| DLAT (Pyruvate dehydrogenase E2, PDH‑E2, dihydrolipoyl acetyltransferase) forms the inner catalytic core of the mammalian pyruvate dehydrogenase complex (PDC), where it integrates glycolytic carbon flux into mitochondrial acetyl‑CoA production and thereby couples carbohydrate oxidation to the citric acid cycle and downstream lipid biosynthesis. PDH‑E2 belongs to the 2‑oxoacid dehydrogenase family and organizes as a dodecahedral assembly of multiple identical inner‑core domains, which provide the scaffold for peripheral E1 (pyruvate dehydrogenase) and E3 (dihydrolipoamide dehydrogenase) as well as the E3‑binding protein (E3BP) that is specific to mammalian PDC and is embedded into the same core architecture. Each E2 polypeptide contains N‑terminal lipoyl domains and an E1‑binding segment connected by flexible linkers to a C‑terminal inner‑core domain; the inner‑core portion adopts a conserved α/β fold organized into trimers at each three‑fold axis, with these trimers further linked along two‑fold axes into a hollow dodecahedral cage that positions active sites toward internal and external solvent channels. Within each trimer, two neighboring E2 subunits cooperate to form a composite acetyltransferase active site, where a serine residue from one subunit and a histidine from the clockwise partner coordinate binding and positioning of the dihydrolipoyl group and coenzyme A, guiding acetyl transfer from the reduced lipoyllysine arm to CoA during the oxidative decarboxylation sequence initiated by E1. The lipoyllysine “swinging arm” approaches the E2 channel from the outer surface through a narrow passage near helix H1, while CoA accesses the same active site from the inner cavity through a separate portal; shaped electrostatic landscapes at the exterior, channel, and interior surfaces favor productive guidance of lipoylated domains and CoA without nonspecific trapping of substrate or product. A mobile β‑turn between βE and βF at the heart of the active‑site channel lacks ordered density and is positioned to act as a dynamic gate that modulates access and release of lipoyl and CoA ligands, supporting tight coupling of acetyl transfer to upstream decarboxylation and downstream NADH production. Along the two‑fold interface, C‑terminal helices form hydrophobic “knobs” that dock into complementary “holes” on the partner trimer; in human E2, a straight H2 helix and terminal H7 helix remodel this interaction to favor a larger intertrimer angle and thus the dodecahedral rather than cubic geometry seen in other family members, a feature that shapes how E1, E3, and E3BP are arranged and regulated around the core. The same structural surfaces that determine the dodecahedral organization also define how lipoyl domains and regulatory proteins approach the core, so the E2 inner architecture directly influences substrate channeling efficiency, responsiveness to pyruvate dehydrogenase kinases and phosphatases, and the integration of E1 and E3 activities into a single catalytic assembly. A closely related inner‑core domain in E3BP shares high sequence and structural similarity with E2 and can occupy equivalent positions in heterotrimers, while lacking the catalytic histidine; E3BP thereby retains substrate‑binding determinants for the lipoyl arm and CoA but alters the catalytic configuration of one active site within the trimer, creating mixed E2/E3BP trimers that modify local acetyltransferase capacity and the distribution of E3 around the core. Modeling and interface analysis indicate that heterotrimers containing two E2 and one E3BP inner‑core domains are energetically favored, with E3BP‑rich interfaces weaker than E2–E2 contacts; this arrangement supports a core in which E3BP subunits are inserted in a limited, patterned manner that preserves overall dodecahedral geometry while tuning E3 recruitment and local catalytic environment. Through this modular organization, PDH‑E2 not only catalyzes acetyl transfer but also determines the spatial pattern of catalytic centers and E3 docking, thereby shaping flux through pyruvate oxidation and influencing how PDC responds to changes in nutrient availability, redox balance, and hormonal input that signal through pyruvate dehydrogenase kinases and phosphatases. The irreversible conversion of pyruvate to acetyl‑CoA, supported by the E2 core, constitutes a major control point in mammalian energy metabolism, and reduction in overall PDC function due to defects in E2, E3BP, or their assembly leads to impaired oxidative metabolism, accumulation of pyruvate and lactate, and neurological dysfunction linked to lactic acidosis and other metabolic disorders. |
