Citrate Synthase: Difference between revisions
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{{STRUCTURE_1cts | PDB=1cts | SCENE= }}Citrate synthase is an enzyme active in the mitochondria, where it is responsible for catalyzing the first reaction of the citric acid cycle (Krebs Cycle): the condensation of acetyl-CoA and oxaloacetate to form citrate. | {{STRUCTURE_1cts | PDB=1cts | SCENE= }}Citrate synthase is an enzyme active in the mitochondria, where it is responsible for catalyzing the first reaction of the citric acid cycle (Krebs Cycle): the condensation of acetyl-CoA and oxaloacetate to form citrate. | ||
'''Structure:''' Citrate synthase exists as a <scene name='Daniel_Eddelman_Sandbox_2/Cts_homodimer/1'>homodimer</scene>. Each identical subunit consists of a large and a small domain, and is comprised almost entirely of α helices (making it an all α protein). In its free enzyme state, citrate synthase exists in “open” form, with its two domains forming a cleft containing the substrate (oxaloacetate) binding site (PDB: [[1cts]]) <ref>PMID:7120407</ref>. When oxaloacetate binds, the smaller domain undergoes an 18° rotation, sealing the oxaloacetate binding site and resulting in the <scene name='Daniel_Eddelman_Sandbox_2/Closed_homodimer/1'>“closed” conformation</scene> (PDB: [[2cts]]). This conformational change not only prevents solvent from reaching the bound | '''Structure:''' Citrate synthase is a single amino acid chain <scene name='Daniel_Eddelman_Sandbox_2/Cts_open_monomer/1'>monomer</scene>. Biologically, however, it exists as a <scene name='Daniel_Eddelman_Sandbox_2/Cts_homodimer/1'>homodimer</scene>. Each identical subunit consists of a large and a small domain, and is comprised almost entirely of α helices (making it an all α protein). In its free enzyme state, citrate synthase exists in “open” form, with its two domains forming a cleft containing the substrate (oxaloacetate) binding site (PDB: [[1cts]]) <ref>PMID:7120407</ref>. When oxaloacetate binds, the smaller domain undergoes an 18° rotation, sealing the oxaloacetate binding site and resulting in the <scene name='Daniel_Eddelman_Sandbox_2/Closed_homodimer/1'>“closed” conformation</scene> (PDB: [[2cts]]). This conformational change not only prevents solvent from reaching the bound substrate, but also generates the acetyl-CoA binding site. This presence of “open” and “closed” forms results in citrate synthase having Ordered Sequential kinetic behavior. | ||
'''Mechanism:''' The reaction mechanism for citrate synthase was proposed by James Remington. In this mechanism, ionizable side chains of citrate synthase participate in acid-base catalysis: His 274, His 320, and Asp 375. First, Asp 375 (a base) removes a proton from the methyl group of acetyl-CoA to form its enol. His 274 stabilizes the acetyl-CoA enolate by forming a hydrogen bond with the enolate oxygen. The enolate then nucleophilically attacks oxaloacetate’s carbonyl carbon, and His 320 donates a proton to oxaloacetate’s carbonyl group in a concerted step, forming citryl-CoA (which remains bound to the enzyme). Finally, citryl-CoA is hydrolyzed to citrate and CoA. | '''Mechanism:''' The reaction mechanism for citrate synthase was proposed by James Remington. In this mechanism, three ionizable side chains of citrate synthase participate in acid-base catalysis: His 274, His 320, and Asp 375. First, Asp 375 (a base) removes a proton from the methyl group of acetyl-CoA to form its enol. His 274 stabilizes the acetyl-CoA enolate by forming a hydrogen bond with the enolate oxygen. The enolate then nucleophilically attacks oxaloacetate’s carbonyl carbon, and His 320 donates a proton to oxaloacetate’s carbonyl group in a concerted step, forming citryl-CoA (which remains bound to the enzyme). Finally, citryl-CoA is hydrolyzed to citrate and CoA. | ||