Connectivity between surface and interior in catalytic subunits of acetylcholinesterases inferred from their X‐ray structures

Abstract

Catalytic activity and function of acetylcholinesterase (AChE; EC 3.1.1.7) have been recognized and studied for over a century and its quaternary and primary structures for about half a century, and its tertiary structure has been known for about 33 years. Clear understanding of relationships between the structure and the function is still pending for this enzyme. Hundreds of crystallographic, static snapshots of AChEs from different sources reveal largely one general backbone conformation with narrow entry into the active center gorge, tightly fit to accept one acetylcholine (ACh) molecule, in contrast to its high catalytic turnover. This short review of available X‐ray structures of AChEs from electric ray Torpedo californica, mouse and human, finds some limited, yet consistent deviations in conformations of selected secondary structure elements of AChE relevant for its function. The observed conformational diversity of the acyl pocket loop of AChE, unlike the large Ω‐loop, appears consistent with structurally dynamic INS data and solution‐based SAXS experiments to explain its dominant role in controlling the size of the active center gorge opening, as well as connectivity between the immediate surroundings of the buried active Ser, and catalytically relevant sites on the AChE surface. image

Recent advances in cholinergic mechanisms: A preface for the ISCM2022 special issue

“…Cholinesterases are always a convergence point in this series of symposia, bringing together biomolecular mechanisms with structural aspects relevant for acetylcholine's physiological function in tested tissues. In this sense, a review by Zoran Radić sheds new light on the apparent paradox of the outstanding catalytic efficiency which evolved to secure effective cholinergic neurotransmission, in spite of being located within the tight geometry of the active center gorge, barely large enough to admit a single molecule of the physiological substrate (Radić, 2023). Dynamic biophysical experiments on PDB‐deposited static X‐ray structural data of AChEs propose the mechanism of connectivity linking the interior and surface of a single catalytic subunit in AChEs.…”

“…Dynamic biophysical experiments on PDB‐deposited static X‐ray structural data of AChEs propose the mechanism of connectivity linking the interior and surface of a single catalytic subunit in AChEs. Briefly, the conformational flexibility of the acyl pocket loop in AChE molecules bears significant functional implications in controlling the accessibility of the active center gorge, regulating its catalytic function (hydrolysis of ACh in synapses), as well as the oligomerization state of this enzyme and the tissue‐dependent diversity of its molecular forms (Radić, 2023).…”

A-agents, misleadingly known as “Novichoks”: a narrative review

“…A research group from the University of California, San Diego, published the most relevant information on the interaction between AChE and A-series molecules. Luedtke et al ( 2021 ) and Radić ( 2021a , 2023 ) used X-ray structural data of recombinant human AChE (hAChE) inhibited by A-234 uploaded into the Protein Data Bank. They conducted a computational study of different OP–hAChE conjugates, including those of A-230, A-232, and A-234.…”

“…They pinpointed that even bulkier structures like A-series agents fit into the hAChE active site without significant steric hindrance onto the hAChE backbone or side chains and can form stabilizing hydrophobic or electrostatic interactions with the choline-binding site. Such stabilization could render them more resistant to nucleophilic reactivation with oxime antidotes (Blumenthal et al 2021 ; Luedtke et al 2021 ; Radić 2021 , 2023 ). They also presented A-234–hAChE conjugate very vividly in virtual reality on YouTube (Radic 2021b ).…”

Activation/Inhibition of Cholinesterases by Excess Substrate: Interpretation of the Phenomenological b Factor in Steady-State Rate Equation

“…The importance of this loop for the productive binding of substrate acyl moiety has long been recognized [ 25 , 26 , 27 ]. Recent crystallographic and small angle X-ray scattering (SAXS) data on AChE conjugates confirmed the conformational plasticity of this loop for the adaptative binding of various ligands, substrates and covalent inhibitors [ 28 , 29 ]. As a result, ChEs display a complex catalytic behavior particularly with positively charged substrates.…”