as a <scene name='Cocaethylene_Synthesis_and_Pathophysiology/Oxyanion_red_pocket/1'>flexible pocket</scene> of residues kitty-corner to the bottom of the active site. | |||