Subunit Interactions in SDH
The interactions between subunits forming a tetramer in SDH is determined by non covalent interaction. These non covalent interactions consists of an hydrophobic effect, hydrogen bonds and electrostatic interactions between the four identical subunits. For homotetrameric proteins such as SDH, the structure is believed to have evolved going from a monomeric to a dimeric and finally toward a tetrameric structure in evolution. The SDH proteins are evolutionary closely related to alcohol dehydrogenase which also belongs to the protein superfamily of medium chain dehydrogenase/reductase enzymes (MDRs). Mammalian ADHs are all dimeric enzymes but certain bacterial ADHs also share a tetrameric quaternary structure. SDH from silver leaf whitefly and yeast ADH1 both lacks a structural zinc site and shares a tetrameric quaternary structure, thus showing a close evolutionary relationship from a structural viewpoint between the two classes of proteins (ADH and SDH).
The general binding process in SDH is described by the gain in free energy which can be determined from the rate of association and dissociation between subunits.
The following image shows the assembly of the four subunits (A,B,C and D) in SDH.
A hydrogen bonding network between subunits has been shown to be important for the stability of the tetrameric quaternary protein structure. For example a study of SDH which used diverse methods such as protein sequence alignments, structural comparisons, energy calculations, gelfiltration experiments and enzyme kinetics experiments, could reveal an important hydrogen bonding network which stabilizes the tetrameric quaternary structure in mammalian SDH.
Read more about this topic: Sorbitol Dehydrogenase
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