The E1 subunit, called the pyruvate dehydrogenase subunit, is either a homodimer (comprising two “ɑ” chains, e.g. in Escherichia coli) or a heterotetramer of two different chains (two “ɑ” and  two “ꞵ” chains). A magnesium ion forms a 4-coordinate complex with three, polar amino acid residues (Asp, Asn, and Tyr) located on the alpha chain, and the thiamine diphosphate (TPP) cofactor directly involved in decarboxylation of the pyruvate.