Mechanism-based cross-linking probes capture the Escherichia coli ketosynthase FabB in conformationally distinct catalytic states.

Ketosynthases (KSs) catalyse essential carbon-carbon bond-forming reactions in fatty-acid biosynthesis using a two-step, ping-pong reaction mechanism. In Escherichia coli, there are two homodimeric elongating KSs, FabB and FabF, which possess overlapping substrate selectivity. However, FabB is essential for the biosynthesis of the unsaturated fatty acids (UFAs) required for cell survival in the absence of exogenous UFAs. Additionally, FabB has reduced activity towards substrates longer than 12 C atoms, whereas FabF efficiently catalyses the elongation of saturated C14 and unsaturated C16:1 acyl-acyl carrier protein (ACP) complexes. In this study, two cross-linked crystal structures of FabB in complex with ACPs functionalized with long-chain fatty-acid cross-linking probes that approximate catalytic steps were solved. Both homodimeric structures possess asymmetric substrate-binding pockets suggestive of cooperative relationships between the two FabB monomers when engaged with C14 and C16 acyl chains. In addition, these structures capture an unusual rotamer of the active-site gating residue, Phe392, which is potentially representative of the catalytic state prior to substrate release. These structures demonstrate the utility of mechanism-based cross-linking methods to capture and elucidate conformational transitions accompanying KS-mediated catalysis at near-atomic resolution.

Structures of the human LONP1 protease reveal regulatory steps involved in protease activation.

The human mitochondrial AAA+ protein LONP1 is a critical quality control protease involved in regulating diverse aspects of mitochondrial biology including proteostasis, electron transport chain activity, and mitochondrial transcription. As such, genetic or aging-associated imbalances in LONP1 activity are implicated in pathologic mitochondrial dysfunction associated with numerous human diseases. Despite this importance, the molecular basis for LONP1-dependent proteolytic activity remains poorly defined. Here, we solved cryo-electron microscopy structures of human LONP1 to reveal the underlying molecular mechanisms governing substrate proteolysis. We show that, like bacterial Lon, human LONP1 adopts both an open and closed spiral staircase orientation dictated by the presence of substrate and nucleotide. Unlike bacterial Lon, human LONP1 contains a second spiral staircase within its ATPase domain that engages substrate as it is translocated toward the proteolytic chamber. Intriguingly, and in contrast to its bacterial ortholog, substrate binding within the central ATPase channel of LONP1 alone is insufficient to induce the activated conformation of the protease domains. To successfully induce the active protease conformation in substrate-bound LONP1, substrate binding within the protease active site is necessary, which we demonstrate by adding bortezomib, a peptidomimetic active site inhibitor of LONP1. These results suggest LONP1 can decouple ATPase and protease activities depending on whether AAA+ or both AAA+ and protease domains bind substrate. Importantly, our structures provide a molecular framework to define the critical importance of LONP1 in regulating mitochondrial proteostasis in health and disease.

Structure and mechanistic analyses of the gating mechanism of elongating ketosynthases.

Ketosynthases (KSs) catalyze carbon-carbon bond forming reactions in fatty acid synthases (FASs) and polyketide synthases (PKSs). KSs utilize a two-step ping pong kinetic mechanism to carry out an overall decarboxylative thio-Claisen condensation that can be separated into the transacylation and condensation reactions. In both steps, an acyl carrier protein (ACP) delivers thioester tethered substrates to the active sites of KSs. Therefore, protein-protein interactions (PPIs) and KS-mediated substrate recognition events are required for catalysis. Recently, crystal structures of Escherichia coli elongating type II FAS KSs, FabF and FabB, in complex with E. coli ACP, AcpP, revealed distinct conformational states of two active site KS loops. These loops were proposed to operate via a gating mechanism to coordinate substrate recognition and delivery followed by catalysis. Here we interrogate this proposed gating mechanism by solving two additional high-resolution structures of substrate engaged AcpP-FabF complexes, one of which provides the missing AcpP-FabF gate-closed conformation. Clearly defined interactions of one of these active site loops with AcpP are present in both the open and closed conformations, suggesting AcpP binding triggers or stabilizes gating transitions, further implicating PPIs in carrier protein-dependent catalysis. We functionally demonstrate the importance of gating in the overall KS condensation reaction and provide experimental evidence for its role in the transacylation reaction. Furthermore, we evaluate the catalytic importance of these loops using alanine scanning mutagenesis and also investigate chimeric FabF constructs carrying elements found in type I PKS KS domains. These findings broaden our understanding of the KS mechanism which advances future engineering efforts in both FASs and evolutionarily related PKSs.

Interfacial plasticity facilitates high reaction rate of E. coli FAS malonyl-CoA:ACP transacylase, FabD.

Fatty acid synthases (FASs) and polyketide synthases (PKSs) iteratively elongate and often reduce two-carbon ketide units in de novo fatty acid and polyketide biosynthesis. Cycles of chain extensions in FAS and PKS are initiated by an acyltransferase (AT), which loads monomer units onto acyl carrier proteins (ACPs), small, flexible proteins that shuttle covalently linked intermediates between catalytic partners. Formation of productive ACP-AT interactions is required for catalysis and specificity within primary and secondary FAS and PKS pathways. Here, we use the Escherichia coli FAS AT, FabD, and its cognate ACP, AcpP, to interrogate type II FAS ACP-AT interactions. We utilize a covalent crosslinking probe to trap transient interactions between AcpP and FabD to elucidate the X-ray crystal structure of a type II ACP-AT complex. Our structural data are supported using a combination of mutational, crosslinking, and kinetic analyses, and long-timescale molecular dynamics (MD) simulations. Together, these complementary approaches reveal key catalytic features of FAS ACP-AT interactions. These mechanistic inferences suggest that AcpP adopts multiple, productive conformations at the AT binding interface, allowing the complex to sustain high transacylation rates. Furthermore, MD simulations support rigid body subdomain motions within the FabD structure that may play a key role in AT activity and substrate selectivity.

Activity Mapping the Acyl Carrier Protein: Elongating Ketosynthase Interaction in Fatty Acid Biosynthesis.

Elongating ketosynthases (KSs) catalyze carbon-carbon bond-forming reactions during the committed step for each round of chain extension in both fatty acid synthases (FASs) and polyketide synthases (PKSs). A small α-helical acyl carrier protein (ACP) shuttles fatty acyl intermediates between enzyme active sites. To accomplish this task, the ACP relies on a series of dynamic interactions with multiple partner enzymes of FAS and associated FAS-dependent pathways. Recent structures of the Escherichia coli FAS ACP, AcpP, in covalent complexes with its two cognate elongating KSs, FabF and FabB, provide high-resolution details of these interfaces, but a systematic analysis of specific interfacial interactions responsible for stabilizing these complexes has not yet been undertaken. Here, we use site-directed mutagenesis with both in vitro and in vivo activity analyses to quantitatively evaluate these contacting surfaces between AcpP and FabF. We delineate the FabF interface into three interacting regions and demonstrate the effects of point mutants, double mutants, and region deletion variants. Results from these analyses reveal a robust and modular FabF interface capable of tolerating seemingly critical interface mutations with only the deletion of an entire region significantly compromising activity. Structure and sequence analyses of FabF orthologs from related type II FAS pathways indicate significant conservation of type II FAS KS interface residues and, overall, support its delineation into interaction regions. These findings strengthen our mechanistic understanding of molecular recognition events between ACPs and FAS enzymes and provide a blueprint for engineering ACP-dependent biosynthetic pathways.

Gating mechanism of elongating ß-ketoacyl-ACP synthases.

Carbon-carbon bond forming reactions are essential transformations in natural product biosynthesis. During de novo fatty acid and polyketide biosynthesis, ß-ketoacyl-acyl carrier protein (ACP) synthases (KS), catalyze this process via a decarboxylative Claisen-like condensation reaction. KSs must recognize multiple chemically distinct ACPs and choreograph a ping-pong mechanism, often in an iterative fashion. Here, we report crystal structures of substrate mimetic bearing ACPs in complex with the elongating KSs from Escherichia coli, FabF and FabB, in order to better understand the stereochemical features governing substrate discrimination by KSs. Complemented by molecular dynamics (MD) simulations and mutagenesis studies, these structures reveal conformational states accessed during KS catalysis. These data taken together support a gating mechanism that regulates acyl-ACP binding and substrate delivery to the KS active site. Two active site loops undergo large conformational excursions during this dynamic gating mechanism and are likely evolutionarily conserved features in elongating KSs.

Unveiling the functional diversity of the alpha/beta hydrolase superfamily in the plant kingdom.

The alpha/beta hydrolase (ABH) superfamily is a widespread and functionally malleable protein fold recognized for its diverse biochemical activities across all three domains of life. ABH enzymes possess unexpected catalytic activity in the green plant lineage through selective alterations in active site architecture and chemistry. Furthermore, the ABH fold serves as the core structure for phytohormone and ligand receptors in the gibberellin, strigolactone, and karrikin signaling pathways in plants. Despite recent discoveries, the ABH family is sparsely characterized in plants, a sessile kingdom known to evolve complex and specialized chemical adaptations as survival responses to widely varying biotic and abiotic ecologies. This review calls attention to the ABH superfamily in the plant kingdom to highlight the functional adaptability of the ABH fold.