<p>This is the C-terminal regulatory (R) domain of alpha-isopropylmalate synthase, which catalyses the first committed step in the leucine biosynthetic pathway [<cite idref="PUB00020847"/>]. This domain, is an internally duplicated structure with a novel fold [<cite idref="PUB00020847"/>]. It comprises two similar units that are arranged such that the two -helices pack together in the centre, crossing at an angle of 34 degrees, sandwiched between the two three-stranded, antiparallel beta-sheets. The overall domain is thus constructed as a beta-alpha-beta three-layer sandwich [<cite idref="PUB00020847"/>]. </p> 2-isopropylmalate synthase LeuA, allosteric (dimerisation) domain