Structure
The NAD-IDH is composed of 3 subunits, is allosterically regulated, and requires an integrated Mg2+ or Mn2+ ion. The closest homologue that has a known structure is the E. coli NADP-dependent IDH, which has only 2 subunits and a 13% identity and 29% similarity based on the amino acid sequences, making it dissimilar to human IDH and not suitable for close comparison. All the known NADP-IDHs are homodimers.
Most isocitrate dehydrogenases are dimers, to be specific, homodimers (two identical monomer subunits forming one dimeric unit). In comparing C. glutamicum and E. coli, monomer and dimer, respectively, both enzymes were found to "efficiently catalyze identical reactions." However, C. glutamicum was recorded as having ten times as much activity than E. coli and seven times more affinitive/specific for NADP. C. glutamicum favored NADP+ over NAD+. In terms of stability with response to temperature, both enzymes had a similar Tm or melting temperature at about 55°C to 60°C. However, the monomer C. glutamicum showed a more consistent stability at higher temperatures, which was expected. The dimer E. coli showed stability at a higher temperature than normal due to the interactions between the two monomeric subunits.
Read more about this topic: Isocitrate Dehydrogenase
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