The structural maturation of penicillin G acylase (PGA) from Kluyvera citrophila (KcPGA) is a tightly regulated autocatalytic process essential for enzyme activation. This transformation involves the intramolecular cleavage of a single peptide bond—Thr289–Ser290—resulting in the release of a 54-residue linker peptide and the formation of the catalytically active heterodimer composed of α and β chains. To capture the molecular architecture of the precursor prior to processing, a slow-processing Ser290Gly mutant was crystallized and analyzed at high resolution, providing unprecedented insights into the conformational state governing activation.
X-ray diffraction data were collected from two crystal forms: one belonging to the triclinic space group P1 with unit-cell parameters a = 54.0, b = 124.6, c = 135.1 Å, α = 104.1°, β = 101.4°, γ = 96.5°, and another in the monoclinic space group C2 with a = 265.1, b = 54.0, c = 249.2 Å, β = 104.4°. The P1 crystal diffracted to 2.5 Å resolution, while the C2 form reached 3.5 Å. Molecular replacement using the Escherichia coli PGA precursor structure (PDB 1e3a) yielded robust initial phases, with strong rotation and translation function scores confirming correct orientation. Initial electron-density maps revealed continuous density for residues 236–289, corresponding to the spacer peptide that connects the α and β chains in the precursor.
The overall fold of the KcPGA precursor closely mirrors that of EcPGA, with a central β-barrel domain flanked by α-helices and a conserved catalytic triad involving Serβ1, Lys299, and Glu317. However, unlike the mature enzyme, the active site cavity remains partially occluded by the intact spacer peptide, which occupies the region where the β-chain N-terminus would normally be exposed. This suggests that the spacer acts as an intrinsic inhibitor, preventing premature activation until appropriate environmental signals are received.
Notably, the side chain of Lys299 is positioned within hydrogen-bonding distance of the backbone carbonyl of Thr289 and the hydroxyl group of Ser290, indicating its potential role in stabilizing the transition state during nucleophilic attack.Ethyl3-(3-Pyridinylcarbamoyl)carbazate Protocol This arrangement supports the hypothesis that Lys299 functions as a general base in facilitating the deprotonation of Ser290, thereby initiating the cleavage reaction.1-Boc-4-bromomethylpiperidine Autophagy The proximity of this residue to the scissile bond, combined with its conservation across bacterial PGAs, underscores its functional importance.PMID:35065992
Refinement of the P1 structure resulted in a model containing 3272 out of 3312 residues, including the His-tag, with Rcryst and Rfree values of 22.9% and 27.5%, respectively. The C2 form, despite lower resolution, confirmed the same structural features, reinforcing the reliability of the observed conformation. Analysis of solvent content (50%) and Matthews coefficient (2.48 ų Da⁻¹) further supports the biological relevance of the asymmetric unit.
These results demonstrate that the unprocessed KcPGA precursor adopts a closed, inactive conformation stabilized by interdomain interactions mediated by the spacer peptide. The crystallographic data provide direct evidence that the activation mechanism is not merely a passive unfolding event but involves specific conformational rearrangements triggered by pH or metal ion binding. This insight opens new avenues for protein engineering aimed at modulating processing rates or enhancing stability under industrial conditions.
In summary, the high-resolution structure of the slow-processing KcPGA precursor offers a detailed snapshot of the enzyme’s inactive state, revealing how structural constraints regulate autocatalytic activation. These findings lay the groundwork for designing next-generation biocatalysts with improved performance in the synthesis of semi-synthetic antibiotics.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com