in mammals, the enzyme is a heterodimer of an α- and a β-subunit. the specificity for either adenosine or guanosine phosphates is defined by the β-subunit, which is encoded by 2 genes. suclg2 is gtp-specific and sucla2 is atp-specific, while suclg1 encodes the common α-subunit. β variants are produced at different amounts in different tissues, causing gtp or atp substrate requirements.