Citric acid is a compound naturally present in citrus fruits and is fundamental to the metabolism of nearly all living organisms. It is a molecule in the pathways that generate cellular energy and a common additive in food and commercial products, used for its properties as an acidulant and preservative.
The Initial Step of the Citric Acid Cycle
The primary route for biological synthesis of citric acid is the first step of the Citric Acid Cycle. This reaction joins two molecules: acetyl-CoA and oxaloacetate. Acetyl-CoA is a carrier molecule with a two-carbon acetyl group derived from breaking down carbohydrates, fats, and proteins, while oxaloacetate is a four-carbon compound that is regenerated at the end of the cycle.
This joining is facilitated by the enzyme citrate synthase, which aligns acetyl-CoA and oxaloacetate to facilitate their chemical bond. The enzyme first binds to oxaloacetate, causing a change in its shape that creates a binding site for acetyl-CoA. This ensures the molecules are positioned correctly for the reaction.
Once both molecules are in place, citrate synthase catalyzes their condensation. The acetyl group from acetyl-CoA is transferred to oxaloacetate, forming a new six-carbon molecule called citrate, the ionized form of citric acid. The enzyme then releases the citrate and returns to its original state, ready to catalyze another reaction.
The Cellular Location of Synthesis
The synthesis of citric acid and the subsequent reactions of its cycle occur in a specialized cell compartment called the mitochondrion. Known as the cell’s “powerhouse,” the mitochondrion is where most cellular respiration takes place in eukaryotes.
A mitochondrion has two membranes: a smooth outer membrane and a highly folded inner membrane. These folds, known as cristae, increase the surface area for chemical reactions. The innermost compartment is a fluid-filled space called the mitochondrial matrix, where the synthesis of citrate and the entire Citric Acid Cycle unfold.
This location promotes metabolic efficiency by concentrating all necessary components in a small volume. The mitochondrial matrix contains the substrates like acetyl-CoA and oxaloacetate, and all the enzymes, including citrate synthase. Housing the cycle within the matrix ensures the products of one reaction are immediately available for the next.
Regulating Citric Acid Production
Cells manage the rate of citric acid synthesis to match their energy requirements, preventing deficits and overproduction. This control is achieved by regulating the enzyme citrate synthase through feedback inhibition. This process is where products of the metabolic pathway signal the enzyme to slow down.
A primary regulator is adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. High ATP levels signify an abundant energy supply. Excess ATP binds to a regulatory site on the citrate synthase enzyme, changing its shape and reducing its affinity for its substrates, which slows citrate production.
Other molecules also modulate citrate synthase activity. High concentrations of NADH, an electron carrier, indicate the energy pathways are running at high capacity and inhibit the enzyme. Succinyl-CoA, an intermediate product from later in the cycle, also acts as an inhibitor by competing with acetyl-CoA for the enzyme’s active site.
Industrial Microbial Fermentation
The large quantities of citric acid used commercially are not extracted from fruit but are produced through microbial fermentation. This industrial process primarily uses a filamentous fungus named Aspergillus niger, which is favored for its ability to produce large amounts of citric acid under controlled conditions.
The dominant method is submerged fermentation, where Aspergillus niger is grown in large, sterilized steel tanks called bioreactors. These tanks are filled with a liquid nutrient medium, or broth, that provides the fungus with the raw materials for growth and citric acid production.
To compel the fungus to overproduce citric acid, manufacturers manipulate the growth conditions. The liquid medium contains a high concentration of a sugar substrate, like molasses or corn syrup. The environment is maintained at a low pH and is limited in certain trace metals, such as manganese, which redirects the fungus’s metabolism toward synthesis and excretion of citric acid for harvesting.