oxalosuccinate

The third box is Step 2, which is the decarboxylation of oxalosuccinate.

The decarboxylation of oxalosuccinate (below center) is a key step in the formation of alpha-ketoglutarate.

After carbon dioxide is split from the oxalosuccinate in the decarboxylation step (below right), the enol will retautomerize to the keto from.

The oxidation of the alpha-C also takes place in this picture where NAD accepts a hydride resulting in oxalosuccinate.

The 3 substrates of this enzyme are ATP, 2-oxoglutarate, and HCO3-, whereas its 3 products are ADP, phosphate, and oxalosuccinate.