Fumarate is a naturally occurring organic molecule found in the cells of most living things, including humans. It is a metabolite, a substance formed during chemical processes that grant cells energy to grow and reproduce. Fumarate is an intermediate compound in a biochemical pathway fundamental to how cells extract energy from nutrients.
Chemical Composition and Geometry
The chemical identity of fumarate is defined by its formula, C₄H₂O₄²⁻, indicating it is composed of four carbon, two hydrogen, and four oxygen atoms, with an overall negative charge. The core of the molecule’s structure is a rigid carbon-carbon double bond (C=C). Attached to the two central carbon atoms are two identical functional groups known as carboxylate groups (-COO⁻).
The arrangement of these groups determines the molecule’s geometry. In fumarate, the carboxylate groups are positioned on opposite sides of the carbon-carbon double bond, a spatial orientation known as a trans configuration. This arrangement forces the main atoms into a flat, planar structure, which minimizes crowding between the bulky carboxylate groups and contributes to the molecule’s stability.
The Isomeric Difference From Maleate
Fumarate shares its chemical formula with another molecule, maleate, yet the two have distinct properties due to a difference in their geometry. Maleate is the cis-isomer, meaning its two carboxylate groups are on the same side of the carbon-carbon double bond, which contrasts with fumarate’s trans configuration. This variation in atomic arrangement leads to significant changes in molecular shape and stability.
The structure of maleate, with its carboxylate groups in close proximity, introduces steric hindrance—a repulsion between bulky groups that forces the molecule into a less stable, higher-energy state. Fumarate avoids this internal strain, making it the more stable of the two isomers. This difference has biological implications, as cellular enzymes are highly specific.
Cellular enzymes have active sites precisely shaped to bind with specific molecules, much like a lock accepts only a specific key. An enzyme designed to interact with the linear, trans shape of fumarate will not be able to properly bind to the bent, cis structure of maleate. This specificity ensures that biochemical pathways proceed correctly.
Function in the Citric Acid Cycle
The structure of fumarate is directly related to its role as an intermediate in the citric acid cycle, also known as the Krebs cycle. This cycle is a series of chemical reactions used by aerobic organisms to release stored energy. Within this pathway, fumarate participates in a hydration reaction, where a water molecule is added across its double bond.
This reaction is catalyzed by a specific enzyme called fumarate hydratase, or fumarase. The enzyme’s active site is conformed to the planar, trans geometry of fumarate, allowing it to bind and facilitate the chemical change. The product of this reaction is another intermediate called malate.
The specificity of fumarate hydratase is absolute; it cannot bind or act upon maleate, fumarate’s cis-isomer. This enzymatic selectivity ensures that only the correct substrate is acted upon. The conversion of fumarate to malate is one of the final steps in the cycle before the pathway regenerates its starting molecule.