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<ref name="gould"> Gould, T.; van de Langemheen, H.; Muñoz-Elías, E.; McKinney, D.; Sacchettini, J.; Dual role of isocitrate lyase 1 in the glyoxylate and methylcitrate cycles in ''Mycobacterium tuberculosis''. ''Molecular Microbiology''. '''2006'''. ''61(4)'':940-947. doi:10.1111/j.1365-2958.2006.05297.x. </ref> | <ref name="gould"> Gould, T.; van de Langemheen, H.; Muñoz-Elías, E.; McKinney, D.; Sacchettini, J.; Dual role of isocitrate lyase 1 in the glyoxylate and methylcitrate cycles in ''Mycobacterium tuberculosis''. ''Molecular Microbiology''. '''2006'''. ''61(4)'':940-947. doi:10.1111/j.1365-2958.2006.05297.x. </ref> | ||
The active site of isocitrate lyase consists of | The active site of isocitrate lyase consists of eight residues: Trp93, Cys191, His193, Ser315, Ser317, Asn313, Thr347, Leu348. Additionally, there are several other amino acid side chains present that form hydrogen bonding opportunities with isocitrate to catalyze the breakdown reaction to glyoxylate and succinate. Ser91, Gly92, Trp93, and Arg228 form hydrogen bonds with glyoxylate while Asn313, Glu295, Arg228, and Gly192 and Trp93, Thr347, Ser315, Ser317, and His193 form hydrogen bonding opportunities with the two carboxylates within succinate. <ref name="sharma"> Sharma, V.; Sharma, S.; Hoener zu Bentrup, K.; McKinney, J.; Russell, D.; ''et. al''; Structure of isocitrate lyase, a persistence factor of ''Mycobacterium tuberculosis''. ''Nat. Struct. Biol.''. '''2000'''. ''7(8)'':663-668. </ref> Additionally, a Mg<sup>2+</sup> ion is needed for further electrostatic stabilization of the extreme negative charge on isocitrate. This Mg<sup>2+</sup> hydrogen bonds to the carboxylate in glyoxylate and one of the carboxylates in succinate. | ||
The catalytic loop of isocitrate lyase consists of residues 185-196. The two most important are Cys191 and His193 as these form a charge relay strong enough to extract a proton from isocitrate. Poor electron density has been observed for residues His193 and Leu194 indicating that this loop is very flexible. <ref name="sharma"> Sharma, V.; Sharma, S.; Hoener zu Bentrup, K.; McKinney, J.; Russell, D.; ''et. al''; Structure of isocitrate lyase, a persistence factor of ''Mycobacterium tuberculosis''. ''Nat. Struct. Biol.''. '''2000'''. ''7(8)'':663-668. </ref> This data backs up the claim that that monomers of the protein are in a structural equilibria between the open and closed forms of the active site. In order for the catalytic loop to shift into the closed position necessary for catalysis, isocitrate must be within the binding pocket. The hydrogen bonding opportunities formed cause a ripple effect that shifts the catalytic loop into a closer position. <ref name="sharma"> Sharma, V.; Sharma, S.; Hoener zu Bentrup, K.; McKinney, J.; Russell, D.; ''et. al''; Structure of isocitrate lyase, a persistence factor of ''Mycobacterium tuberculosis''. ''Nat. Struct. Biol.''. '''2000'''. ''7(8)'':663-668. </ref> This shift also causes the C-terminal domain of the subunit (residues 411-428) to move into the former position of the catalytic loop. The C-terminal domain is stabilized by an electrostatic interaction with Lys189. This combined movement locks the active site residues into a proper orientation for lysis of a C-C bond within isocitrate. <ref name="sharma"> Sharma, V.; Sharma, S.; Hoener zu Bentrup, K.; McKinney, J.; Russell, D.; ''et. al''; Structure of isocitrate lyase, a persistence factor of ''Mycobacterium tuberculosis''. ''Nat. Struct. Biol.''. '''2000'''. ''7(8)'':663-668. </ref> | |||