Solution structure and dynamics of the lipoic acid-bearing domain of human mitochondrial branched-chain α-keto acid dehydrogenase complex

The lipoyl-bearing domain (LBD) of the transacylase (E2) subunit of the branched-chain α-keto acid dehydrogenase complex plays a central role in substrate channeling in this mitochondrial multienzyme complex. We have employed multidimensional heteronuclear NMR techniques to determine the structure and dynamics of the LBD of the human branched-chain α-keto acid dehydrogenase complex (hbLBD). Similar to LBD from other members of the α-keto acid dehydrogenase family, the solution structure of hbLBD is a flattened β-barrel formed by two four-stranded antiparallel β-sheets. The lipoyl Lys⁴⁴ residue resides at the tip of a β-hairpin comprising a sharp type I β-turn and the two connecting β-strands 4 and 5. A prominent V-shaped groove formed by a surface loop, L1, connecting β1- and β2-strands and the lipoyl lysine β-hairpin constitutes the functional pocket. We further applied reduced spectral density functions formalism to extract dynamic information of hbLBD from ¹⁵N-T₁, ¹⁵N-T₂, and (¹H-¹⁵N) nuclear Over-hauser effect data obtained at 600 MHz. The results showed that residues surrounding the lipoyl lysine region comprising the L1 loop and the Lys⁴⁴ β-turn are highly flexible, whereas β-sheet S1 appears to display a slow conformational exchange process.