Fumarase 2: Difference between revisions

Fumarase, also known as fumarate hydratase, functions as an enzyme in the metabolic pathway known as the Kreb’s cycle, or citric acid cycle.  As the seventh step in the pathway, fumarase catalyzes the reversible reaction converting fumarate to S-malate.  It metabolizes Fumarate in the cytosol, which becomes a byproduct of the urea cycle and amino acid catabolism. It catalyzes the addition of water to make S-Malate; therefore, the mechanism of fumarase in the reaction involves hydration of fumarate in order to form S-malate.

Fumarase is classified as an all alpha protein which belongs to the L-aspartase/fumarase family, and the enzyme specifically consists of four identical subunits which form a tetramer (see image on right).  From the four subunits, fumarase has three domains which comprise two binding sites: the active site and B site.  Although the active site has a mostly solid structure and shifts very little when it binds, the B site shifts substantially more upon binding, and this shift helps regulate affinity for molecule binding at the active site.

Fumarase has the ability to catalyze the hydration of fumarate to malate or the dehydration of malate to fumarate.  The mechanism of fumarase in the hydration and dehydration reaction pathways remains simple, only involving three steps.  In the dehydration reaction, fumarase deprotonates a carbon atom on malate to form a carbanion.  This deprotonation results in an aci-carboxylate intermediate. After the intermediate forms, the acidic proton from the initial step removes the hydroxide group from the aci-carboxylate intermediate to form fumarase which then detaches from the active site of fumarase, completing the reaction <ref name="Weaver, et al."> Weaver,T.  Structure of free fumarase C from ''Escherichia coli''. ''Acta Crystallographica'' (2005), '''D61''', 1395-1401. ['''http://dx.doi.org/10.1107/S0907444905024194''' doi:10.1107/S0907444905024194]</ref>.  In the <scene name='Vas_Sandbox_1/Active_site/1'>active site</scene>, amino acid residues involved in binding the substrate are located on three subunits: Thr100, Ser139, Ser140, and Asn141 from the b-subunit, Thr187 and His188 on the d-subunit, and Lys324 and Asn326 of the c-subunit <ref name="Beeckmans, et al."> Beeckmans, S. & Van Driessche, E.  Pig heart fumarase contains two distinct substrate-binding sites differing in affinity.  ''Journal of Biological Chemistry'' (1998), '''273'''(48), 31661-31669.</ref>.  The B site is located in a π-helix turn between the active site and solvent, and it includes residues Arg126, Lys127, Val128, His129, Pro130, Asn131, and  Asp132 all on the b-subunit.  Two hydrogen bonds initiate the binding of Asn131 and Asp132 residues with S-malate.