Anthony Noles Sandbox: Difference between revisions

The <scene name='Anthony_Noles_Sandbox/Secondary_structure/1'>secondary structure</scene> consists of numerous alternating alpha helices and beta sheets (SCOP classification α/β alternating). The tertiary structure is somewhat bilobed with the active site in the middle, and, since there is only one subunit, there is no quaternary structure. Aconitase consists of four domains, three of which are tightly packed while the fourth is more flexible. <ref name="Frishman">Frishman, D., and Hentze, M.W., "Conservation of aconitase residues revealed by multiple sequence analysis: Implications for structure/function relationships." European Journal of Biochemistry, 1996, 239, 197-200.</ref> Aconitase contains a <scene name='Anthony_Noles_Sandbox/Fe-scluster/2'>4Fe-4S iron-sulfur cluster</scene>. This iron sulfur cluster does not participate in redox as most do, but holds the OH goup of citrate to facilitate its elimination.<ref>PMID:16407072 </ref> It is at this 4Fe-4S site that catalysis occurs and citrate or <scene name='Anthony_Noles_Sandbox/Fe-scluster_bound_isocitrate/8'>isocitrate</scene> is bound. The rest of the <scene name='Anthony_Noles_Sandbox/Fe-scluster_w_active_site/5'>active site (manually rotate this scene to see the proximity of each residue to the 4Fe-4S cluster)</scene> is made up of residues Gln72, Asp100, His101, Asp165, Ser166, His167, His147, Glu262, Asn258, Cys358, Cys421, Cys424, Cys358, Cys421, Asn446, Arg447, Arg452, Asp568, Ser642, Ser643, Arg644, Arg580. <ref name="Beinert">Beinert, H., Kennedy, M. C., Stout, C.D. “Aconitase as Iron−Sulfur Protein, Enzyme, and Iron-Regulatory Protein.” Chem. Rev. 1996, 96, 2335−2373.</ref>

Substrate-free aconitase contains a [4Fe-4S]²⁺ cluster with hydroxyl bound to one of the Fe. Upon binding of substrate the bound hydroxyl is protonated. A hydrogen bond from <scene name='Anthony_Noles_Sandbox/His101/3'>His101</scene> to the isocitrate hydroxyl is donated to form water. Alternatively, the proton could be donated by <scene name='Anthony_Noles_Sandbox/His167/3'>His167</scene> as this histidine is hydrogen bonded to a H2O molecule. His167 is also hydrogen bonded to the bound H2O in the [4Fe-4S] cluster. Both <scene name='Anthony_Noles_Sandbox/His_101_and_167/4'>His101 and His167</scene> are paired with carboxylates (<scene name='Anthony_Noles_Sandbox/Asp100_and_glu262/2'>Asp100 and Glu262</scene>, respectively) and are likely to be protonated. The conformational change associated with substrate binding reorients the cluster. <ref name="Beinert" />  The residue which removes a proton from citrate or isocitrate is <scene name='Anthony_Noles_Sandbox/Ser642/3'>Ser642</scene>. <ref name="Beinert" /> This causes the cis-Aconitate intermediate (seen below), which consists of a double bond, which is a direct result of the deprotonation. Then, there is a rehydration of the double bond of cis-aconitate to form isocitrate (if the original substrate was citrate). To better understand this, consider this process as stages, seen below.