Citrate Synthase

Overview

Citrate synthase is an enzyme active in all examined cells, where it is most often responsible for catalyzing the first reaction of the citric acid cycle (Krebs Cycle or the tricarboxylic acid [TCA] cycle): the condensation of acetyl-CoA and oxaloacetate to form citrate. Although in eukaryotes it is a mitochondrial enzyme, and in fact, is often used as a enzyme marker for intact mitochondria, it is encoded by nuclear DNA^[1]. The standard free energy change (ΔG°’) for the citrate synthase reaction is -31.5kJ/mol ^[2]. This negative free energy value means that citrate synthase is likely to function far from equilibrium under physiological conditions, and is thus a rate-determining enzyme in the citric acid cycle. See also:

• Krebs cycle carbons
• Krebs cycle importance
• Krebs cycle overview
• Krebs cycle reactions
• Citric Acid Cycle
• Krebs cycle step 1
• Glyoxylate cycle

2-methylcitrate synthase catabolizes propionate to succinate and pyruvate^[3].

Structure

Biologically, citrate synthase exists as a homodimer of a single amino acid chain monomer. 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 an “open” form of the homodimer, with its two domains forming a cleft containing the substrate (oxaloacetate) binding site (PDB: 1cts) ^[4][5]. When oxaloacetate binds, the smaller domain undergoes an 18° rotation, sealing the oxaloacetate binding site^[6] and resulting in the closed conformation of the homodimer (PDB: 2cts)^[4]. The dramatic conformational change is best illustrated via a morph between the "open" and "closed" states, and be sure to view the morph from the side as well to get a full sense of the structural change. The 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 ^[2].

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Mechanism

Three side chains in each of the two active sites of the dimer contribute directly to the chemistry of catalysis. Focusing on a single active site in the closed conformation, one can easily observe that these three side chains and the two substrates are together in an arrangement favorable for reaction. (By contrast, the active site residues are significantly farther apart in the open conformation; the difference in the distance is ~5Å along the axis that changes the most during the conformation shift.) Pause/Continue Animation

The reaction mechanism for citrate synthase was proposed by Remington and colleagues^[4][7] and is illustrated here in three dimensions using structures resembling key states of the reaction^[8]. In this mechanism, three ionizable side chains in the active site of citrate synthase participate in acid-base catalysis: His 274, His 320, and Asp 375. Citrate synthase is among one of the few enzymes that can directly form a carbon-carbon bond without the presence of metal ion cofactors.

• In step one, Asp 375 acts as a base removing a proton from the methyl group of acetyl-CoA, resulting in acetyl-CoA forming its enol; His 274 (magenta) stabilizes the acetyl-CoA enol by forming a hydrogen bond with the enol's oxygen. (See the reaction scheme below for a more thorough accounting of the chemistry.)
• In step two (condensation), the enol of acetyl-CoA then nucleophilically attacks oxaloacetate’s carbonyl carbon, and His 320 (cyan) acts as an acid donating a proton to oxaloacetate’s carbonyl group in a concerted step, forming citryl-CoA as acetyl-CoA and oxaloacetate become covalently linked; citryl-CoA remains bound to the enzyme at this step in the actual reaction although that linkage is not represented in the 3D structure.
• Finally, citryl-CoA is hydrolyzed to citrate and CoA as a nearby water is deprotonated by His 320 and the oxygen attacks citryl-CoA. Release of the CoA and the citrate involves return of the closed conformation to the open conformation.
  

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Regulation

Perhaps the most crucial regulators of the citrate synthase reaction are its substrates, acetyl-CoA and oxaloacetate. Both are present in the mitochondria at concentrations below saturation of citrate synthase. The metabolic flux is controlled by substrate availability, so controlling the levels of acetyl-CoA and oxaloacetate in the mitochondria controls the rate of reaction. Furthermore, citrate synthase is inhibited by NADH, citrate (which competes with oxaloacetate), and succinyl-CoA (an example of competitive feedback inhibition) ^[9]. In many plants, bacteria and fungi, such as the peroxisomes of baker's yeast, citrate synthase plays a role in the glyoxylate cycle^[10][11][12].

• Citrate Synthase Closed Form (Monomer) 2cts
• 'Citrate Synthase Open Form (Monomer) 1cts

3D structures of Citrate Synthase

Citrate Synthase 3D structures