Citrate Synthase

Citrate synthase 'closed' form (2cts) and the reaction

The Structure and Mechanism of Citrate Synthase

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.

Structure: Biologically, citrate synthase exists as a

of a single amino acid chain . 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 , with its two domains forming a cleft containing the substrate (oxaloacetate) binding site (PDB: 1cts) ^[3][4]. When oxaloacetate binds, the smaller domain undergoes an 18° rotation, sealing the oxaloacetate binding site^[5] and resulting in the (PDB: 2cts)^[3]. The dramatic conformational change is best illustrated via , and be sure to view 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].

Mechanism: 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 in an arrangement favorable for reaction. (By contrast, in the open conformation; the difference in the distance is ~5Å along the axis that changes the most during the conformation shift.)

The reaction mechanism for citrate synthase was proposed by Remington and colleagues^[3][6] and is illustrated here in three dimensions using structures resembling key states of the reaction^[7]. In this mechanism, three ionizable side chains in the active site of citrate synthase participate in acid-base catalysis: . 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, , 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 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 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 . Release of the involves return of the to the open conformation.
  

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, (which competes with oxaloacetate), and succinyl-CoA (an example of competitive feedback inhibition) ^[8]. In many plants, bacteria and fungi, such as the peroxisomes of baker's yeast, citrate synthase plays a role in the glyoxylate cycle^[9][10][11].

• 2cts
• 1cts