Oxalosuccinate in Step 4 of the Krebs cycle undergoes decarboxylation. In the presence of oxalosuccinate decarboxylase enzyme, oxalosuccinate is changed into α-ketoglutarate.

In step 5 of Kreb's cycle -carbon compound, α – Ketoglutarate undergoes simultaneous dehydrogenation and decarboxylation in the presence of enzyme α – ketoglutarate dehydrogenase complex. This enzyme complex contains TPP, Lipoic Acid, Mg2+, and trans – succinylase. NAD+ and CoA are required. The products formed are 4 – carbon compound succinyl CoA, NADH2 and CO2.