Fumarate hydratase, also called fumarase, is the enzyme that reversibly interconverts fumarate plus water with L-malate. The activity is EC4.2.1.2:In the hydration direction, water adds across the carbon-carbon double bond of fumarate. In the reverse direction, water is eliminated. Its lyase classification describes this transformation; the mere presence of water does not make the reaction hydrolysis.[ref]
Fumarate hydratase in IUBMB Enzyme Nomenclature. Reaction and accepted/other names
For an invented inventory, begin with ten micromoles of fumarate and let six micromoles undergo hydration. Four micromoles of fumarate remain and six micromoles of L-malate form. Each retains a four-carbon skeleton, so the counted carbon inventory remains micromoles of carbon atoms. The chosen extent does not determine chemical equilibrium or enzyme kinetics.
A 1995 X-ray crystallography study of Escherichia coli fumarase C found a four-subunit assembly at resolution. A competitive tungstate derivative helped locate the active site, which included atoms from three of the four protein subunits.[ref]
The multisubunit active site of fumarase C from Escherichia coli by T. M. Weaver and others. Abstract, tetrameric structure and multisubunit active-site assignment
The example shows why counting subunits and locating a catalytic site are different tasks: several chains can cooperate to build one local reaction environment. New to topics? Read the docs here!