text: Bacterial and mammalian SCSs are made up of α and β subunits.  In E. coli two αβ heterodimers link together to form an αβ heterotetrameric structure.  However, mammalian mitochondrial SCSs are active as αβ dimers and do not form a heterotetramer.  
  The E. coli SCS heterotetramer has been crystallized and characterized in great detail.  As can be seen in Image 2, the two α subunits (pink and green) reside on opposite sides of the structure and the two β subunits (yellow and blue) interact in the middle region of the protein.  The two α subunits only interact with a single β unit, whereas the β units interact with a single α unit (to form the αβ dimer) and the β subunit of the other αβ dimer.  A short amino acid chain links the two β subunits which gives rise to the tetrameric structure.
  The crystal structure of Succinyl-CoA synthetase alpha subunit (succinyl-CoA-binding isoform) was determined by Joyce et al. to a resolution of 2.10 A, with PDB code 1CQJ. [http://www.rcsb.org/pdb/explore.do?structureId=1CQJ].