is the decarboxylation of oxalosuccinate. In this step, the carboxyl group oxygen is deprotonated by a nearby tyrosine, and those electrons flow down to C2. Carbon dioxide, the leaving group, detaches from the beta carbon of isocitrate (C3) and the electrons flow to the ketone oxygen attached to the alpha carbon, granting a negative charge to the associated oxygen atom and forming an alpha-beta unsaturated double bond between carbons 2 and 3. The fourth and final box illustrates step 3, which is the saturation of the alpha-beta unsaturated double bond that formed in the previous step. The negatively charged oxygen (attached to the alpha-carbon) donates its electrons, reforming the ketone double bond and pushing another lone pair (the one that forms the double bond between the alpha and beta carbons) "off" the molecule. This lone pair, in turn, picks up a proton from the nearby tyrosine. This reaction results in the formation of alpha-ketoglutarate, NADH + H+/NADPH + H+, and CO2. Detailed mechanism Two aspartate amino acid residues (below left) are interacting with two adjacent water molecules (w6 and w8) in the Mn2+ isocitrate porcine IDH complex to deprotonate the alcohol off the alpha-carbon atom. The oxidation of the alpha-C also takes place in this picture where NAD+ accepts a hydride resulting in oxalosuccinate. Along with the sp3 to sp2 stereochemical change around the alpha-C, there is a ketone group that is formed from the alcohol group. The formation of this ketone double bond allows for resonance to take place as electrons coming down from the leaving carboxylate group move towards the ketone. The decarboxylation of oxalosuccinate (below center) is a key step in the formation of alpha-ketoglutarate. In this reaction, the lone pair on the adjacent Tyrosine hydroxyl abstracts the proton off the carboxyl group. This carboxyl group is also referred to as the beta subunit in the isocitrate molecule. The deprotonation of the carboxyl group causes the lone pair of electrons to move down making carbon dioxide and separating from oxalosuccinate. The electrons continue to move towards the alpha carbon pushing the double bond electrons (making the ketone) up to abstract a proton off an adjacent lysine residue. An alpha-beta unsaturated double bond results between carbon 2 and three. As you can see in the picture, the green ion represents either Mg2+ or Mn2+, which is a cofactor necessary for this reaction to occur. The metal-ion forms a