Crystal Structure of the Pyrococcus horikoshii Isopropylmalate Isomerase Small Subunit Provides Insight into the Dual Substrate Specificity of the Enzyme

Recent studies have implied that the isopropylmalate isomerase small subunit of the hyperthermophilic archaea Pyrococcus horikoshii (PhIPMI-s) functions as isopropylmalate isomerase in the leucine biosynthesis pathway, and as homoaconitase (HACN) in the lysine biosynthesis pathway via α-aminoadipic acid. PhIPMI is thus considered a key to understanding the fundamental metabolism of the earliest organisms. We describe for the first time the crystal structure of PhIPMI-s, which displays dual substrate specificity. The crystal structure unexpectedly shows that four molecules create an interlocked assembly with intermolecular disulfide linkages having a skewed 222 point-group symmetry. Although the overall fold of the PhIPMI-s monomer is related closely to domain 4 of the aconitase (ACN), one α-helix in the ACN structure is replaced by a short loop with relatively high temperature factor values. Because this region is essential for discriminating the structurally similar substrate based on interactions with its diversified γ-moiety, the loop structure in the PhIPMI-s must be dependent on the presence of a substrate. The flexibility of the loop region might be a structural basis for recognizing both hydrophobic and hydrophilic γ-moieties of two distinct substrates, isopropylmalate and homocitrate.