This is the C-terminal regulatory (R) domain of alpha-isopropylmalate synthase, which catalyses the first committed step in the leucine biosynthetic pathway [1]. This domain, is an internally duplicated structure with a novel fold [1]. 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 [1].

1: Crystal structure of LeuA from Mycobacterium tuberculosis, a key enzyme in leucine biosynthesis. Koon N, Squire CJ, Baker EN; Proc Natl Acad Sci U S A 2004;101:8295-8300. PMID:15159544