= Fumarate hydratase
= Fumarase
{synonym}
<Fumarate hydratase>, also called <fumarase>, is the <enzyme> that reversibly interconverts <fumarate> plus <water> with <L-malate>. The activity is EC4.2.1.2:
$$
\mathrm{fumarate}+\mathrm{H_2O}\rightleftharpoons\text{L-malate}.
$$
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>.<Fumarate hydratase in IUBMB Enzyme Nomenclature>[Reaction and accepted/other names]{ref}
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 $4(4+6)=40$ 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 $2.0\,\text{Å}$ resolution. A competitive <tungstate> derivative helped locate the <active site>, which included atoms from three of the four <protein subunits>.<The multisubunit active site of fumarase C from Escherichia coli by T. M. Weaver and others>[Abstract, tetrameric structure and multisubunit active-site assignment]{ref} The example shows why counting subunits and locating a catalytic site are different tasks: several chains can cooperate to build one local reaction environment.
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